Chapter 25 The History of Life on Earth PDF

The fossil record provides a “deep time” perspective on evolutionary history. Chapter 25 Macro...

The fossil record provides a “deep time” perspective on evolutionary history. Chapter 25 Macroevolutionary phenomena: mass extinctions, the role of the physical The History of Life on Earth environment in the origin of species, and adaptive radiations, all came to the https://phys.org/news/2016-03-secrets-shark-bay-stromatolites.html attention of biologists via the study of fossils and the rocks that preserve them. The leading image is a photograph of bacterial colonies called stromatolites. Stromatolites were first described as fossils by paleontologists before they were found as living colonies. This way of life dates back 3.5 billion years: some of the oldest fossils are stromatolites. Living stromatolites require specific conditions. https://www.abc.net.au/news/2011-11-11/stromatolites-under-threat-from-climate-change/3660416 Figure 25.1 Major evolutionary events informed by earth history Stromatolites result from cyanobacteria Over time, layers are laid down as (photosynthetic bacteria) forming film-like sediment settles on the plaque and forces colonies (plaques/biofilms) on shallow sea the cyanobacteria to migrate upwards, and or lake bottoms. Plaques are sticky and over time the stromatolite grows upward. sediment in the water will accumulate on Paleontologists infer that ancient the top of the plaque. The cyanobacteria cyanobacteria formed stromatolite then migrate upwards and form a new colonies in the same manner 3.5 billion layer. years ago (in the Archean Eon). https://www.nps.gov/articles/stromatolites-of-capitol-reef-national-park.htm https://mediakron.bc.edu/livingearth/topic-17-earth-and-lifes-evolution-through- geologic-time/stromatolites-sunlight-mud-cyanobacteria-mat https://www.geologyin.com/2014/04/stromatolite.html Unlike the Shark Bay stromatolites, which There is no geochemical evidence of are exposed at low tide, paleontologists atmospheric oxygen during most of the infer that Archean stromatolites were Archean Eon (starting 4 billion years ago) never exposed out of water. Why? Because and only of trace amounts towards the the ozone layer largely protects land end of the Archean (2.5 billion years ago). organisms from harmful UV radiation. Ozone forms from atmospheric oxygen. Accordingly, uniformitarianism is of limited use when reconstructing the earliest life https://www.canada.ca/en/environment-climate-change/services/air- on Earth. pollution/issues/ozone-layer/depletion-impacts/about.html Conditions on early Earth made Geological research of the Earth and its the origin of life possible moon indicates that the sun and planets formed 4.6 billion years ago. Earth initially Direct evidence of life on early Earth formed as a sphere of molten rock. The comes from fossils of stromatolites and surface gradually cooled by 4.5 billion other microorganisms, preserved in rocks years ago with the formation of igneous that have been dated to 3.5 billion years rocks and minerals. Volcanoes spewed out ago by geologists. How did cellular life CO2, CH4, H2O and other gases. come to be? https://www.sci.news/othersciences/geophysics/plate- tectonics-zircons-10744.html#google_vignette There was no atmospheric O2. Because Some of the atmospheric CO2 diffused into Earth is in the “Goldilocks zone” of the the ocean, reacted with the water, became solar system, most of the H2O condensed carbonic acid, and then transformed to out of the atmosphere to form an ocean. dissolved minerals such as CaCO3. Earth’s orbit within the habitable zone results in an average insolation of about CaCO3 precipitates out of solution and 1000 W/m2, whereas that of Venus is 2600 rains down onto the ocean floor. This W/m2 and that of Mars in 590 W/m2. forms layers (strata) that over time will form into limestone, a sedimentary rock. https://kardashev.fandom.com/wiki/Goldilocks_zone https://iedro.org/articles/ocean-acidification-and-the-short-term-marine-carbon-cycle/ In this manner, carbon from the air and the Thus, conditions were set on the early ocean is locked up into carbonate rocks, a Earth that were conducive to the origin of mechanism that prevents CO2 from cellular life. The conditions are: liquid accumulating in the atmosphere and water with dissolved substances, sources creating a greenhouse effect. of energy (sunlight, lightning, geothermal energy), enormous stretches of time, and There were no continents or other major no atmospheric O2. land masses, but volcanic islands resulted as oceanic plates collided. https://www.latimes.com/science/sciencenow/la-sci-sn-early-earth- bombardment-20140730-story.html The absence of atmospheric O2 is critical In the 1920s, Soviet scientist Alexander because O2 is an active oxidizing agent. Oparin inferred that early life required a Paleobiologist Richard Cowen estimated reducing environment (i.e. O2 is not that 95% of the O2 generated by plants and present). other photosynthetic organisms is lost to mineral oxidation. Our cells and tissue Drawing from what was then known of the would oxidize in air if not for the giant gas planets, he hypothesized that the mitochondria in our cells consuming O2. atmosphere of the early Earth was made up of CO2, CH4, H2O, and N2. https://vinepair.com/articles/what-is-oxidation-in-wine/ A natural source of energy (he proposed British biologist J.B.S. Haldane came to the sunlight) could result in the interaction of same conclusion independently a few these gases and result in the formation of years later. simple organic molecules. Haldane coined the term “primordial Thus, the organic ingredients for more soup” to convey vividly the concept of the complex molecules could be generated early ocean as a kind of gigantic liquid naturally from abiotic ingredients. petri dish. Oparin and Haldane separately arrived at Stanley Miller was Harold Urey’s graduate what has been formalized as the first of student at the University of Chicago. four steps in the origination of early cells: the synthesis of small, simple organic They set up a simple apparatus in which molecules, such as amino acids and they collected a mixture of gases ( CO2, nitrogenous bases, from abiotic reactants. CH4, H2O, and NH3 ) thought to This idea was tested in a simple lab approximate a reducing atmosphere experiment by Stanley Miller in 1953. according to Oparin and Haldane. Liquid H2O simulated the ocean. https://astronoo.com/en/biographies/stanley-miller.html To simulate lightning, they subjected the Two years after his famous experiment, gases to electrical sparks.Miller and Urey Stanley Miller repeated the basic premise detected several different amino acids: but this time tried to replicate a volcanic glycine, alacine, serine, isoserine, valine, eruption, which would have produced and 11 other amino acids. Later alternative lightning and the abiotic precursors ( CO2, experiments using mixtures of CO2 and N2 CH4, H2O, and N2 ) for simple organic as the primary gases (in addition to liquid molecules. This experiment also yielded H2O to stand in for the ocean) also amino acids and other simple organic produced amino acids. molecules. https://arstechnica.com/science/2021/10/scientists-recreated-classic- https://www.wired.com/2016/02/sergio-tapiro-lava-ash-lightning-perfect-volcano-photo/ origin-of-life-experiment-and-made-a-new-discovery/ Miller froze samples from his volcanic eruption experiment, and these were reanalyzed in 2008. The research team found the same simple organic molecules as did Miller, but found 10 additional different organic molecules, including phenylalanine, homoserine, and ornithine. Figure 25.2 Amino acid synthesis in a simulated volcanic eruption Miller’s initial work and the 2008 revisit of Alkaline vents release water that ranges his 1955 samples lead researchers to 40º to 90º C. This is too warm for most suggest that the first organic compunds on modern cellular life, but there are living Earth formed near volcanoes. prokaryotes called thermophiles that live in such environments. Another source for organic compounds may have been warm, deep-sea alkaline There are other deep sea vents called volcanic vents. “black smokers” but they release 300º to 400º C water. https://geologyscience.com/geology/black-smokers/ Abiotic Synthesis of Macromolecules In addition to the abiotic amino-acid synthesis in Miller and Urey’s (1953) and later experiments, research has shown that RNA nucleotides (adenine, guanine, Figure 25.3 Did life originate in cytosine, uracil) can occur spontaneously deep-sea alkaline vents? from simple precursor molecules. Researchers have also produced polymers (macromolecules) of amino acids or nucleotides by dripping solutions of these molecules on hot sand, clay, or rock. The polymers of these building blocks formed spontaneously, without the help of enzymes or ribosomes. Figure 25.4 Self assembly of vesicles in the presence of a catalyst  Protocells DNA replication itself requires an assemblage of complex enzymes and a supply of nucleotides that have been Diagnostic features of cellular life are provided by the cell’s metabolism. reproduction and energy processing (metabolism). Nucleic acids, such as DNA, This suggests that self-replicating carry genetic information, including the molecules and a metabolic source of instructions that cells need for replication. building blocks may have appeared together in early protocells. The necessary condition for protocells may have been met by vesicles, which are fluid-filled compartments enclosed by a membrane-like structure. Experiments show that abiotically produced vesicles can exhibit certain properties of life, including simple replication and metabolism. Figure 25.4 Vesicle self-replication and absorption of RNA The rate of vesicle replication increases in Self-replicating RNA the presence of montmorillorite clay, which forms from weathered volcanic ash. Whereas living cells use DNA as the nucleic acid to record genetic information, some Vesicles can also absorb montmorillorite viruses instead use RNA, and all clay particles on which RNA and other metabolizing cells use RNA in gene organic molecules have attached. These expression. RNA and DNA use most of the findings are the result of lab research. same nucleotides, but RNA is single stranded whereas DNA is double stranded.  Evolutionary biologists infer that RNA Ribozymes are able to function as catalysts originated as the nucleic acid that encoded because the strands exhibit three the genome of ancient cells, and that RNA dimensionality. The exact shape is was later supplanted by DNA because, determined by the sequence of being double stranded, it is more stable. nucleotides. Another reason that RNA preceded DNA is because some RNAs act as enzyme-like Natural selection can act on variation of catalysts, known as ribozymes. sequences in ribozymes and favour those https://www.nature.com/articles/418222a that replicate the fastest. [Google Image search for “www.nature.com ribozyme”] A ribozyme encased with other organic DNA may have evolved if an RNA strand molecules in a vesicle would be more served as a template on which nucleotides efficient that a free-floating ribozyme were being linked but thymine was because the vesicle would concentrate the substituted for uracil. reactants. In addition to being more stable physically, Vesicle-encased ribozymes capable of self- the double line of code of DNA facilitates replication would have been the first the correction of coding errors during protocells. nucleic acid replication. https://en.wikipedia.org/wiki/Replisome The Fossil Record Fossils are the petrified remains of organisms, either body parts or their The fossil record of life is sparse for the activities, of the past. first 2 billion years of Earth history. There are chemical fossils that preserve the Bone, teeth, leaves, and wood preserves metabolic activity of early life from 3.9 more readily that soft body parts. What is billion years ago. Stromatolites appear 400 preserved can range from the entire million years later. Cell “body” fossils skeleton of an animal to a single bone or a appear approx. 30 million years after that. fragment of a bone. https://www.photonics.com/Articles/Image_Software_Focuses_on_Earths_Oldest_Fossils/a16573  Some fossils are preserved in ash (tracks) and some in amber (fossilised resin), but most are preserved in sedimentary rocks. Sedimentary rocks result from the deposition of rock particles (sand, mud, silt) by water (usually), or from minerals (CaCO3) that precipitate out of water, and then petrified over the course of time. https://en.wikipedia.org/wiki/Stratum https://www.geologypage.com/2015/12/geological-folds.html Figure 25.5 Fossils galore Petrification involves minerals percolating Sediments facilitate the formation of from water in the enclosing sediment fossils because they can cover and protect partially to fully replacing the original remains from scavengers and/or bacterial organic matter (bone mineral, etc.). decomposition. Minerals may also precipitate out of the Water that percolates through sediments water seeping through sediments and carries minerals that contribute to the form crystals in bone hollows (such as process of fossilization. marrow cavities). Sedimentation generally occurs over long The fossil record is not a complete record time periods, leading to layers of deposits of past life and never will be. This is that, once petrified, become strata. because various biases influence the fossil formation. Marine organisms have a better Older fossils are from lower strata and fossil record than land-living organisms younger fossils from upper strata. Relative because they were living in (or flying over) ages of fossils can be worked out by water. comparing fossiliferous strata from two or more sites (localities). https://www.pinterest.com/pin/marine-fossils-kentucky--178736678934728664/ https://natureroamer.com/marine-fossils-guide/ https://science8sc.weebly.com/law-of-superposition.html Plants and animals that lived in coastal How Rocks and Fossils are Dated lowlands have a better fossil record than those that lived in uplands because their To be as informative as possible, fossils remains are likely to be covered in sand, need to be collected with contextual mud, or silt. information. Fossils found when still partially buried are ideal because the Upland environments are usually not surrounding rock allows the stratigraphic conducive to fossil preservation because of position of the fossil to be logged and increased erosion and weathering. clues about the depositional environment. Most fossils are preserved in sedimentary Radioactive isotopes are atoms of a rocks, the exact age of which cannot be chemical element that decay, i.e. they lose directly determined. subatomic particles and transform into a stable isotope of a different element. If the right samples can be collected from strata, then radiometric dating can be used Particular radioactive isotopes of an to determine a numerical age. This element decay at the same rate. The term method is based on the decay of half life refers to the time required for half radioactive isotopes. of a certain radioactive isotope to decay. For example, carbon has two stable isotopes (12C, 13C) and a single radioactive isotope (14C). Carbon-14 (“parent isotope”) decays to nitrogen-14 (“daughter” isotope) and has a half life of 5,730 years. Carbon samples (e.g. wood) up to approx. 75,000 years old can dated radiometrically (“carbon dated”). Figure 25.6 Radiometric dating Radiometric dates older than that require If you have fossils preserved in strata that a sample with zircon minerals from are sandwiched between layers of volcanic igenous rock or volcanic ash. ash beds, and you have samples from the ash beds dated radiometrically, then you Zircons contain isotopes of uranium. For can state that the fossiliferous strata are example, uranium-235 has a half life of beween “X” and “Y” years old. 703.8 million years and uranium-238 has a Geochronologists have made recent half life of 4.5 billion years. (The oldest advances to date major time boundaries. known minerals on Earth are zircons.) https://stratigraphy.org/chart https://www.abc.net.au/science/articles/2014/02/24/3950076.htm Early 19th century geologists began constructing the geological record on the basis of the fossils they found in Table 25.1 sedimentary rocks. The geological record Originally the ages of major boundaries were estimations based on sedimentation rates. Radiometric dating during the 20th century provided absolute dates. The Origin of New Groups For example, mid-19th century naturalists of Organisms suspected that birds and reptiles were closely related. In On the Origin of Species, Looking at living organisms only, major Darwin predicted that intermediate forms morphological gaps are evident between would be found in the fossil record. major groups of organisms that available evidence indicated that they are closely Two years later the first skeleton of the related. early bird Archaeopteryx lithographica was collected and described. https://www.livescience.com/24745-archaeopteryx.html In another example of transitional forms found in the fossil record, fossils shed light on the large morphological gap the lies between mammals and other tetrapods, i.e. reptiles and amphibians. Study of the fossil records reveals that mammals are the only living members of a group of tetrapods called synapsids. Figure 25.7 Exploring the Origin of Mammals Context: in addition to many soft-tissue Reptiles and amphibians have multipartite traits, living mammals are distinct from lower jaws, a quadrate-articular jaw joint, other living tetrapods in that the lower jaw undifferentiated teeth primitively, and is formed by a single bone (the dentary) on each ear has a single bone (stapes). each side; the jaw joint (hinge) is formed by the dentary and the squamosal bones; Paleontology reveals that mammalian teeth are differentiated; and there are skeletal traits evolved in non-mammalian three ear ossicles (stapes, incus, and synapsids (including the ancestors of malleus) in each ear. mammals) in step-wise fashion. https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2018.00278/full 300 Ma synapsid The oldest mammal fossils are approx. 165 million years old (165 Ma) and the oldest synapsid fossils are 315 Ma. That means the first half of synapsid evolutionary history can be studied only from fossils. The fossil record indicates a rich early diversity of non-mammalian 265 Ma synapsid synapsids. Figure 25.7 Exploring the Origin of Mammals Skeletons of early synapsids were reptile- Approx. 35 million years later (i.e. 265 Ma) like. 300 million year old (Ma) synapsids appeared synapsids with true canine teeth had relatively a small temporal fenestra and a larger temporal fenestra, suggesting and a quadrate-articular jaw joint (hinge). increased bite forces. Figure 25.7 300 Ma synapsid Figure 25.7 265 Ma synapsid 255 Ma synapsid By 255 Ma, synapsids with proportionately larger temporal fenestra and larger dentary bones had evolved. Note the 220 Ma synapsid incipient coronoid process. 195 Ma synapsid Figure 25.7 Exploring the Origin of Mammals Figure 25.7 255 Ma synapsid Also by this time, the insertion of the jaw By 220 Ma mammal-line synapsids had adductor musculature had moved from evolved a double jaw joint, with the the top of the lower jaw onto the side as squamosal (purple) and the dentary well, creating a pocket (*) on the dentary. (brown) adding to the jaw joint. * Figure 25.7 255 Ma synapsid Figure 25.7 255 Ma synapsid  In addition, the coronoid process of the By 195 Ma, the squamosal (purple) and dentary became greatly enlarged, with a the dentary (brown) alone formed the jaw shallow area for increased jaw muscle joint (hinge); the quadrate and the attachment on the side. articular were relocated in the ear and have become the incus and the malleus. coronoid process Figure 25.7 195 Ma synapsid Figure 25.7 220 Ma synapsid https://digimorph.org/specimens/Didelphis_virginiana/ Key Events in the History of Life Until the end of the 1900s, rocks older than the Phanerozoic were referred to Key events in life’s history include the simply as “pre-Cambrian” after the earliest origins of unicellular and multicellular geological period of the Phanerozoic. organisms and the colonization of the land. The Phanerozoic Eon (542 Ma to The pre-Cambrian is now subdivided into present) encompasses most of the time Hadean, Archean, and Proterozoic eons. animals have existed on Earth. Stromatolites of the Archean Eon mark the first appearance of prokaryotes. Figure 25.8 Visualizing the Scale of Geologic Time: Hadean and Archaean eons Figure 25.8 Visualizing the Scale of Geologic Time The oldest stromatolites are 3.5 billion The oldest stromatolites years old. These are trace fossils of and single-celled fossils bacterial biofilms. are preserved in marine rocks, which indicate the The oldest unequivocal fossil cells are 3.47 first cells lived in the billion years old. They are the size of oceans. modern bacteria and feature helical body The presence of oxygen-producing form like some living bacteria. Note: actual bacteria even earlier (3.8 billion years ago) cell structure is not preserved. is suggested by banded iron formations. https://en.wikipedia.org/wiki/Banded_iron_formation In BIFs, red bands of chert are interbedded Most banded iron formations (BIFs) date with layers of iron oxides. from approx. 3.8 to 1.85 billion years ago. Iron ions (Fe+2) were dissolved in sea water The iron oxides formed when O2, released during this time and there is no evidence by cyanobacteria, reacted with iron for atmospheric O2. dissolved in the ancient oceans, and Fe2O3 or Fe3O4 rained down on to the ocean O2 may have been dissolved in upper floor. The alternating banding is due to levels of shallow seas, but the oceans were seasonal changes. largely anoxic (lacking O2). Starting 2.7 to 2.5 billion years ago, O2 was By 1.8 billion years ago, O2 began to gas out increasingly produced but remained low of the oceans, but was absorbed by land (0.02 and 0.04 atm) and was absorbed by surfaces, evidenced by terrestrial red beds oceans and sea beds. (iron particles in sand, mud, etc. grains are oxidized on land). This marks the start of the Oxygen Revolution, Then, 850 million years ago, O2 began to a.k.a. the Great accumulate in the atmosphere, and Oxidation Event. reached 10% by the start of the Cambrian. http://www.luckysci.com/2014/09/easy-science-the-great-oxygenation-event/  The rising concentration of atmospheric O2 must have doomed many prokaryotic groups. O2 acts as a free radical and readily oxidizes organic molecules. Some prokaryotic groups survived by living in anaerobic (O2-less) habitats, such as wetlands. Others adapted to the new conditions. Figure 25.9 The Rise of Atmospheric Oxygen These proto-eukaryotes possibly preyed Some other prokaryote lineages evolved upon aerobic bacteria, engulfing them via a cellular respiration, which uses O2 as a process called phagocytosis. fuel, becoming aerobic. Still other lineages may have survived by living close to aerobic bacteria. One of these non-aerobic lineages (“proto- eukaryotes”) evolved membranes and the ability to change them. https://www.britannica.com/science/phagocytosis In one instance the proto- The origin of the mitochondrion is eukaryote did not digest reconstructed as the first episode of the bacterium, and primary endosymbiosis. instead they formed a mutually beneficial relationship. This process, endosymbiosis, resulted in host cell and endosymbiote becoming the ancestral eukaryote. Figure 25.10 Primary endosymbiosis The first step is the evolution of infolding The resulting “proto-eukaryote” can alter of the plasma membrane. The next step is the shape of its membranes, facilitating engulfment of the nuclear region, resulting phagocytosis, which it practiced on food in the nucleus and other organelles. particles and bacteria. Figure 25.10 Primary endosymbiosis Figure 25.10 Primary endosymbiosis At least one “proto-eukaryote” did not The larger cell became the host and the digest a phagocytosized aerobic aerobic bacterium became the bacterium, instead setting up a symbiotic endosymbiont. relationship. Figure 25.10 Primary endosymbiosis Figure 25.10 Primary endosymbiosis The result is a fully eukaryotic cell, a hybrid This ancestral eukaryote was of host cell with endosymbionts heterotrophic: it obtained energy for its (mitochondria) that use O2 as fuel for metabolism by breaking down organic cellular respiration and neutralize its free molecules it ingested. radical effects. The eukaryotic cell design was Cellular respiration provides the eukaryote advantageous and early eukaryotes with 11x more energy than the primitive diversified into several lineages. metabolic pathway glycolysis. One of these eukaryote lineages then The heterotrophic eukaryote host cell did underwent endosymbiosis again, but this not digest the engulfed photosynthetic time with a photosynthetic bacterium. bacterium, and a new symbiotic relationship was created. Figure 25.10 Primary endosymbiosis Figure 25.10 Primary endosymbiosis The Origin of Multicellurity The endosymbiont, this time a photosynthetic bacterium, became a After the first eukaryotes appeared, a plastid (chloroplast), and the host cell great range of unicellular heterotrophs became the first algal cell. and autotrophs evolved, most giving rise to the diversity of single-celled The algal cell is autotrophic: the plastids eukaryotes living today, collectively called convert sunlight energy into sugars, which protists, such as the planktonic diatoms, can drawn into cellular respiration, or foraminiferans, and radiolarians. made into other organic molecules. https://thenaturalhistorian.com/2015/02/26/forams-vs-diatoms-testing-young-earth-flood-geology-hypotheses/ https://www.pinterest.com/pin/74168725089073044/ Larger and more diverse multicellular Figure 25.11 Ediacaran (a) alga and (b) the animal Kimberella. eukaryotes do not appear for another 635 million years: in the Ediacaran Period of the late Proterozoic Eon. Fossils of the Ediacaran biota included soft-bodied animals and plants, some of which grew over 1 m tall (long). Some fossils are of unknown relationships. http://www.ediacaran.org/dickinsonia.html http://www.ediacaran.org/newfoundland-canada.html  The Ediacaran marks the first appearances DNA analyses suggest that sponges of major animal groups such as sponges, evolved by 700 Ma and bilaterians cnidarians, and possibly molluscs. Tiny (animals with left & right sides: chordates, shelled organisms appear at the end of the arthropods, etc.) had evolved by 670 Ma. period. Mistaken Point in NL is radiometrically dated to 565 Ma. Molecular clock estimates, based on DNA analyses, agree with the fossil record on The Proterozoic Eon ends 539 Ma and is the age of animals. succeeded by the Phanerozoic Eon. The first chapter of the Phanerozoic (Paleozoic Era) is the Cambrian Period. The first unequivocal molluscs appear at the beginning of the Cambrian Period. Ten million years into the Cambrian (529 Ma) the first brachiopods appear, as do the first reefs (formed by archaeocyathid sponges). https://www.sciencephoto.com/media/547390/view/camarotoechi-brachiopod-fossils Figure 25.12 Earliest appearances of major animal groups By approx. 520 Ma the first arthropods The available evidence suggests that have appeared in the form of trilobites. predation was a major selective force. The next 15 million years is marked by the Two major fossil sites characterize the appearances of major taxa, such as Cambrian Explosion: Chengjiang and the chordates, and many distinct invertebrate Burgess Shale. fossils that indicate a period of Both preserve rich fossil faunas of soft- evolutionary “experimentation” called the bodied organisms and a diversity of Cambrian Explosion. animals with mineralized exoskeletons https://en.wikipedia.org/wiki/Trilobite (shells, carapaces, etc.). https://www.livescience.com/planet-earth/evolution/did-the-cambrian-explosion-really-happen  A few of the predatory arthropods reached Uniquely Cambrian arthropods, such as 1 m in length. Wiwaxia from the Burgess Shale, exhibited spiky extensions that were probably The carapaces of trilobites were more defensive. mineralized (hardened with CaCO3) than https://burgess-shale.rom.on.ca/fossils/wiwaxia-corrugata/ their antennae and legs, suggesting a defensive function for the carapace. https://dinopedia.fandom.com/wiki/Hallucigenia https://es.pinterest.com/pin/335729347229667980/ https://www.newscientist.com/article/2311012-burst-of-animal-evolution-altered-chemical-make-up-of-earths-mantle/ https://en.wikipedia.org/wiki/Opabinia The Colonization of Land The oldest plant body fossils are 430 Ma, and the oldest animal fossils (millipedes) The colonization of land my multicellular are 428 Ma. organisms is another major milestone in the history of life. Modern plants form symbiotic relationships with fungi (called The oldest terrestrial multicellular fossils mycorrhizae). Fossils of 410 Ma plants are plant spores approx. 468 million years preserve evidence (arbuscules) of old. mycorrhizae. Plate Tectonics Nineteenth century geographers noted that the profiles of coastlines of eastern North and South America corresponded to those of western Europe and Africa on maps. The concept of continental drift was proposed by Alfred Wegener in 1915 to explain these observations. Figure 25.13 Evidence of an ancient symbiosis Continental drift was supported by In 1964, a 9.2 magnitude earthquake stratigraphic studies of the opposing rocked Alaska, causing some areas to coastlines and the presence of similar subside (lower) and others to rise. In one fossils on both sides of the Atlantic Ocean. area, land rose an average of 2 m with a maximum increase of 11.6 m (38 feet). Despite that evidence, continental drift was not widely accepted and most The idea that Earth’s crust is made of rocky scientists continued to believe in the fixed plates that float atop molten rock was positions of continents into the 1960s. proposed to explain this phenomena. Plate tectonics became accepted as the primary mechanism that explained continental drift, and it explained earthquakes, volcanism, and tsunamis. The seemingly “rock solid” continents on which we live move over time, usually only a few centimeters a year, some as much as 5.5 cm a year. Figure 25.15 Cutaway view of the Earth; crust thickness is exaggerated. The previous positions of land masses are reconstructed using stratigraphy (study of strata on complimentary coastlines) and paleomagnetism. Paleomagnetism is based on the study of iron particles in rock, which align with the magnetic north as volcanic rock cools. Figure 25.16 Earth’s major tectonic plates  Many important geological processes, In other cases, two plates are sliding past including the formation of mountains and each other. As a result, earthquakes are islands, occur at plate boundaries. common to the regions along the sliding contact. In some cases, plates are moving away from each other. The North American and In many cases two plates collide directly. If Eurasian plates are moving apart at a rate this occurs between oceanic plates, one of about 2 cm per year. plate slides over the other, driving the overlapped edge down into the mantle. https://www.britannica.com/place/Aleutian-Islands The edge of the overridden plate starts to When an continental plate collides with a melt, and the molten rock rises upwards oceanic plate, the latter is overridden by and punches holes along the margin of the the former because it is denser. The overriding plate, resulting in volcanic margin of the continental plate crumples, archipelagos. resulting in mountain ranges. Volcanoes The margin of the overriding plate is usually erupt along the coast and persist usually convex (seen from above) resulting for 10s of 1000s of years, e.g. Mount St. in island arcs. Helens in WA state. https://www.zmescience.com/feature-post/natural-sciences/geology-and-paleontology/earth- https://en.wikipedia.org/wiki/Mount_St._Helens dynamics/convergent-boundaries/ Continental drift can split up widespread Over long time scales the effect of populations and result in speciation via continental drift is dramatic. There have allopatric speciation. been three occasions ( 1 billion years ago, Some terrestrial groups evolve on an 600 Ma, and 255 Ma) that all the isolated land mass, and then can disperse continents have accreted (come together) when continental plates collide or land to form supercontinents, which later broke bridges develop between them, e.g. the apart. Alfred Wegener named the last one Panama land bridge (isthmus). (255 Ma) Pangea. https://latinalista.com/new-headline/scientists-make-starting-discovery-of-when- https://www.openculture.com/2014/07/map-showing- panama-linked-north-and-south-america where-todays-countries-would-be-located-on-pangea.html  Organisms are affected by the climate change that results when a continent shifts its location. Figure 25.17 The history of continental drift durin the Phanerozoic Eon For example, what is Nova Scotia was positioned a few degrees south of the equator 300 Ma. Movement and rotation of the North American plate sine then has resulted in Nova Scotia now lying 45º N. In another example, Antarctica was home to temperate forests, mammals, and large ostrich-like birds until Australia broke away 34-29 Ma and started drifting north. This resulted in the formation of the Antarctic Circumpolar Current, insulating Antarctica from warm currents and starting icehouse conditions. https://en.wikipedia.org/wiki/Antarctic_Circumpolar_Current Figure 25.18 Mass extinction and the diversity of life Mass extinction events record periods Five mass extinction events (a.k.a. the “Big when assemblages of species became Five”) are recognized today. extinct more-or-less simultaneously. There is a large amount of evidence that On a global scale, environmental change is the end-Mesozoic extinction event (66 Ma, so rapid and disruptive that entire a.k.a. the end-Cretaceous extinction event) ecosystems collapse and several million was caused by an extraterrestrial impact years are needed for recovery. (asteroid or comet).