Explorations: Una Invitación Abierta a la Antropología Biológica, 2ª Edición - Capitulo 7: Piedras y Huesos PDF

Inicio Leer Iniciar sesión Buscar en el libro... ¿Quieres crear o adaptar libros como este? Obtenga más información sobre cómo Pressbooks apoya las prácticas de publicación abierta. EXPLORACIONES: UNA INVITACIÓN ABIERTA A LA ANTROPOLOGÍA BIOLÓGICA, 2ª EDICIÓN CONTENIDO 7. Piedras y huesos: estudiando el registro fósil Sarah S. King, Ph.D., Colegio Comunitario de Cerro Coso Kara Jones, M.A., estudiante de doctorado, Universidad de Nevada en Las Vegas Este capítulo es una revisión del "Capítulo 7: Entendiendo el Contexto Fósil" de Sarah King y Lee Anne Zajicek. En Explorations: An Open Invitation to Biological Anthropology, primera edición, editado por Beth Shook, Katie Nelson, Kelsie Aguilera y Lara Braff, que está licenciado bajo CC BY-NC 4.0. Objetivos de aprendizaje Identificar los diferentes tipos de fósiles y describir cómo se forman. Discutir los métodos de datación relativa y cronométrica, el tipo de material que analizan y sus aplicaciones. Describir los métodos utilizados para reconstruir entornos pasados. Interprete un sitio utilizando los métodos descritos en este capítulo. Estudio de fósiles: un proceso en evolución Mary Anning y la Edad de las Maravillas Mary Anning (1799-1847) es probablemente la cazadora de fósiles más famosa de la que nunca hayas oído hablar (Figura 7.1). Anning vivió toda su vida en Lyme Regis, en la costa de Dorset, Inglaterra. Como mujer, nacida en el seno de una familia pobre, con una educación mínima (incluso para los estándares del siglo XIX), las probabilidades estaban en contra de que Anning se convirtiera en científica (Emling 2009, xii). Fue notable que Anning fuera capaz de influir en los grandes científicos de la época con sus descubrimientos de fósiles y sus posteriores hipótesis sobre la evolución. Figura 7.1: Una pintura al La época en que vivió Anning fue un período notable en la historia óleo de Mary Anning y su de la humanidad debido a la Revolución Industrial en Gran Bretaña. perro, Tray, antes de 1845. Además, los descubrimientos científicos de los siglos XVIII y XIX La "Costa Jurásica" de Lyme Regis está al fondo. Nótese sentaron las bases para grandes avances en el conocimiento y la que Anning está señalando comprensión sobre los seres humanos y el mundo natural. Apenas un fósil. Crédito: Mary un siglo antes, Sir Isaac Newton había desarrollado sus teorías sobre Anning por B. J. Donne de la física y se había convertido en el presidente de la Royal Society de Sociedad Geológica / Londres (Dolnick 2011, 5). En este marco, la búsqueda de descubrim- NHMPL es de dominio público. ientos intelectuales y científicos se convirtió en una vocación popu- lar para muchos individuos, la gran mayoría de los cuales eran hom- bres ricos (figura 7.2). A pesar de las expectativas de la sociedad geor- giana inglesa de lo contrario, Anning se convirtió en un exitoso cazador de fósiles, así como en un geólogo y anatomista autodidacta. La geología de Lyme Regis, con sus acantilados de piedra cal- iza, proporcionó un telón de fondo fortuito para el trabajo de toda la vida de Anning. Ahora lla- mada la "Costa Jurásica", Lyme Regis siempre ha sido una rica fuente de restos fosilizados (Figura 7.3). Continuando con la pasión de su padre por la búsqueda de fósiles, Anning recorrió los acan- Figure 7.2: A Walk at Dusk, 1830–1835, by Caspar tilados desmoronados después de las tormentas David Friedrich, is a painting likely of a dolmen, a en busca de restos fosilizados y conchas. El tra- megalithic (large rock) tomb. Dolmens were built bajo era físicamente exigente y francamente throughout Europe, five to six thousand years peligroso. En 1833, mientras buscaba fósiles, ago. Scholars were fascinated by the ancient Anning perdió a su amado perro en un desliza- world, which was an accepted part of Earth’s history, even if explanation defied nonsecular miento de tierra y casi pierde su propia vida en el thought. Credit: A Walk at Dusk object 93.PA.14 proceso (Emling 2009). by Casper David Friedrich German, 1774–1840, Paul Getty Museum, is in the public domain and Around part of the Getty Open Content Program. the age of ten, Anning located and excavated a complete fossilized skeleton of an ichthyosaurus (“fish lizard”). She eventually found Pterodactylus macronyx and a 2.7-meter Plesiosaurus, considered by many to be her greatest dis- covery (Figure 7.4). These discoveries proved that there had been significant changes in the way living things ap- Figure 7.3: The “Jurassic Coast” of Lyme Regis: the home of fossil hunter Mary peared throughout the history of the world. Like many of Anning. Credit: Lyme-regis-coast-sea- her peers, including Darwin, Anning had strong religious cliffs-924431 by jstarj has been convictions. However, the evidence that was being found designated to the public domain (CC0) in the fossil record was contradictory to the Genesis story under a Pixabay License. in the Bible. In The Fossil Hunter: Dinosaurs, Evolution, and the Woman Whose Discoveries Changed the World, Anning’s biographer Shelley Emling (2009, 38) notes, “the puzzling attributes of Mary’s fossil [ichthyosaurus] struck a blow at this belief and eventually helped pave the way for a real under- standing of life before the age of humans.” Intellectual and scientific debate now had physical evidence to support the theory of evolution, which would eventually result in Darwin’s seminal work, On the Origin of Species (1859). Anning’s discoveries and theories were appreciated and advo- cated by her friends, intellectual men who were associated with the Geological Society of London. Regrettably, this orga- nization was closed to women, and Anning received little offi- cial recognition for her contributions to the fields of natural history and paleontology. It is clear that Anning’s knowledge, diligence, and uncanny luck in finding magnificent specimens of fossils earned her unshakeable credibility and made her a peer to many antiquarians (Emling 2009). Fossil hunting is still providing evidence and a narrative of the story of Earth. Mary Anning recognized the value of fossils in understanding natural history and relentlessly championed her theories to the brightest minds of her day. Anning’s ability to creatively think “outside the box”—skillfully assimilating knowledge from multiple academic fields—was her gift to our Figure 7.4: Plesiosaurus, illustrated and described by Mary Anning in present understanding of the fossil record. Given how pro- an undated handwritten letter. foundly Anning has shaped how we, in the modern day, think Credit: Autograph letter about the origins of life, it is surprising that her contributions concerning the discovery of have been so marginalized. Anning’s name should be on the tip plesiosaurus by Mary Anning of everyone’s tongue. Fortunately, at least in one sense of the (1799–1847) from the Wellcome Collection is under a CC BY 4.0 word, it is. The well-known tongue twister, below, may have License. been written about Mary Anning: She sells sea-shells on the sea-shore. The shells she sells are sea-shells, I’m sure. For if she sells sea-shells on the sea-shore Then I’m sure she sells sea-shore shells. —T. Sullivan (1908) Developing Modern Methods As Mary Anning’s story suggests, scientists in Europe were working at a time dominated by western Christian tradition. Literal interpretations of the bible did not allow for the long, slow processes of geological or evolutionary change to operate. However, many scientists were mak- ing observations that did not fit the biblical narrative. During the 18th century, Scotsman James Hutton’s work on the formation of Earth provided a much longer timeline of events than previ- ous biblical interpretations would allow. Hutton’s theory of Deep Time was crucial to the under- standing of fossils. Deep Time gave the history of Earth enough time—4.543 billion years—to encompass continental drift , the evolution of species, and the fossilization process. A second Scotsman, Charles Lyell, propelled Hutton’s work into his own theory of uniformitarianism , the doctrine that Earth’s geologic formations are the work of slow geologic forces. Lyell’s three-vol- ume work, Principles of Geology (1830–1833), was influential to naturalist Charles Darwin (see Chapter 2 for more information on Darwin’s work). In fact, Lyell’s first volume accompanied Darwin on his five-year voyage around the world on the HMS Beagle (1831–1836). The concepts proposed by Lyell gave Darwin an opportunity to apply his working theories of evolution by nat- ural selection and a greater length of time with which to work. These resulting theories were important scientific discoveries and paved the way for the “Age of Wonder” (Holmes 2010, xvi). The work of Anning, Darwin, Lyell, and many others laid the foundation for the modern methods we use today. Though anthropology is focused on humans and our primate rela- tives (and not on dinosaurs, as many people wrongly as- sume), you will see that methods developed in paleontology, geology, chemistry, biology, and physics are often applied in Figure 7.5: Murexsul (Miocene): This anthropological research. In this chapter, you will learn about fossil was found at the Naval the primary methods and techniques employed by biological Weapons Center, China Lake, anthropologists to answer questions about fossils , the min- California, in 1945. The fossil was eralized copies of once-living organisms (Figure 7.5). buried deep in the strata and was pulled out of the ground along with Ultimately, these answers provide insights into human evo- a crashed “Fat Boy” missile after lution. Pay close attention to ways in which modern biologi- atomic missile testing (S. Brubaker, cal anthropologists use other disciplines to analyze evidence personal communication, March 9, and reconstruct past activities and environments. 2018). Credit: Murexsul (Figure 7.6) from the Maturango Museum, Ridgecrest, California, by Sarah S. Earth: It’s Older than Dirt King and Lee Anne Zajicek is under a CC BY-NC 4.0 License. Scientists have developed precise and accurate dating meth- ods based on work in the fields of physics and chemistry. Using these methods, scientists are able to establish the age of Earth as well as approximate ages of the organisms that have lived here. Earth is roughly 4.6 billion years old, give or take a few hundred million years. The first evidence for a living organism appeared around 3.5 billion years ago ( bya ). The scale of geologic time can seem downright overwhelming. In order to orga- nize and make sense of Earth’s past, geologists break up that time into subunits, which are hu- man-made divisions along Earth’s timeline. The largest subunit is the eon. An eon is further di- vided into eras, and eras are divided into periods. Finally, periods are divided into epochs (see Figure 7.6; Williams 2004, 37). Currently, we are living in the Phanerozoic eon, Cenozoic era, Quaternary period, and probably the Holocene epoch—though there is academic debate about the current epoch (see below). Figure 7.6: The Geologic time scale is shown here, with periods broken into eons, eras, periods, and in some cases epochs. Some life forms and geological events are noted for each period. Credit: Geologic Time Scale, by National Park Service, designed by Trista Thornberry-Ehrlich and Rebecca Port, adapted from ones from USGS and the International Commission on Stratigraphy, is in the public domain. [Image Description]. These divisions are based on major changes and events recorded in the geologic record. Events like significant shifts in climate or mass extinctions can be used to mark the end of one geologic time unit and the beginning of another. However, it is important to remember that these bor- ders are not real in a physical sense; they are helpful organizational guidelines for scientific re- search. There can be debate regarding how the boundaries are defined. Additionally, the meth- ods we use to establish these dates are refined over time, occasionally leading to shifts in estab- lished chronology (see the discussion on calibration in the radiocarbon dating section below). For instance, the current epoch has been traditionally known as the Holocene. It began almost twelve thousand years ago ( kya ) during the warming period after that last major ice age. Today, there is evidence to indicate human-driven climate change is warming the world and changing the environmental patterns faster than the natural cyclical processes. This has led some scien- tists within the stratigraphic community to argue for a new epoch beginning around 1950 with the Nuclear Age called the Anthropocene (Monastersky 2015; Waters et al. 2016). Nobel Laureate Paul Crutzen places the beginning of the Anthropocene much earlier—at the dawn of the Industrial Revolution, with its polluting effects of burning coal (Crutzen and Stoermer 2000, 17–18). Geologist William Ruddiman argues that the epoch began 5,000–8,000 years ago with the advent of agriculture and the buildup of early methane gasses (Ruddiman et al. 2008). Regardless of when the Anthropocene started, the major event that marks the boundary is the warming temperatures and mass extinction of nonhuman species caused by human activity (Figure 7.7). Researchers now declare that “human activity now rivals geologic forces in influenc- ing the trajectory of the Earth System” (Steffen et al. 2018, 1). Fossils: The Taphonomic Process Most of the evidence of human evolution comes from the study of the dead. To obtain as much information as possible from the remains of once-living creatures, one Figure 7.7: The Chooz Nuclear Power, in a must understand the processes that occur after death. valley in Ardennes, France, is a reminder This is where taphonomy comes in (Figure 7.8). that human activity affects the planet Taphonomy is the study of what happens to an organ- greatly. Credit: Chooz Nuclear Power ism after death (Komar and Buikstra 2008, 189; Stodder Plant-9361 by Raimond Spekking is under 2008). It includes the study of how an organism be- a CC BY-SA 4.0 License. comes a fossil. However, as you’ll see throughout this book, the majority of organisms never make it through the full fossilization process. Taphonomy is important in biological anthropology, especially in subdisciplines like bioarchaeol- ogy (the study of human remains in the archaeological record) and zooarchaeology (the study of faunal remains from archaeological sites). It is so important that many scientists have recreated a variety of burial and decay experiments to track taphonomic change in modern contexts. These contexts can then be used to understand the taphonomic patterns seen in the fossil record (see Reitz and Wing 1999, 122–141). Taphonomic analysis can also give us important insights into the development of complex thought and ritual in human evolution. In Chapter 11, you will see the first evidence of rec- ognized burial practices in hominins. Taphonomy helped to establish whether these burials were simply the result of natural processes or intentionally constructed by humans (Klein 1999, 395; Straus 1989). Deliberate burials often include the body placed in a specific position, such as supine (on the back) with arms crossed over the chest or in a flexed position (think fetal position) facing a particular direction. If bones Figure 7.8: Taphonomy focuses on what happens to the remains of an have evidence of a carnivore or rodent gnawing on them, it organism, like this coyote, after can be inferred that the remains were exposed to scavengers death. Credit: Coyote remains after death. (Figure 7.14) by Sarah S. King is under a CC BY-NC 4.0 License. Going back further in time, taphonomic evidence may tell us how our ancestors died. For instance, several australopithecine fossils show evidence of carni- vore tooth marks and even punctures from saber-toothed cats, indicating that we weren’t al- ways the top of the food chain. The Bodo Cranium, a Homo erectus cranium from Middle Awash Valley, Ethiopia, shows cut marks made by stone tools, indicating an early example of possible defleshing activity in our human ancestors (White 1986). At the archaeological site of Zhoukoudian, researchers used taphonomy to show that the highly fragmented remains of at least 51 Homo erectus individuals were scavenged by Pleistocene cave hyenas (Boaz et al. 2004). The damage on Skull VI was described as “elongated, raking bite marks, isolated puncture bite marks, and perimortem breakage consistent with patterns of modern hyaenid bone modifica- tion” (Boaz et al. 2004). Additionally, a fresh burnt equid cranium was discovered which supports the theory of mobile hominid scavenging and fire use at the site (Boaz et al. 2004). Special Topic: Bog Bodies and Mummies Preservation is a key topic in anthropological research, since we can only study the evidence that gets left behind in the fossil and archaeological record. This chapter is concerned with the fossil record; however, there are other forms of preserved remains that provide anthropologists with information about the past. You’ve undoubtedly heard of mummification, likely in the context of Egyptian or South American mummies. However, bog bodies and ice mummies are further exam- ples of how remains can be preserved in special circumstances. It is important to note that fos- silization is a process that takes much longer than the preservation of bog bodies or mummies. Bog bodies are good examples of wetland preservation. Peat bogs are formed by the slow accu- mulation of vegetation and silts in ponds and lakes. Individuals were buried in bogs throughout Europe as far back as 10 kya, with a proliferation of activity from 1,600 to 3,200 years ago (Giles 2020; Ravn 2010). When they were found thousands of years later, they resembled recent buri- als. Their hair, skin, clothing, and organs were exceptionally well preserved, in addition to their bones and teeth (Eisenbeiss 2016; Ravn 2010). Preservation was so good in fact that archaeolo- gists could identify the individuals’ last meals and re-create tattoos found on their skin. Extreme cold can also halt the natural decay process. A well-known ice mummy is Ötzi, a Copper Age man dating to around 5,200 years ago found in the Alps (Vanzetti et al. 2012; Vidale et al. 2016). As with the bog bodies, his hair, skin, clothing, and organs were all well preserved. Recently, archaeologists were able to identify his last meal (Maixner et al. 2018). It was high in fat, which makes sense considering the extremely cold environment in which he lived, as meals high in fat assist in cold tolerance (Fumagalli et al. 2015). In the Andes, ancient peoples would bury human sacrifices throughout the high peaks in a sacred ritual called Capacocha (Wilson et al. 2007). The best-preserved mummy to date is called the “Maiden” or “Sarita” because she was found at the summit of Sara Sara Volcano. Her remains are over 500 years old, but she still looks like the 15-year-old girl she was at the time of her death, as if she had just been sleeping for 500 years (Reinhard 2006). Finally, arid environments can also contribute to the preservation of organic remains. As dis- cussed with waterlogged sites, much of the bacteria that is active in breaking down bodies is al- ready present in our gut and begins the putrefaction process shortly after death. Arid environ- ments deplete organic material of the moisture that putrefactive bacteria need to function (Booth et al. 2015). When that occurs, the soft tissue like skin, hair, and organs can be preserved. It is similar to the way a food dehydrator works to preserve meat, fruit, and vegetables for long- term storage. There are several examples of arid environments spontaneously preserving human remains, including catacomb burials in Austria and Italy (Aufderheide 2003, 170, 192–205). Fossilization Fossils only represent a tiny fraction of creatures that existed in the past. It is extremely difficult for an organism to become a fossil. After all, organisms are designed to deteriorate after they die. Bacteria, insects, scavengers, weather, and environment all aid in the process that breaks down organisms so their elements can be returned to Earth to maintain ecosystems (Stodder 2008). Fossilization , therefore, is the preservation of an organism against these natural decay processes (Figure 7.9). Figure 7.9: A simplified illustration of the fossilization process beginning at an organism’s death. In this example, the individual begins to decompose and then is covered by water and sediments, both protecting it and creating an environment for perimineralization. Sediments accumulate over time. Erosion eventually exposes the fossil, leading to its eventual discovery by paleoanthropologists. Credit: Fossilization process (Figure 7.15) original to Explorations: An Open Invitation to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0 License. For fossilization to occur, several important things must happen. First, the organism must be protected from things like bacterial activity, scavengers, and temperature and moisture fluctua- tions. A stable environment is important. This means that the organism should not be exposed to significant fluctuations in temperature, humidity, and weather patterns. Changes to moisture and temperature cause the organic tissues to expand and contract repeatedly, which will even- tually cause microfractures and break down (Stodder 2008). Soft tissue like organs, muscle, and skin are more easily broken down in the decay process; therefore, they are less likely to be pre- served. Bones and teeth, however, last much longer and are more common in the fossil record (Williams 2004, 207). Wetlands are a particularly good area for preservation because they allow for rapid permanent burial and a stable moisture environment. That is why many fossils are found in and around an- cient lakes and river systems. Waterlogged sites can also be naturally anaerobic (without oxy- gen). Much of the bacteria that causes decay is already present in our gut and can begin the de- composition process shortly after death during putrefaction (Booth et al. 2015). Since oxygen is necessary for the body’s bacteria to break down organic material, the decay process is signifi- cantly slowed or halted in anaerobic conditions. The next step in the fossilization process is sediment accumulation. The sediments cover and protect the organism from the environment. They, along with water, provide the minerals that will eventually become the fossil (Williams 2004, 31). Sediment accumulation also provides the pressure needed for mineralization to take place. Lithification is when the weight and pressure of the sediments squeeze out extra fluids and replace the voids that appear with minerals from the surrounding sediments. Finally, we have permineralization. This is when the organism is fully replaced by minerals from the sediments. A fossil is really a mineral copy of the original organism (Williams 2004, 31). Types of Fossils Plants Plants make up the majority of fossilized materials. One of the most common plants existing today, the fern, has been found in fossilized form many times. Other plants that no longer exist or the early ancestors of modern plants come in fossilized forms as well. It is through these fossils that we can discover how plants evolved and learn about the climate of Earth over different periods of time. Figure 7.10: An exquisite piece of Another type of fossilized plant is petrified wood. This fossil petrified wood. Credit: is created when actual pieces of wood—such as the trunk of PetrifiedWood at the Petrified Forest National Park by Jon Sullivana tree—mineralize and turn into rock. Petrified wood is a has been designated to the public combination of silica, calcite, and quartz, and it is both heavy domain (CC0). and brittle. Petrified wood can be colorful and is generally aesthetically pleasing because all the features of the original tree’s composition are illuminated through mineralization (Figure 7.10). There are a number of places all over the world where petrified wood “forests” can be found, but there is an excellent assemblage in Arizona, at the Petrified Forest National Park. At this site, evidence relating to the environment of the area some 225 mya is on display. Human/Animal Remains We are more familiar with the fossils of early animals because natural history museums have ex- hibits of dinosaurs and extinct mammals. However, there are a number of fossilized hominin re- mains that provide a picture of the fossil record over the course of our evolution from primates. The term hominins includes all human ancestors who existed after the evolutionary split from chimpanzees and bonobos, some six to seven mya. Modern humans are Homo sapiens, but ho- minins can include much earlier versions of humans. One such hominin is “Lucy” (AL 288-1), the 3.2 million-year-old fossil of Australopithecus afarensis that was discovered in Ethiopia in 1974 (Figure 7.11). Until recently, Lucy was the most complete and oldest hominin fossil, with 40% of her skeleton preserved (see Chapter 9 for more information about Lucy). In 1994, an Australopithecus fossil nicknamed “Little Foot” (Stw 573) was located in the World Heritage Site at Sterkfontein Caves (“the Cradle of Humankind”) in South Africa. Little Foot is more com- plete than Lucy and possibly the oldest fossil that has so far been found, dating to at least 3.6 million years (Granger et al. 2015). The ankle bones of the fossil were extricated from the matrix of concrete-like rock, revealing that the bones of the an- kles and feet indicate bipedalism (University of Witwatersrand 2017). Both the Lucy and Little Foot fossils date back to the Pliocene (5.8 to 2.3 mya). Older hominin fossils from the late Miocene (7.25 to 5.5 mya) have been located, although they are much less complete. The oldest hominin fossil is a fragmentary skull named Sahelanthropus tchadensis, found in Northern Chad and dating to circa seven mya (Lebatard et al. 2008). It is through the discovery, dating, and study of primate and early hominin Figure 7.11: “Lucy” (AL 288-1), fossils that we find physical evidence of the evolutionary time- Australopithecus afarensis. Credit: line of humans. Lucy blackbg by 120 is under a CC BY 2.5 License. Asphalt Figure 7.12: This is a recreation of how animals tragically came to be trapped in the asphalt lake at the La Brea Tar Pits. Credit: Mammoth Tragedy at La Brea Tar Pits (5463657162) by KimonBerlin is under a CC BY-SA 2.0 License. Asphalt, a form of crude oil, can also yield fossilized remains. Asphalt is commonly referred to in error as tar because of its viscous nature and dark color. A famous fossil site from California is La Brea Tar Pits in downtown Los Angeles (Figure 7.12). In the middle of the busy city on Wilshire Boulevard, asphalt (not tar) bubbles up through seeps (cracks) in the sidewalk. The La Brea Tar Pits Museum provides an incredible look at the both extinct and extant animals that lived in the Los Angeles Basin 40,000–11,000 years ago. These animals became entrapped in the asphalt during the Pleistocene and perished in place. Ongoing excavations have yielded Figure 7.13: The fearsome millions of fossils, including megafauna such as American jaws of the saber-toothed mastodons and incomplete skeletons of extinct species of dire cat (Smilodon fatalis) found at the La Brea Tar Pits. wolves, Canis dirus, and the saber-toothed cat, Smilodon fatalis Credit: Smilodon saber- (Figure 7.13). Fossilized remains of plants have also been found in toothed tiger skull (La Brea the asphalt. The remains of one person have also been found at the Asphalt, Upper Pleistocene; tar pits. Referred to as La Brea Woman, the remains were found in Rancho La Brea tar pits, southern California, USA) 11914 and were subsequently dated to around 10,250 years ago. The La Brea Woman was a likely female individual who was 17–28 years by James St. John is under a CC BY 2.0 License. old at the time of her death, with a height of under five feet (Spray 2022). She is thought to have died from blunt force trauma to her head, famously making her Los Angeles’s first documented homicide victim (Spray 2022). (Learn more about her in the Special Topic box, “Necropolitics,” below.) Between the fossils of animals and those of plants, paleontologists have a good idea of the way the Los Angeles Basin looked and what the climate in the area was like many thousands of years ago. Igneous Rock Most fossils are found in sedimentary rock. This type of rock has been formed from deposits of minerals over millions of years in bodies of water on Earth’s surface. Some examples include shale, limestone, and siltstone. Sedimentary rock typically has a layered appearance. However, fossils have been found in igneous rock as well. Igneous rock is volcanic rock that is created from cooled molten lava. It is rare for fossils to survive molten lava, and it is estimated that only 2% of all fossils have been found in igneous rock (Ingber 2012). Part of a giant rhinocerotid skull dating back 9.2 mya to the Miocene was discovered in Cappadocia, Turkey, in 2010. The fossil was a re- markable find because the eruption of the Çardak caldera was so sudden that it simply dehy- drated and “baked” the animal (Antoine et al. 2012). Trace Fossils Depending on the specific circumstances of weather and time, even footprints can become fos- silized. Footprints fall into the category of trace fossils , which includes other evidence of biologi- cal activity such as nests, burrows, tooth marks, and shells. A well-known example of trace fos- sils are the Laetoli footprints in Tanzania (Figure 7.14). More recently, archaeological investiga- tions in North America have revealed fossil footprints which rewrite the history of people in the Americas at White Sands, New Mexico. You can read more about the Laetoli and White Sands footprints in the Dig Deeper box below. Other fossilized footprints have been dis- covered around the world. At Pech Merle cave in the Dordogne region of France, ar- chaeologists discovered two fossilized foot- prints. They then brought in indigenous trackers from Namibia to look for other footprints. The approach worked, as many other footprints belonging to as many as five individuals were discovered with the Figure 7.14: A few early hominin footprints fossilized at expert eyes of the trackers (Pastoors et al. Laetoli. Credit: NHM – Laetoli Fußspuren by Wolfgang Sauber is under a CC BY-SA 4.0 License. 2017). These footprints date back 12,000 years (Granger Historical Picture Archive 2018). Some of the more unappealing but still-fascinating trace fossils are bezoars and coprolite. Bezoars are hard, concrete-like substances found in the intestines of fossilized creatures. Bezoars start off like the hair balls that cats and rabbits accumulate from grooming, but they be- come hard, concrete-like substances in the intestines. If an animal with a hairball dies before ex- pelling the hair ball mass and the organism becomes fossilized, that mass becomes a bezoar. Coprolite is fossilized dung. One of the best collections of coprolites is affectionately known as the “Poozeum.” The collection includes a huge coprolite named “Precious” (Figure 7.15). Coprolite, like all fossilized materials, can be in matrix —meaning that the fossil is embedded in secondary rock. As unpleasant as it may seem to work with coprolites, remember that the organic material in dung has mineralized or has started to mineralize; therefore, it is no longer soft and is gener- ally not smelly. Also, just as a doctor can tell a lot about health and diet from a stool sample, an- thropologists can glean a great deal of information from coprolite about the diets of ancient ani- mals and the environment in which the food sources existed. For instance, 65 million-year-old grass phytoliths (microscopic silica in plants) found in dinosaur coprolite in India revealed that grasses had been in existence much earlier than scientists initially believed (Taylor and O’Dea 2014, 133). Pseudofossils Pseudofossils are not to be mistaken for fake fossils, which have vexed scientists from time to time. A fake fossil is an item that is deliberately manipulated or manufactured to mislead scientists and the general public. In contrast, pseudofossils are not misrepresen- tations but rather misinterpretations of rocks that look like true fossilized remains (S. Brubaker, personal com- Figure 7.15: An extremely large coprolite munication, March 9, 2018). Pseudofossils are the re- named “Precious.” Credit: Precious the Coprolite Courtesy of the Poozeum by sult of impressions or markings on rock, or even the Poozeum is under a CC BY-SA 4.0 License. way other inorganic materials react with the rock. A common example is dendrites, the crystallized deposits of black minerals that resemble plant growth (Figure 7.16). Other examples of pseudofossils are unusual or odd-shaped rocks that include various concretions and nodules. An expert can exam- ine a potential fossil to see if there is the requisite internal structure of organic material such as bone or wood that would qualify the item as a fossil. Figure 7.16: A beautiful example of dendrites, a type of pseudofossil. It’s easy to see how the black crystals look like plant growth. Credit: Dendrites (Figure 7.25) from the Maturango Museum, Ridgecrest, California, by Sarah S. King and Lee Anne Zajicek is under a CC BY-NC 4.0 License. Dig Deeper: Trace Fossils The Power of Poop Coprolites found in Paisley Caves, Oregon, in the United States are shedding new light on some of the earliest occupants in North America. Human coprolites are distinguished from animal co- prolites through the identification of fecal biomarkers using lipids, or fats, and bile acids (Shillito et al. 2020a). Paisley Caves have 16,000 years of anthropogenic, or human-caused, deposition, with some coprolites having been dated as old as 12.8kya (Blong et al. 2020). Over 285 radiocar- bon dates have been recorded from the site (Shillito et al. 2020a), making Paisley Caves one of the most well-dated archaeological sites in the United States. Coprolite analysis can be summa- rized in three levels, macroscopic, microscopic, and molecular. This can also be understood as an- alyzing the morphology (macroscopic), contents (microscopic), and residues (molecular) (Shillito et al. 2020b). Each of these levels adds a different layer of information. Coprolite shape is infor- mative through what can be seen macroscopically, such as ingestions of basketry or cordage, small gravels and grains, and general shape. The contents of coprolites may be of the most inter- est to scientists because certain plants and animals can signal past environments as well as food procurement methods. Coprolites from Paisley Caves have included small pebbles and obsidian chips from butchering game, grinding plants, and general food preparation as well as small bits of fire cracked rock likely from cooking in hearths (Blong 2020). Additionally, rodent bones in co- prolites included crania and vertebrae, which suggests whole consumption (Taylor et al. 2020). Insect remains are present in the coprolites as well, such as ants, Jerusalem crickets, June beetles, and darkling beetles (Blong 2020). In all, the coprolites of Paisley Caves have provided an invalu- able resource to anthropologists to study the past climate and lifeways of early humans in the Americas. Coprolites can also signal past health, which is a study known as paleopathology. A study by Katelyn McDonough and colleagues (2022) focused on the identification of parasites in coprolites at Bonneville Estates Rockshelter in eastern Nevada and their link to the greater Great Basin dur- ing the Archaic, a period of time spanning 8,000–5,000 years ago. According to the study, para- sites such as Acanthocephalans (thorny-headed worms) have been affecting the Great Basin for at least the last 10,000 years. Acanthocephalans are endoparasites, meaning parasites that live inside of their hosts. They are found worldwide and seem to have been concentrated in the Great Basin in the past. Bonneville Estates Rockshelter has been visited by humans for over 13,000 years, with parasite identification going back to nearly 7,000 years. The species identified at Bonneville Estates is Moniliformis clarki. This species parasitizes crickets and insects, a popular food source during the Archaic in the Great Basin. The parasite uses intermediate hosts to get to mammals and birds as definitive hosts. Crickets and beetles have been recorded as food materi- als in Paisley Caves as well. Insects have remained an important dietary staple for people of the Great Basin and are consumed raw, dried, brined, or ground into flour. Insects that remain un- cooked or undercooked have a higher risk for transmission of parasites. Symptoms associated with Acanthocephalans infection are intense intestinal discomfort, anemia, and anorexia, leading to death. It is hypothesized that the consumption of basketry, cordage, and charcoal (which was also identified at Paisley Caves), sometimes associated with parasite-infected coprolites, may have been a method of treatment for the infection. Interestingly, present day infections from this parasite are rising after remaining quite rare, as detection of the parasite is occurring in insect farms. Walking to the Past In 1974, British anthropologist Mary Leakey discovered fossilized animal tracks at Laetoli (Figure 7.17), not far from the important paleoanthropological site at Olduvai Gorge in Tanzania. A few years later, a 27-meter trail of hominin footprints were discovered at the same site. These 70 footprints, now referred to as the Laetoli Footprints, were created when early humans walked in wet volcanic ash. Before the impressions were obscured, more volcanic ash and rain fell, sealing the footprints. These series of environmental events were truly extraordinary, but they fortu- nately resulted in some of the most famous and revealing trace fossils ever found. Dating of the footprints indicate that they were made 3.6 mya (Smithsonian National Museum of Natural History 2018). Just as forensic scientists can use footprints to identify the approximate build of a potential sus- pect in a crime, archaeologists have read the Laetoli Footprints for clues to these early humans. The footprints clearly indicate bipedal ho- minins who had similar feet to those of modern humans. Analysis of the gait through computer simulation revealed that the hominins at Laetoli walked similarly to the way we walk today (Crompton 2012). More recent analyses confirm the similarity to modern hu- mans but also indicate a gait Figure 7.17: Location of Laetoli site in Tanzania, Africa, with Olduvai that involved more of a flexed Gorge nearby. Credit: Laetoli and Olduvai Gorge sites (Figure 7.26) limb than that of modern hu- original to Explorations: An Open Invitation to Biological Anthropology by Elyssa Ebding at GeoPlace, California State mans (Hatala et al. 2016; University, Chico is under a CC BY-NC 4.0 License. Raichlen and Gordon 2017). The relatively short stride implies that these hominins had short legs—unlike the longer legs of later early humans who migrated out of Africa (Smithsonian National Museum of Natural History 2018). In the context of Olduvai Gorge, where fossils of Australopithecus afarensis have been located and dated to the same timeframe as the footprints, it is likely that these newly discovered impressions were left by these same hominins. The footprints at Laetoli were made by a small group of as many as three Australopithecus afarensis, walking in close proximity, not unlike what we would see on a modern street or side- walk. Two trails of footprints have been positively identified with the third set of prints appear- ing smaller and set in the tracks left by one of the larger individuals. While scientific methods have given us the ability to date the footprints and understand the body mechanics of the ho- minin, additional consideration of the footprints can lead to other implications. For instance, the close proximity of the individuals implies a close relationship existed between them, not unlike that of a family. Due to the size variation and the depth of impression, the footprints seem to have been made by two larger adults and possibly one child. Scientists theorize that the weight being carried by one of the larger individuals is a young child or a baby (Masao et al. 2016). Excavation continues at Laetoli today, resulting in the discovery of two more footprints in 2015, also believed to have been made by Au. afarensis (Masao et al. 2016). But it is not just human evolution studies that can benefit from the analysis of fossil footprints. A re- cent discovery of fossilized foot- prints has rewritten what we know about the peopling of the Americas. It was originally thought that humans had been in the Americas for at least the last 15,000 years by crossing through the ice-free corridor (IFC) between the Cordilleran and Laurentide ice sheets in present-day Alaska and Canada. Figure 7.18: Tularosa Basin, New Mexico. Credit: Map of Tularosa Basin by the United States Geological Survey is in the public However, fossil footprints from domain. the Tularosa Basin of New Mexico (see Figure 7.18) discov- ered in 2021 have challenged this theory. The footprints, dated between 22,860 (∓320) and 21,130 (∓250) years ago (nps.gov) based on Ruppia cirrhosa grass seeds located above and be- low the footprints, have shown humans have been in the Americas for much longer than previ- ously thought. These footprints represent an adolescent individual and toddler walking through the lakebed at White Sands (see Figure 7.19), New Mexico, alongside both giant ground sloths and mammoths (Barras 2022; Wade 2021). Also present in the lakebed are footprints of camels and dire wolves (nps.gov 2022; Wade 2021). Figure 7.19: Excavation of fossil footprints from New Mexico. Credit: Images of White Sands National Park Study Site Footprints by the USGS Climate Research and Development Program is in the public domain. The IFC model was upheld by a group of theorists known as “Clovis First,” who believed the mi- gration of people into the Americas was recent and was represented archaeologically through the Clovis projectile point toolkit. Subsequent discoveries at sites such as Cactus Hill on the east coast of the United States and Monte Verde, Chile, have demonstrated that this model wouldn’t have worked. Because these sites are as old as 20,000 years and 18,500 years respectively, the IFC would have been frozen over and impassable (Gruhn 2020). Other models have been adopted to account for this, such as the coastal migration model down the west coast of North America. The more-likely migration scenario seems to be neither of these as more discoveries or antiquity continue to emerge. People may instead have migrated into the Americas before the last glacial maximum began, around 25,500–19,000 years ago. According to Indigenous knowl- edge, they have always been here. With the discovery of the White Sands footprints, it is known that humans have been in the Americas for at least 20,000 years. This discovery also reveals the importance of recognizing knowledge beyond that which is pro- duced by the European scientific tradition. Rather than framing science in a way that runs counter to Indigenous knowledge, it can be thought that science is catching up with it. For in- stance, the Acoma Pueblo people have the word for camel in their vocabulary. This was dis- missed by scientists who assumed the word was for describing camels that were introduced to the United States in the past 100 years. However, the discovery of the White Sands footprints also included the footprints of Pleistocene camels in the same strata. Therefore, the fact that the Acoma Pueblo people have had a word for camel likely refers the Pleistocene-age megafauna camel, Camelops hesternus, rather than Camelus dromedarius or Camelus bactrianus, two present-day camel species (which are actually descendants of Camelops hesternus). Therefore, the existence of the Acoma Pueblo word for camel is not like an anomaly but rather a testament to the fact that Acoma Pueblo ancestors walked beside C. hesternus on this continent 20,000 years ago. These footprints challenge the “ice-free corridor” expansion model, as the bridge con- necting present-day Alaska and Russia into Canada would have been covered in an impenetrable ice sheet at this time. The discovery of these footprints urges scientists to reconsider further in- vestigations at well-known Terminal Pleistocene/Early Holocene dry lake beds in the Southwestern and Mojave deserts—and to include Indigenous knowledge in their work rather than ignore it. Special Topic: Necropolitics What are necropolitics? Necropolitics is an application of critical theory that describes how “gov- ernments assign differential value to human life” and similarly how someone is treated after they die (Verghese 2021). How is someone’s death political? Consider the La Brea Woman example from the section on asphalt above. The La Brea Woman’s discovery was controversial, not because she is the only person to be found in the tar pits or be- cause of her age but also because of necropolitics. The La Brea Woman was collected in 1914 and her body was housed on display at the George C. Page Museum in Los Angeles against the wishes of the Chumash and the Tongva, two tribes whose ancestral lands include Los Angeles. The museum decided to display a skull cast instead to meet the request of the tribes which in- cluded a separate postcranial skeleton from a different individual. The updated display itself was wrought with other ethical issues, as a cast of her skull was “attached to the ancient remains of a Pakistani female that was dyed dark bronze, the femurs shortened to approximate the stature of native people” (Cooper 2010). In both cases, neither the individuals or their descendent communi- ties consented to the display or grotesque modification of human remains. According to an inter- view conducted by LA Weekly (Cooper 2010) with Cindi Alvitre, former chair of the Gabrielino- Tongva Tribal Council, the display of Indigenous human remains is akin to voyeurism. She states “It’s disheartening to me because it’s very inappropriate to display any human remains. The things we do to fill the imagination of visitors. It violates human rights.” It is important to listen to the wishes of Indigenous people and center their values when conducting work with their an- cestors. A good source for considering places to look for archaeological research ethics before conducting fieldwork (and ideally during your research design) is the Society for American Archaeology’s ethics principle list, as well as following the Indigenous Archaeology Collective. Indigenous remains are now protected in the United States due to legislation such as Native American Graves Protection and Repatriation Act (NAGPRA). You can read more about this in Chapter 15: Bioarchaeology and Forensic Anthropology. Before the passing of NAGPRA, tribes had little agency over how the bodies of their ancestors were treated by anthropologists and museums, including decisions about sampling and destructive tests. Now when archaeological field work is conducted on federal land, tribes must be consulted before work begins. This con- sultation process often includes what to do if human remains are encountered. Indigenous tribes are multifaceted and multivocal; each has its own rules about how to handle the remains of their ancestors. In some cases, all work on the project must be halted after the discovery of human re- mains. Other tribes allow for work to continue if the remains are moved and reburied. Some tribes are open to radiometric dating if it aligns with their beliefs in the afterlife. Each tribe is dif- ferent, and each tribe deserves to have its wishes respected. Voices From the Past: What Fossils Can Tell Us Given that so few organisms ever become fossilized, any anthropologist or fossil hunter will tell you that finding a fossil is extremely exciting. But this is just the beginning of a fantastic mys- tery. With the creative application of scientific methods and deductive reasoning, a great deal can be learned about the fossilized organism and the environment in which it lived, leading to enhanced understanding of the world around us. Dating Methods Context is a crucial concept in paleoanthropology and archaeology. Objects and fossils are inter- esting in and of themselves, but without context there is only so much we can learn from them. One of the most important contextual pieces is the dating of an object or fossil. By being able to place it in time, we can compare it more accurately with other contemporary fossils and artifacts or we can better analyze the evolution of a fossil species or artifacts. To answer the question “How do we know what we know?,” you have to know how archaeologists and paleoanthropol- ogists establish dates for artifacts, fossils, and sites. Though accurate dating is important for context and analysis, we must consider the impact. Many of the chronometric dating methods used by anthropologists require the removal of small samples from artifacts, bones, soils, and rock. Thus these techniques are considered destructive. How much of an artifact are you willing to destroy to get your date? Sharon Clough, a Senior Environmental Officer at Cotswold Archaeology, addressed this issue in a case study from her re- search. She stated that “the benefit of a date did not outweigh the destruction of a valuable and finite resource” (Clough 2020). The resource in question was human remains. When considering our dating options, we want to be sure that we do as little harm as possible, especially in the case of human remains (read more about this issue in the Special Topic box, “Necropolitics”). Dating techniques are divided into two broad categories: relative dating methods and chrono- metric (sometimes called absolute) dating methods. Relative Dating Relative dating methods are used first because they rely on simple observational skills. In the 1820s, Christian Jürgensen Thomsen at the National Museum of Denmark in Copenhagen devel- oped the “three-age” system still used in European archaeology today (Feder 2017, 17). He cate- gorized the artifacts at the museum based on the idea that simpler tools and materials were most likely older than more complex tools and materials. Stone tools must predate metal tools because they do not require special technology to develop. Copper and bronze tools must pre- date iron because they can be smelted or worked at lower temperatures, etc. Based on these ob- servations, he categorized the artifacts into Stone Age, Bronze Age, and Iron Age. The restriction of relative dating is that you don’t know specific dates or how much time passed between different sites or artifacts. You simply know that one artifact or fossil is older than an- other. Thomsen knew that Stone Age artifacts were older than Bronze Age artifacts, but he couldn’t tell if they were hundreds of years older or thousands of years older. The same is true with fossils that have differences of ages into the hundreds of millions of years. The first relative dating technique is stratigraphy (Figure 7.20). You might have already heard this term if you have watched documentaries on archaeological excavations. That’s because this method is still being used today. It provides a solid foundation for other dating techniques and gives important context to artifacts and fossils found at a site. Stratigraphy is based on the Law of Superposition first proposed by Nicholas Steno in 1669 and further explored by James Hutton (the previously mentioned “Father” of Deep Time). Essentially, superposition tells us that things on the bottom are older than things on the top (Williams 2004, 28). Notice on Figure 7.20 that there are distinctive layers piled on top of each other. It stands to reason that each layer is older than the one immediately on top of it (Hester et al. 1997, 338). Think of a pile of laundry on the floor. Over the course of a week, as dirty clothes get tossed on that pile, the shirt tossed down on Monday will be at the bottom of the pile while the shirt tossed down on Friday will be at the top. Assuming that the laundry pile was undisturbed throughout the week, if the clothes were picked up layer by layer, the clothing choices that week could be reconstructed in the order that they were worn. Another relative dating technique is bios- tratigraphy. This form of dating looks at the context of a fossil or artifact and compares it to the other fossils and biological remains (plant and animal) found in the same strati- graphic layers. For instance, if an artifact is found in the same layer as wooly mammoth remains, you know that it must date to around the last ice age, when wooly mam- moths were still abundant on Earth. In the absence of more specific dating techniques, early archaeologists could prove the great an- tiquity of stone tools because of their associ- ation with extinct animals. The application of this relative dating technique in archaeology was used at the Folsom site in New Mexico. In 1927, a stone spear point was discovered embedded in the rib of an extinct species of bison. Because of the undeniable association between the artifact and the ancient animal, Figure 7.20: An illustration of a stratigraphic cross- there was scientific evidence that people had section. The objects at a lower strata are older than occupied the North American continent since the one above. Credit: Stratigraphic cross-section antiquity (Cook 1928). (Figure 7.28) original to Explorations: An Open Invitation to Biological Anthropology by Mary Nelson Similar to biostratigraphic dating is cultural is under a CC BY-NC 4.0 License. dating (Figure 7.21). This relative dating tech- nique is used to identify the chronological re- lationships between human-made artifacts. Cultural dating is based on artifact types and styles (Hester et al. 1997, 338). For instance, a pocket knife by itself is difficult to date. However, if the same pocket knife is discovered surrounded by cassette tapes and VHS tapes, it is logical to as- sume that the artifact came from the late 20th century like the cassette and VHS tapes. The pocket knife could not be dated earlier than the late 20th century because the tapes were made no earlier than 1977. In the Thomsen example above, he was able to identify a relative chronol- ogy of ancient European tools based on the artifact styles, manufacturing techniques, and raw materials. Cultural dating can be used with any human-made artifacts. Both cultural dating and biostratigraphy are most effective when researchers are already familiar with the time periods for the artifacts and animals. They are still used today to identify general time periods for sites. Chemical dating was developed in the 19th cen- tury and represents one of the early attempts to use soil composition and chemistry to date arti- facts. A specific type of chemical dating is fluo- rine dating , and it is commonly used to compare the age of the soil around bone, antler, and teeth located in close proximity (Cook and Ezra- Cohn 1959; Goodrum and Olson 2009). While this technique is based on chemical dating, it only provides the relative dates of items rather than their absolute ages. For this reason, fluo- rine dating is considered a hybrid form of rela- tive and chronometric dating methods (which will be discussed next). Soils contain different amounts of chemicals, and those chemicals, such as fluorine, can be absorbed by human and animal bones buried in the soil. The longer the remains are in the soil, the more fluorine they will absorb (Cook and Ezra-Cohn 1959; Goodrum and Olson 2009). A Figure 7.21: Charts of typology, like these sample of the bone or antler can be processed representing items from the Bronze Age, are used and measured for its fluorine content. to classify artifacts and illustrate cultural material Unfortunately, this absorption rate is highly assemblages. Credit: Bronze Age implements, sensitive to temperature, soil pH, and varying ornaments and pottery (Period II) by Wellcome Collection is under a CC BY 4.0 License. fluorine levels in local soil and groundwater (Goodrum and Olson 2009; Haddy and Hanson 1982). This makes it difficult to get an accurate date for the remains or to compare remains be- tween two sites. However, this technique is particularly useful for determining whether different artifacts come from the same burial context. If they were buried in the same soil for the same length of time, their fluorine signatures would match. Chronometric Dating Unlike relative dating methods, chronometric dating methods provide specific dates and time ranges. Many of the chronometric techniques we will discuss are based on work in other disci- plines such as chemistry and physics. The modern developments in studying radioactive materi- als are accurate and precise in establishing dates for ancient sites and remains. Many of the chronometric dating methods are based on the measurement of radioactive decay of particular elements. Elements are materials that cannot be broken down into more simple materials without losing their chemical identity (Brown et al. 2018, 48). Each element consists of an atom that has a specific number of protons (positively charged particles) and electrons (neg- atively charged particles) as well as varying numbers of neutrons (particles with no charge). The protons and neutrons are located in the densely compacted nucleus of the atom, but the major- ity of the volume of an atom is space outside the nucleus around which the electrons orbit (see Figure 7.22). Elements are classified based on the number of protons in the nucleus. For example, carbon has six protons, giving it an atomic number 6. Uranium has 92 protons, which means that it has an atomic number 92. While the num- ber of protons in the atom of an element do not vary, the number of neutrons may. Atoms of a given element that have different numbers of neutrons are known as iso- topes. The majority of an atom’s mass is determined by the pro- tons and neutrons, which have more than a thousand Figure 7.22: Simplified illustration of an atom. Credit: Atom Diagram by AG times the mass of an electron. Due to the different num- Caesar is under a CC BY-SA 4.0 License. bers of neutrons in the nucleus, isotopes vary by nuclear/atomic weight (Brown et al. 2018, 94). For in- stance, isotopes of carbon include carbon 12 (12C), carbon 13 (13C), and carbon 14 (14C). Carbon al- ways has six protons, but 12C has six neutrons whereas 14C has eight neutrons. Because 14C has more neutrons, it has a greater mass than 12C (Brown et al. 2018, 95). Most isotopes in nature are considered stable isotopes and will remain in their normal structure indefinitely. However, some isotopes are considered unstable isotopes (sometimes called ra- dioisotopes) because they spontaneously release energy and particles, transforming into stable isotopes (Brown et al. 2018, 946; Flowers et al. 2018, section 21.1). The process of transforming the atom by spontaneously releasing energy is called radioactive decay. This change occurs at a predictable rate for nearly all radioisotopes of elements, allowing scientists to use unstable iso- topes to measure time passage from a few hundred to a few billion years with a large degree of accuracy and precision. The leading chronometric method for archaeology is radiocarbon dating (Figure 7.23). This method is based on the decay of 14C, which is an unstable isotope of carbon. It is created when nitrogen 14 (14N) interacts with cosmic rays, which causes it to capture a neutron and convert to 14C. Carbon 14 in our atmosphere is absorbed by plants during photosynthesis, a process by which light energy is turned into chemical energy to sustain life in plants, algae, and some bacte- ria. Plants absorb carbon dioxide from the atmosphere and use the energy from light to convert it into sugar that fuels the plant (Campbell and Reece 2005, 181–200). Though 14C is an unstable isotope, plants can use it in the same way that they use the stable isotopes of carbon. Animals get 14C by eating the plants. Humans take it in by eating plants and animals. After death, organisms stop taking in new carbon, and the unstable 14C will begin to decay. Carbon 14 has a half- life of 5,730 years (Hester et al. 1997, 324). That means that in 5,730 years, half the amount of 14C will convert back into 14N. Because the pattern of radioactive decay is so reliable, we can use 14C to accurately date sites up to 55,000 years old (Hajdas et al. 2021). However, 14C can only be used on the remains of biological organisms. This includes charcoal, shell, wood, plant material, and bone. This method involves destroying a small sample of the material. Earlier methods of radiocarbon dating re- Figure 7.23: A graphic illustrating how 14C is created in the atmosphere, quired at least 1 gram of ma- is absorbed by living organisms, and ends up in the archaeological record. Credit: Radiocarbon dating (Figure 7.32) original to Explorations: terial, but with the introduc- An Open Invitation to Biological Anthropology by Mary Nelson is under tion of accelerator mass a CC BY-NC 4.0 License. spectrometry (AMS), sample sizes as small as 1 milligram can now be used (Hajdas et al. 2021). This significantly reduces the destructive nature of this method. The use of radiocarbon dating at Denisova Cave in modern-day Russia revealed an astounding find, the first dated first-generation individual with a Neanderthal mother and Denisovan father. Vivian Slon and colleagues (2018) sequenced the genome, which revealed the individual’s hybrid genetic background, and radiocarbon dated the remains, revealing the sub-adult was over 50,000 years old (Slon et al. 2018). As mentioned before, 14C is unstable and ultimately decays back into 14N. This decay is happen- ing at a constant rate (even now, inside your own body!). However, as long as an organism is alive and taking in food, 14C is being replenished in the body. As soon as an organism dies, it no longer takes in new 14C. We can then use the rate of decay to measure how long it has been since the organism died (Hester et al. 1997, 324). However, the amount of 14C in the atmosphere is not stable over time. It fluctuates based on changes to the earth’s magnetic field and solar ac- tivity. In order to turn 14C results into accurate calendar years, they must be calibrated using data from other sources. Annual tree rings (see discussion of dendrochronology below), foraminifera from stratified marine sediments, and microfossils from lake sediments can be used to chart the changes in 14C as “calibration curves.” The radiocarbon date obtained from the sample is com- pared to the established curve and then adjusted to reflect a more accurate calendar date (see Figure 7.24). The curves are updated over time with more data so that we can continue to refine radiocarbon dates (Törnqvist et al. 2016). The most recent calibration curves were released in 2020 and may change the dates for some existing sites by hundreds of years (Jones 2020). Figure 7.24: This is a simplified example of a calibration curve, showing how the radiocarbon age (y axis) is compared with the calibration curve to produce calibrated dates (x axis). Credit: Radiocarbon Date Calibration Curve by HowardMorland is under a CC BY-SA 3.0 License. [Based on information from Reimer et al. 2004. Radiocarbon 46: 1029-58.] [Image Description]. As you will see in the hominin chapters (Chapters 9–12), 55,000 years is only a tiny fragment of human evolutionary history. It is insignificant in the context of the age of our planet. In order to date even older fossils, other methods are necessary. Potassium-argon (K-Ar) dating and argon-argon (Ar-Ar) dating can reach further back into the past than radiocarbon dating. Used to date volcanic rock, these techniques are based on the de- cay of unstable potassium 40 (40K) into argon 40 (40Ar) gas, which gets trapped in the crys- talline structures of volcanic material. It is a method of indirect dating. Instead of dating the fos- sil itself, K-Ar and Ar-Ar dates volcanic layers around the fossil. It will tell you when the volcanic eruption that deposited the layers occurred. This is where stratigraphy becomes important. The date of the surrounding layers can give you a minimum and maximum age of the fossil based on where it is in relation to those layers. This technique was used at Gesher Benot Ya’aqo in the Jordan Valley, dating early stratigraphic deposits of basalt flows to 100,000 years old (Bar-Yosef and Belmaker 2011). The site is unique because early layers of occupation with an Acheulean handaxe industry were made primarily of basalt, which is an uncommon material for this tool technology (see Chapter 10 for a full discussion of this tool technology). The benefit of this dat- ing technique is that 40K has a half-life of circa 1.3 billion years, so it can be used on sites as young as 100 kya and as old as the age of Earth. As you will see in later chapters, it is particularly useful in dating early hominin sites in Africa (Michels 1972, 120; Renfrew and Bahn 2016, 155). Another benefit to this technique is that it does not damage precious fossils because the sam- ples are taken from the surrounding rock instead. However, this method is not without its flaws. A study by J. G. Funkhouser and colleagues (1966) and Raymond Bradley (2015) demonstrated that igneous rocks with fluid inclusions, such as those found in Hawai‘i, can release gasses in- cluding radiogenic argon when crushed, leading to incorrectly older dates. This is an example of why it is important to use multiple dating methods in research to detect anomalies. Uranium series dating is based on the decay chain of unstable isotopes of uranium. It uses mass spectrometry to detect the ratios of uranium 238 (238U), uranium 234(234U), and thorium 230 (230Th) in carbonates (Wendt et al. 2021). Thorium accumulates in the carbonate sample through radiometric decay. Thus, the age of the sample is calculated from the difference between a known initial ratio and the ratio present in the sample to be dated. This makes uranium series ideal for dating carbonate rich deposits such as carbonate cements from glacial moraine de- posits, speleothems (deposits of secondary minerals that form on the walls, floors, and ceilings of caves, like stalactites and stalagmites), marine and lacustrine carbonates from corals, caliche, and tufa, as well as bones and teeth (University of Arizona, n.d.; van Calsteren and Thomas 2006). Due to the timing of the decay process, this dating technique can be used from a few years up to 650k (Wendt et al. 2021). Since many early hominin sites occur in cave environments, this dating technique can be very powerful. This method has also been used to develop more ac- curate calibration curves for radiocarbon dating. However, the accuracy of this method depends on knowing the initial ratios of the elements and ruling out possible contamination (Wendt et al. 2021). It also involves the destruction of a small sample of material. Fission track dating is another useful dating technique for sites that are millions of years old. This is based on the decay of radioactive uranium 238 (238U). The unstable atom of 238U fissions at a predictable rate. The fission takes a lot of energy and causes damage to the surrounding rock. For instance, in volcanic glasses we can see this damage as trails in the glass. Researchers in the lab take a sample of the glass and count the number of fission trails using an optical mi- croscope. As 238U has a half-life of 4,500 million years, it can be used to date rock and mineral material starting at just a few decades and extending back to the age of Earth. As with K-Ar, ar- chaeologists are not dating artifacts directly. They are dating the layers around the artifacts in which they are interested (Laurenzi et al. 2007). Luminescence dating , which includes thermoluminescence and a related technique called opti- cally stimulated luminescence, is based on the naturally occurring background radiation in soils. Pottery, baked clay, and sediments that include quartz and feldspar are bombarded by radiation from the soils surrounding it. Electrons in the material get displaced from their orbit and trapped in the crystalline structure of the pottery, rock, or sediment. When a sample of the material is heated to 500°C (thermoluminescence) or exposed to particular light wavelengths (optically stimulated luminescence) in the laboratory, this energy gets released in the form of light and heat and can be measured (Cochrane et al. 2013; Renfrew and Bahn 2016, 160). You can use this method to date artifacts like pottery and burnt flint directly. When attempting to date fossils, you may use this method on the crystalline grains of quartz and feldspar in the surrounding soils (Cochrane et al. 2013). The important thing to remember with this form of dating is that heating the artifact or soils will reset the clock. The method is not necessarily dating when the object was last made or used but when it was last heated to 500°C or more (pottery) or exposed to sunlight (sediments). Luminescence dating can be used on sites from less than 100 years to over 100,000 years (Duller 2008, 4). As with all archaeological data, context is crucial to understand- ing the information. Like thermoluminescence dating, electron spin resonance dating is based on the measurement of accumulated background radiation from the burial environment. It is used on artifacts and rocks with crystalline structures, including tooth enamel, shell, and rock—those for which ther- moluminescence would not work. The radiation causes electrons to become dislodged from their normal orbit. They become trapped in the crystalline matrix and affect the electromagnetic en- ergy of the object. This energy can be measured and used to estimate the length of time in the burial environment. This technique works well for remains as old as two million years (Carvajal et al. 2011, 115–116). It has the added benefit of being nondestructive, which is an important con- sideration when dealing with irreplaceable material. Not all chronometric dating methods are based on unstable isotopes and their rates of decay. There are several other methods that make use of other natural biological and geologic pro- cesses. One such method is known as dendrochronology (Figure 7.25), which is based on the natural growth patterns of trees. Trees create concentric rings as they grow; the width of those rings depends on environmental conditions and season. The age of a tree can be determined by counting its rings, which also show records of rainfall, droughts, and forest fires. Tree rings can be used to date wood artifacts and ecofacts from archaeological sites. This first requires the creation of a profile of trees in a particular area. The Laboratory of Tree-Ring Research at the University of Arizona has a comprehensive and ongoing catalog of tree profiles (see University of Arizona n.d.). Archaeologists can then compare wood artifacts and ecofacts with existing timelines, provided the tree rings are visible, and find where their artifacts fit in the pattern. Dendrochronology has been in use since the early 20th century (Dean 2009, 25). The Northern Hemisphere chronology stretches back nearly 14,000 years (Reimer et al. 2013, 1870) and has been used successfully to date southwestern U.S. sites such as Pueblo Bonito and Aztec Ruin (Dean 2009, 26). Dendrochronological evidence has helped calibrate radiocarbon dates and even provided direct evidence of global warming (Dean 2009, 26–27). In Australia, dendrochronology, along with other environmental reconstruction methods, has been used to show that the Indigenous people had sophisticated land manage- ment systems before the arrival of British invaders. According to the work of Michael-Shawn Fletcher and colleagues (2021), there was a significant encroach- ment of the rainforests and tree species into grasslands after the British invasion. Prior to this time, Indigenous people managed the landscape through con- trolled burns at regular intervals. This practice created climate-resistant grass- lands that were biodiverse and provided predictable food supplies for humans and other animals. Under European land man- agement, there have been negative im- pacts on biodiversity and climate re- silience and an increase in catastrophic wildfires (Fletcher et al. 2021). This dating method does have its difficul- ties. Some issues are interrupted ring growth, microclimates, and species growth variations. This is addressed through using multiple samples, statistical analysis, and calibration with other dating methods. Despite these limitations, den- drochronology can be a powerful tool in dating archaeological sites (Hillam et al. 1990; Kuniholm and Striker 1987). Environmental Reconstruction As you read in Chapter 2, Charles Darwin, Figure 7.25: Dendrochronology uses the variations in tree Jean-Baptiste Lamarck, Alfred Russel rings to create timelines. Credit: Dendrochronology Wallace, and others recognized the impor- (Figure 7.34) original to Explorations: An Open Invitation to Biological Anthropology by Mary Nelson is under a CC tance of the environment in shaping the BY-NC 4.0 License. evolutionary course of animal species. To understand what selective processes might be shaping evolutionary change, we must be able to reconstruct the environment in which the organism was living. One of the ways to do that is to look at the plant species that lived in the same time range as the species in which you are interested. One way to identify ancient flora is to analyze sediment cores from water and other protected sources. Pollen gets released into the air and some of that pollen will fall on wetlands, lakes, caves, and so forth. Eventually it sinks to the bottom of the lake and forms part of the sediment. This happens year after year, so subsequent layers of pollen build up in an area, creating strata. By taking a core sample and analyzing the pollen and other organic material, an archaeologist can build a timeline of plant types and see changes in the vegetation of the area (Hester et al. 1997, 284). This can even be done over large areas by studying ocean bed cores, which accumulate pollen and dust from large swaths of neighboring continents. While sediment coring is one of the more common ways to reconstruct past environments, there are a few other methods. These have been recently employed at Holocene Lake Ivanpah, a pale- olake that straddles the California and Nevada border in the United States. This lake was origi- nally thought to have been completely dry around 9,300–7,800 kya (Sims and Spaulding 2017). However, analyzing core samples using soil identification, sediment chemistry, subsurface stratigraphy, and geomorphology (the study of the physical characteristics of the Earth’s sur- face) revealed deposition of three recent lake fillings during this period in the forms of additional hardpan, or lake bottom, playas, bedded or layered fine-grained (wetland) sediments, and buried beaches below the surface (Sims and Spaulding 2017; Spaulding and Sims 2018). These discover- ies are important because they have not been integrated into interpretation of the local archaeo- logical record, as it was assumed that the lake had been dry for thousands of years. Sedimentological analyses such as coring and those listed above can provide great insight into past climates and are accomplished in a minimally destructive way. Another way of reconstructing past environments is by using stable isotopes. Unlike unstable isotopes, stable isotopes remain constant in the environment throughout time. Plants take in the isotopes through photosynthesis and ground water absorption. Animals take in isotopes by drinking local water and eating plants. Stable isotopes can be powerful tools for identifying where an organism grew up and what kind of food the organism ate throughout its life. They can even be used to identify global temperature fluctuations. Global Temperature Reconstruction Oxygen isotopes are a powerful tool in tracking global temperature fluctuations throughout time. The isotopes of Oxygen 18 (18O) and Oxygen 16 (16O) occur naturally in Earth’s water. Both are stable isotopes, but 18O has a heavier atomic weight. In the normal water cycle, evaporation takes water molecules from the surface to the atmosphere. Because 16O is lighter, it is more likely to be part of this evaporation process. The moisture gathers in the atmosphere as clouds that eventually may produce rain or snow and release the water back to the surface of the planet. During cool periods like glacial periods (ice ages), the evaporated water often comes down to Earth’s surface as snow. The snow piles up in the winter but, because of the cooler summers, does not melt off. Instead, it gets compacted and layered year after year, eventually resulting in large glaciers or ice sheets covering parts of Earth. Since 16O, with the lighter atomic weight, is more likely to be absorbed in the evaporation process, it gets locked up in glacier for- mation. The waters left in oceans would have a higher ratio of 18O during these periods of cooler global temperatures (Potts 2012, 154–156; see Figure 7.26). The microorganisms that live in the oceans, foraminifera, absorb the water from their environment and use the oxygen isotopes in their body structures. When these organ- isms die, they sink to the ocean floor, con- tributing to the layers of sediment. Scientists can extract these ocean cores and sample the remains of foraminifera for their 18O and Figure 7.26: This graph depicts how temperatures of 16O ratios. These ratios give us a good ap- the sea have fluctuated greatly over the course of the history of the planet. Credit: Oxygen in deep sea proximation of global temperatures deep sediment carbonate (Figure 2) by NASA Goddard into the past. Cooler temperatures indicate Institute for Space Studies originally from “Science higher ratios of 18O (Potts 2012, 154–156). Briefs: Cold Climates, Warm Climates: How Can We Tell Past Temperatures?” by Gavin Schmidt, is in the Diet Reconstruction public domain. [Image Description]. You may be familiar with the saying “you are what you eat.” When it comes to your teeth and bones, this adage is literal. Stable isotopes can also be used to reconstruct animal diet and migration patterns. Living organisms absorb ele- ments from ingested plants and water. These elements are used in tissues like bones, teeth, skin, hair, and so on. By analyzing the stable isotopes in the bones and teeth of humans and other an- imals, we can identify the types of food they ate at different stages of their lives. Plants take in carbon dioxide from the atmosphere during photosynthesis. We’ve already dis- cussed this using the example of the unstable isotope 14C; however, this absorption also takes place with the stable isotopes of 12C and 13C. During photosynthesis, some plants incorporate carbon dioxide as a three-carbon molecule (C3 plants) and some as a four-carbon molecule (C4 plants). On the one hand, C3 plants include certain types of trees and shrubs that are found in relatively wet environments and have lower ratios of 13C compared to 12C. C4 plants, on the other hand, include plants from drier environments like savannahs and grasslands. C4 plants have higher ratios of 13C to 12C than C3 plants (Renfrew and Bahn 2016, 312). These ratios remain stable as you go up the food chain. Therefore, you can analyze the bones and teeth of an animal to identify the 13C/12C ratios and identify the types of plants that animal was eating. The ratios of stable nitrogen isotopes 15N and 14N can also give information about the diet of fossilized or deceased organisms. Though initially absorbed from water and soils by plants, the nitrogen ratios change depending on the primary diet of the organism. An animal who has a mostly vegetarian diet will have lower ratios of 15N to 14N, while those further up the food chain, like carnivores, will have higher ratios of 15N. Interestingly, breastfeeding infants have a higher nitrogen ratio than their mothers, because they are getting all of their nutrients through their mother’s milk. So nitrogen can be used to track life events like weaning (Jay et al. 2008, 2). A marine versus terrestrial diet will also affect the nitrogen signatures. Terrestrial diets have lower ratios of 15N than marine diets. In the course of human evolution, this type of analysis can help us identify important changes in human nutrition. It can help anthropologists figure out when meat became a primary part of the ancient human diet or when marine resources began to be used. The ratios of stable nitrogen isotopes can also be used to determine a change in status, as in the case of the Llullaillaco children (the “ice mummies”) found in the Andes Mountains. For in- stance, the nitrogen values in hair from the Llullaillaco Maiden showed a significant positive shift that is associated with increased meat consumption in the last 12 months of her life (Wilson et al. 2007). Although the two younger children had little changes in their diets in the last year of their short lives, the changes in their nitrogen values were significant enough to suggest that the improvement in their diets may have been attributed to the Incas’ desire to sacrifice healthy, high-status children” (Faux 2012, 6). Migration Stable isotopes can also tell us a great deal about where an individual lived and whether they migrated during their lifetime. The geology of Earth varies because rocks and soils have different amounts or ratios of certain elements in them. These variations in the ratios of isotopes of cer- tain elements are called isotopic signatures. They are like a chemical fingerprint for a geographi- cal region. These isotopes get into the groundwater and are absorbed by plants and animals liv- ing in that area. Elements like strontium, oxygen, and nitrogen, among others, are then used by the body to build bones and teeth. If you ate and drank local water all of your life, your bones and teeth would have the same isotopic signature as the geographical region in which you lived. However, many people (and animals) move around during their lifetimes. Isotopic signatures can be used to identify migration patterns in organisms (Montgomery et al. 2005). Teeth develop in early childhood. If the isotopes of teeth are analyzed, these isotopes would resemble those found in the geographic area where an individual lived as a child. Bones, however, are a different story. Bones are constantly changing throughout life. Old cells are removed and new cells are de- posited to respond to growth, healing, activity change, and general deterioration. Therefore, the isotopic signature of bones will reflect the geographical area in which an individual spent the last seven to ten years of life. If an individual has different isotopic signatures for their bones and teeth, it could indicate a migration some time during their life after childhood. Recent work involving stable isotope analysis has been done on the cremation burials from Stonehenge, in Wessex, England (Figure 7.27). Much of the archaeological work at Stonehenge in the past focused on the build- ing and development of the monument itself. That is partly because most of the burials at the monument were cremated remains, which are difficult to study because of their fragmentary nature and the chemical alter- ations that bone and teeth undergo when Figure 7.27: Stonehenge continues to provide clues to heated. The cremation process complicates its mysterious existence with recent research using the oxygen and carbon isotopes. However, isotope ratios. Credit: Stonehenge (Figure 7.37) by the researchers determined that strontium Sarah S. King is under a CC BY-NC 4.0 License. would not be affected by heating and could still be analyzed in cranial fragments. Using the remains of 25 individuals, they compared their strontium signatures to the geology of Wessex and other regions of the UK. Fifteen of those in- dividuals had strontium signatures that matched the local geology. This means that in the last ten or so years of their lives, they lived and ate food from around Stonehenge. However, ten of the individuals did not match the local geologic signature. These individuals had strontium ratios more closely aligned with the geology of west Wales. Archaeologists find this particularly inter- esting because in the early phases of Stonehenge’s construction, the smaller “blue stones” were brought 200 km from Wales in a feat of early engineering. These larger regional connections show that Stonehenge was not just a site of local importance. It dominated a much larger region of influence and drew people from all over ancient Britain (Snoeck et al. 2018). Special Topic: Cold Case Naia In 2007, cave divers exploring the Sistema Sac Actun in the Yucatán Peninsula in Mexico (see Figure 7.28 and 7.29) discovered the bones of a 15- to 16-year-old female human along with the bones of various extinct ani- mals from the Pleistocene (Collins et al. 2015). The site was named Hoyo Negro (“Black Hole”). The human bones belonged to a Paleo- American, later named “Naia” after a Greek water nymph. Examination of the partially fossilized remains revealed a great deal about Naia’s life, and the radiocarbon dating of her tooth enamel indicated that she lived some 13,000 years ago (Chatters et al. Figure 7.28: Map of Mexico showing the Yucatan Peninsula and the 2014). Naia’s arms were not locations of Hoyo Negro and Sistema Sac Actun. Credit: Hoyo Negro overly develo

Explorations: Una Invitación Abierta a la Antropología Biológica, 2ª Edición - Capitulo 7: Piedras y Huesos PDF

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