Biopsychology: Evolution, Genetics, and Experience (PDF)

Chapter 2 Evolution, Genetics, and Experience Thinking about the Biology of Behavior pyrozhenka/Shutterstock Chapter Overview and Learning Objectives Thinking about the LO 2.1 Describe the origins of the physiological–psychologic...

Chapter 2 Evolution, Genetics, and Experience Thinking about the Biology of Behavior pyrozhenka/Shutterstock Chapter Overview and Learning Objectives Thinking about the LO 2.1 Describe the origins of the physiological–psychological and Biology of Behavior: From nature–nurture ways of thinking. Dichotomies to Interactions LO 2.2 Explain why thinking about the biology of behavior in terms of traditional physiological–psychological and nature–nurture dichotomies is flawed. Human Evolution LO 2.3 Describe the origins of evolutionary theory. LO 2.4 Explain the evolutionary significance of social dominance and courtship displays. LO 2.5 Summarize the pathway of evolution from single-cell organisms to humans. LO 2.6 Describe nine commonly misunderstood points about evolution. 44 M02_PINE1933_11_GE_C02.indd 44 22/01/2021 10:38  Evolution, Genetics, and Experience 45 LO 2.7 Describe how research on the evolution of the human brain has changed over time. Fundamental Genetics LO 2.8 Explain how Mendel’s work with pea plants has informed us about the mechanisms of inheritance. LO 2.9 Understand the structure and function of chromosomes. LO 2.10 Describe the process of gene expression. LO 2.11 Discuss several ways in which modern advances have changed our understanding of genetic processes. LO 2.12 Define epigenetics, and explain how it has transformed our understanding of genetics. Epigenetics of Behavioral LO 2.13 Discuss what insights into the genetics of behavior were gained Development: Interaction from early research on selective breeding. of Genetic Factors and LO 2.14 Explain how classic research on phenylketonuria (PKU) has Experience informed our understanding of the genetics of behavior. Genetics of Human LO 2.15 Explain why it is important to distinguish between the d ­ evelopment Psychological Differences of individuals and the development of individual differences. LO 2.16 Explain heritability estimates and how they are commonly misinterpreted. LO 2.17 Describe two ways that twin studies can be used to study the interaction of genes and experience (i.e., nature and nurture). We all tend to think about things in ways that have been The Origins of Dichotomous ingrained in us by our zeitgeist (pronounced “TSYTE- gyste”), the general intellectual climate of our culture. That is Thinking why this is a particularly important chapter for you. You see, LO 2.1 Describe the origins of the physiological– you are the intellectual product of a zeitgeist that promotes psychological and nature–nurture ways of ways of thinking about the biological bases of behavior that thinking. are inconsistent with the facts. The primary purpose of this The tendency to think about behavior in terms of dichoto- chapter is to help you bring your thinking about the biology mies is illustrated by two kinds of questions commonly of behavior in line with modern ­biopsychological science. asked about behavior: (1) Is it physiological, or is it ­psychological? (2) Is it inherited, or is it learned? Both questions have proved to be misguided, yet they are Thinking about the among the most common kinds of questions asked in bio- psychology classrooms. That is why we are dwelling on Biology of Behavior: From them here. Dichotomies to Interactions IS IT PHYSIOLOGICAL, OR IS IT PSYCHOLOGICAL? We tend to ignore the subtleties, inconsistencies, and com- The idea that human processes fall into one of two catego- plexities of our existence and to think in terms of simple, ries, physiological or psychological, has a long history in mutually exclusive dichotomies: right–wrong, good–bad, many cultures. For much of the history of Western cultures, attractive–unattractive, and so on. The allure of this way of truth was whatever the Church decreed to be true. Then, in thinking is its simplicity. about 1400, things started to change. The famines, plagues, M02_PINE1933_11_GE_C02.indd 45 22/01/2021 10:38  46 Chapter 2 and marauding armies that had repeatedly swept Europe... Give me a dozen healthy infants, well-formed, during the Dark Ages subsided, and interest turned to and my own specified world to bring them up in and art, commerce, and scholarship—this was the period of I’ll guarantee to take any one at random and train him the Renaissance, or rebirth (1400–1700). Some Renais- to become any type of specialist I might select—doctor, sance scholars were not content to follow the dictates of lawyer, artist, merchant-chief and, yes even beggar-man and thief. (Watson, 1930, pp. 103–104) the Church; instead, they started to study things directly by observing them—and so it was that modern science At the same time experimental psychology was tak- was born. ing root in North America, ethology (the study of ani- Much of the scientific knowledge that accumulated mal behavior in the wild) was becoming the dominant during the Renaissance was at odds with Church dictates. approach to the study of behavior in Europe. European However, the conflict was resolved by the prominent French ethology, in contrast to North American experimental philosopher René Descartes (pronounced “day-CART”). psychology, focused on the study of instinctive ­behaviors Descartes (1596–1650) advocated a philosophy that, in a (behaviors that occur in all like members of a species, sense, gave one part of the universe to science and the other even when there seems to have been no opportunity for part to the Church. He argued that the universe is com- them to have been learned), and it emphasized the role of posed of two elements: (1) physical matter, which behaves nature, or inherited factors, in behavioral development. according to the laws of nature and is thus a suitable object Because instinctive behaviors are not learned, the early of scientific investigation—the human body, including the ethologists assumed they are entirely inherited. They brain, was assumed to be entirely physical, and so were were wrong, but then so were the early experimental nonhuman animals; and (2) the human mind (soul, self, psychologists. or spirit), which lacks physical substance, controls human behavior, obeys no natural laws, and is thus the appropriate purview of the Church. Cartesian dualism, as Descartes’s philosophy became Problems with Thinking about the known, was sanctioned by the Roman Church, and so Biology of Behavior in Terms of the idea that the human brain and the mind are s­ eparate Traditional Dichotomies ­entities became even more widely accepted. It has survived to this day, despite the intervening centuries LO 2.2 Explain why thinking about the biology of of ­scientific progress. Most people now understand that behavior in terms of traditional physiological– human behavior has a physiological basis, but many psychological and nature–nurture dichotomies still cling to the dualistic assumption that there is a cat- is flawed. egory of human activity that somehow transcends the The physiological-or-psychological debate and the nature- human brain. or-nurture debate are based on incorrect ways of thinking about the biology of behavior, and a new generation of questions is directing the current boom in biopsychological IS IT INHERITED, OR IS IT LEARNED? The tendency to research (see Churchland, 2002). What is wrong with these think in terms of dichotomies extends to the way people old ways of thinking about the biology of behavior, and think about the development of behavioral capacities. For what are the new ways? centuries, scholars have debated whether humans and other animals inherit their behavioral capacities or acquire them through learning. This debate is commonly referred to as PHYSIOLOGICAL-OR-PSYCHOLOGICAL THINKING the nature–nurture issue. RUNS INTO DIFFICULTY. Not long after Descartes’s Most of the early North American experimental psy- mind–brain dualism was officially sanctioned by the Roman chologists were totally committed to the nurture (learning) Church, it started to come under public attack. side of the nature–nurture issue. The degree of this commit- In 1747, Julien Offray de la Mettrie anonymously pub- ment is illustrated by the oft-cited words of John B. Watson, lished a pamphlet that scandalized Europe.... La Mettrie the father of behaviorism: fled to Berlin, where he was forced to live in exile for the rest of his life. His crime? He had argued that thought was We have no real evidence of the inheritance of [behavioral] produced by the brain—a dangerous assault, in the eyes of traits. I would feel perfectly confident in the ultimately his contemporaries. (Corsi, 1991, cover) favorable outcome of careful upbringing of a healthy, well-formed baby born of a long line of crooks, murderers There are two lines of evidence against p­ hysiological- and thieves, and prostitutes. Who has any evidence to the or-psychological thinking (the assumption that some contrary? aspects of human psychological functioning are so M02_PINE1933_11_GE_C02.indd 46 22/01/2021 10:38  Evolution, Genetics, and Experience 47 complex that they could not possibly be the product of a physical brain). The first line is composed of the many The Case of the Man Who Fell demonstrations that even the most complex psychologi- Out of Bed cal changes (e.g., changes in self-awareness, memory, or emotion) can be produced by damage to, or stimulation When he awoke, Dr. Sacks’s patient felt fine—that is, until he of, parts of the brain (see Farah & Murphy, 2009). The touched the thing in bed next to him. It was a severed human leg, all hairy and still warm! At first, the patient was confused. second line of evidence is composed of demonstrations Then he figured it out. One of the nurses must have taken it from that some nonhuman species, particularly primate spe- the autopsy department and put it in his bed as a joke. Some cies, possess some abilities (e.g., complex problem solv- joke; it was disgusting. So, he threw the leg out of the bed, but ing) that were once assumed to be purely psychological somehow he landed on the floor with it attached to him. and thus purely human (see Bartal, Decety, & Mason, The patient became agitated and desperate, and Dr. Sacks 2011). The ­following two cases illustrate these two kinds tried to comfort him and help him back into the bed. Making one of evidence. Both cases deal with self-awareness, which last effort to reduce the patient’s confusion, Sacks asked him is widely regarded as one hallmark of the human mind where his left leg was, if the one attached to him wasn’t it. Turning (see Apps & Tsakiris, 2014). pale and looking like he was about to pass out, the patient replied The first case is Oliver Sacks’s (1985) account of “the that he had no idea where his own leg was—it had disappeared. man who fell out of bed.” This patient was suffering from asomatognosia, a deficiency in the awareness of parts of one’s own body. Asomatognosia typically involves the The second case describes G. G. Gallup’s research on self- left side of the body and usually results from damage to awareness in chimpanzees (see Gallup, 1983; Parker, Mitchell, the right frontal and parietal lobes (see Feinberg et al., 2010; & Boccia, 1994). The point of this case is that even nonhumans, Figure 2.1). The point here is that, although the changes in which are assumed by some people to have no mind, are capa- self-awareness displayed by the patient were very complex, ble of considerable psychological complexity—in this case, they were clearly the result of brain damage: Indeed, the full self-awareness. Although their brains are less complex than the range of human experience can be produced by manipula- brains of humans, some species are capable of high levels of tions of the brain. psychological complexity (see Gomez-Marin & Mainen, 2016). The Case of the Chimps with Mirrors* Figure 2.1 Asomatognosia often involves damage to the right frontal and parietal lobes. One way of assessing an organism’s self-awareness is to con- front it with a mirror. Invariably, the first reaction of a chimpanzee Right parietal lobe to a mirror is to respond as if it were seeing another chimpanzee. However, after a day or two, it starts to act as if it were self-aware. It starts to use the mirror to groom itself, inspect parts of its body, and experiment with its reflection by making faces and assuming unusual postures while monitoring the results in the mirror. In an attempt to provide even more convincing evidence of self-awareness, Gallup (1983) devised a clever test. Each chim- panzee was anesthetized, and its eyebrow was painted with a red, odorless, dye. Following recovery from anesthesia, the mirror was Right frontal lobe reintroduced. Upon seeing its painted eyebrow in the mirror, each chimpanzee repeatedly touched the marked area on its eyebrow while watching the image (see Figure 2.2.) Moreover, there was over a threefold increase in the time that the chimps spent looking in the mirror, and several kept touching their eyebrows and smell- ing their fingers. We suspect that you would respond pretty much the same way if you saw y­ ourself in the mirror with a red spot on your face. (continued ) * “Toward a Comparative Psychology of Mind” by G. G. Gallup, Jr., American Journal of Primatology 2:237–248, 1983. Copyright © 1983 John Wiley & Sons, Inc. M02_PINE1933_11_GE_C02.indd 47 22/01/2021 10:38  48 Chapter 2 result-of-experience version is fundamentally flawed. The Figure 2.2 The reactions of chimpanzees to their own images suggest that they are self-aware. In this photo, problem is that it is based on the premise that genetic factors the chimpanzee is reacting to the bright red, ­odorless and experiential factors combine in an additive fashion—that dye that was painted on its eyebrow ridge while it was a behavioral capacity, such as intelligence, is created by com- anesthetized. bining some amount of genetics with some amount of expe- rience rather than through the interaction of genetics and experience. Once you learn more about how genetic factors and experience interact, you will better appreciate the folly of this assumption. For the time being, however, let us illustrate its weakness with a metaphor embedded in an anecdote. The Case of the Thinking Student One of my students told me (JP) she had read that intelligence was one-third genetic and two-thirds experience, and she won- dered whether this was true. I responded by asking her the following question: “If I wanted to get a better understanding of music, would it be reasonable for me to begin by asking how much of it came from the musician and how much of it came from the instrument?” “That would be dumb,” she said. “The music comes from The Povinelli Group LLC both; it makes no sense to ask how much comes from the musi- cian and how much comes from the instrument. Somehow the music results from the interaction of the two together. You would have to ask about the interaction.” “That’s exactly right,” I said. “Now, do you see why... ” Since Gallup’s demonstration, many other species have “Don’t say any more,” she interrupted. “I see what you’re passed what is now known as the mirror self-­recognition test. getting at. Intelligence is the product of the interaction of genes These include Asian elephants, orangutans, and E ­ uropean and experience, and it is dumb to try to find how much comes magpies, to name a few. We humans pass the mirror self- from genes and how much comes from experience.” recognition test only once we have reached 15 to 24 months “Yes!” I thought. of age. NATURE-OR-NURTURE THINKING RUNS INTO The point of this metaphor, in case you have forgotten, ­DIFFICULTY. The history of nature-or-nurture thinking is to illustrate why it is inappropriate to try to understand can be summed up by paraphrasing Mark Twain: “Reports interactions between two factors by asking how much each of its death have been greatly exaggerated.” Each time it factor contributes. We would not ask how much a musician has been discredited, it has resurfaced in a slightly modified and how much her instrument contributes to producing form. First, factors other than genetics and learning were music; we would not ask how much the water and how shown to influence behavioral development; factors such as much the temperature contributes to evaporation; and we the fetal environment, nutrition, stress, and sensory stimu- would not ask how much a male and how much a female lation all proved to be influential. This led to a broadening contributes to reproduction. Similarly, we shouldn’t ask of the concept of nurture to include a variety of experien- how much genetic and how much experiential factors con- tial factors in addition to learning. In effect, it changed the tribute to behavioral development. The answers to all these nature-or-nurture dichotomy from “genetic factors or learn- questions lie in understanding the nature of the interactions ing” to “genetic factors or experience.” (see Sung et al., 2014; Uher, 2014). The importance of think- Next, it was argued convincingly that behavior always ing about development in terms of interactions will become develops under the combined control of both nature and even more apparent later in this chapter. nurture (see Johnston, 1987; Rutter, 1997), not under the con- trol of one or the other. Faced with this point, many people A MODEL OF THE BIOLOGY OF BEHAVIOR. So far in merely substituted one kind of nature-or-nurture thinking this module, you have learned why people tend to think for another. They stopped asking, “Is it genetic, or is it the about the biology of behavior in terms of dichotomies, result of experience?” and started asking, “How much of it and you have learned some of the reasons why this way of is genetic, and how much of it is the result of experience?” thinking is inappropriate. Now, let’s look at a way of think- Like earlier versions of the nature-or-nurture question, ing about the biology of behavior that has been adopted by the how-much-of-it-is-genetic-and-how-much-of-it-is-the- most biopsychologists. It is illustrated in Figure 2.3. Like M02_PINE1933_11_GE_C02.indd 48 22/01/2021 10:38  Evolution, Genetics, and Experience 49 Figure 2.3 A schematic illustration of the way in which most biopsychologists think about the biology of behavior. EVOLUTION 1 Evolution influences the pool of behavior- influencing genes available to the members of each species. GENES 2 Each individual’s genes initiate a unique program of neural development. EXPERIENCE 3 Experience modifies the expression of an individual’s genes. 4 The development of each individual’s nervous system depends on its interactions with its environment (i.e., on its experience). 5 Each individual’s current behavioral capacities and tendencies are determined by CURRENT ORGANISM CURRENT SITUATION its unique patterns of neural activity, some of which are experienced as thoughts, feelings, memories, etc. 6 Each individual’s current behavior arises out of interactions among its ongoing patterns of CURRENT neural activity and their perception of the BEHAVIOR current situation. 7 The success of each individual’s behavior influences the likelihood that their genes will be passed on to future generations. other powerful ideas, it is simple and logical. This model boils down to the single premise that all behavior is the product of interactions among three factors: (1) the organ- Human Evolution ism’s genetic endowment, which is a product of its evolu- In this module, you will explore how brain and behavior tion; (2) its experience; and (3) its perception of the current have been shaped by evolutionary processes. As an entry situation. Please examine the model carefully and consider point to the topic, and to provide some background, you its implications. will first learn about the history of the study of evolution. The module then builds upon that foundation by providing you with an overview of several key aspects of the role of Journal Prompt 2.1 evolution in brain and behavior. Moreover, you will learn Imagine you are a biopsychology instructor. One of your about some of the most commonly misunderstood aspects students asks you whether depression is physiological about evolution. or psychological. What would you say? Darwin’s Theory of Evolution The next three modules of this chapter deal with three LO 2.3 Describe the origins of evolutionary theory. elements of this model of behavior: evolution, genetics, and the interaction of genetics and experience in behavioral Modern biology began in 1859 with the publication of development. The final module of the chapter deals with Charles Darwin’s On the Origin of Species. In this monumen- the genetics of human psychological differences. tal work, Darwin described his theory of evolution—the M02_PINE1933_11_GE_C02.indd 49 22/01/2021 10:38  50 Chapter 2 single most influential theory in the biologi- Figure 2.4 Four kinds of evidence supporting the theory that cal sciences. Darwin was not the first to sug- species evolve. gest that species evolve (undergo systematic change) from preexisting species, but he was the first to amass a large body of supporting Fossil records change evidence and the first to suggest how evolu- systematically through tion occurs (see Bowler, 2009). geological layers. Darwin presented three kinds of evi- Illustrated here is the evolution of the dence to support his assertion that species hominin skull. evolve: (1) He documented the evolution of fossil records through progressively more recent geological layers. (2) He described striking structural similarities among living species (e.g., a human’s hand, a bird’s wing, and a cat’s paw), There are striking structural which suggested that they had evolved similarities among diverse from common ancestors. (3) He pointed to living species (e.g., between a human arm and a bat’s the major changes that had been brought wing). about in domestic plants and animals by programs of selective breeding. However, the most convincing evidence of evolu- tion comes from direct observations of rapid evolution in progress (see Barrick & Lenski, 2013). For example, Grant (1991) observed evolution of the finches of the Major changes have been created in Galápagos Islands—a population stud- domestic plants and ied by Darwin himself (see Lamichhaney animals by programs et al., 2015)—after only a single season of of selective breeding. drought. Figure 2.4 illustrates these four kinds of evidence. Darwin argued that evolution occurs through natural selection (see Pritchard, Evolution has been observed 2010). He pointed out that the members in progress. For example, an of each species vary greatly in their struc- 18-month drought on one of ture, physiology, and behavior and that the Galápagos Islands left only large, difficult-to-eat the heritable traits associated with high seeds, which increased the rates of survival and reproduction are chance that a bird with a the most likely ones to be passed on to long beak would survive and reproduce. future generations (see Kingsley, 2009). He argued that natural selection, when repeated for generation after generation, leads to the evolution of species that are better adapted so it initially met with resistance. Although resistance still to surviving and reproducing in their particular envi- exists, virtually none comes from people who understand ronmental niche. Darwin called this process natural selec- the evidence (see Short & Hawley, 2015). tion to emphasize its similarity to the artificial selective breeding practices employed by breeders of domestic Evolution is both a beautiful concept and an important one, more crucial nowadays to human welfare, to medical animals. Just as horse breeders create faster horses by science, and to our understanding of the world than ever selectively breeding the fastest of their existing stock, before [see Mindell, 2009]. It’s also deeply persuasive—a nature creates fitter animals by “selectively” breeding theory you can take to the bank... the supporting evidence the fittest. Fitness, in the Darwinian sense, is the ability is abundant, various, ever increasing, and easily available of an organism to survive and contribute its genes to the in museums, popular books, textbooks, and a mountain- next generation. ous accumulation of scientific studies. No one needs to, Darwin’s theory of evolution was at odds with the vari- and no one should, accept evolution merely as a matter of ous dogmatic views embedded in the 19th-century zeitgeist, faith. (Quammen, 2004, p. 8) M02_PINE1933_11_GE_C02.indd 50 22/01/2021 10:38  Evolution, Genetics, and Experience 51 Evolution and Behavior attributed these advantages to the fact that high-ranking female chimpanzees are more likely to maintain access to LO 2.4 Explain the evolutionary significance of social productive foraging areas (see Pusey & Schroepfer-Walker, dominance and courtship displays. 2013). Some behaviors play an obvious role in evolution. For example, the ability to find food, avoid predation, or defend COURTSHIP DISPLAY. An intricate series of courtship one’s young obviously increases an animal’s ability to pass displays precedes copulation in many species. The male on its genes to future generations. Other behaviors play a approaches the female and signals his interest. His sig- role that is less obvious but no less important—for example, nal (which may be olfactory, visual, auditory, or tactual) social dominance and courtship displays, which are dis- may elicit a signal in the female, which may elicit another cussed here. response in the male, and so on, until copulation ensues. But copulation is unlikely to occur if one of the pair fails to react SOCIAL DOMINANCE. The males of many species estab- appropriately to the signals of the other. lish a stable hierarchy of social dominance through combative Courtship displays are thought to promote the evolu- encounters with other males (see Qu et al., 2017). In some tion of new species. Let us explain. A species is a group of species, these encounters often involve physical damage; in organisms reproductively isolated from other organisms; others, they involve mainly posturing and threatening until that is, the members of a species can produce fertile off- one of the two combatants backs down. The dominant male spring only by mating with members of the same species usually wins encounters with all other males of the group; (see de Knijff, 2014). A new species begins to branch off the number two male usually wins encounters with all from an existing species when some barrier discourages males except the dominant male; and so on down the line. breeding between a subpopulation of the existing species Once a hierarchy is established, hostilities diminish because and the remainder of the species. Once such a reproductive the lower-ranking males learn to avoid or quickly submit to barrier forms, the subpopulation evolves independently the more dominant males. Because most of the fighting goes of the remainder of the species until cross-fertilization on between males competing for positions high in the social becomes impossible (see Arnegard et al., 2014; Roesti hierarchy, low-ranking males fight little, and the lower lev- & Salzburger, 2014). els of the hierarchy tend to be only vaguely recognizable. The reproductive barrier may be geographic; for exam- Why is social dominance an important factor in evolu- ple, a few birds may fly together to an isolated island, where tion? One reason is that in many species, dominant males many generations of their offspring breed among them- copulate more than nondominant males and thus are more selves and evolve into a separate species. ­Alternatively—to effective in passing on their characteristics to future genera- get back to the main point—the reproductive barrier may tions. McCann (1981) studied the effect of social dominance on be behavioral. A few members of a species may develop the rate of copulation in 10 bull elephant seals that cohabited the same breeding beach. These massive animals challenge Figure 2.5 Two massive bull elephant seals challenge one another. Dominant bull elephant seals copulate more frequently than those lower in the dominance hierarchy. each other by raising themselves to full height and pushing chest to chest. Usu- 40 Percentage of Total Observed Copulations ally, the smaller of the two backs down; if it does not, a vicious neck-biting battle ensues. McCann found that the domi- nant male accounted for about 37 per- 30 cent of the copulations during the study, whereas poor number 10 accounted for only about 1 percent (see Figure 2.5). 20 Another reason why social domi- nance is an important factor in evolu- tion is that in some species, dominant 10 females are more likely to produce more and healthier offspring. For example, Pusey, Williams, and Goodall (1997) 0 found that high-ranking female chim- 1 2 3 4 5 6 7 8 9 10 Others panzees produced more offspring and Rank of Bull Elephant Seal that these offspring were more likely to survive to sexual maturity. They Francois Gohier/Science Source M02_PINE1933_11_GE_C02.indd 51 22/01/2021 10:38  52 Chapter 2 different courtship displays, and these may form a repro- shell-covered eggs and to be covered by dry scales. Both of ductive barrier between themselves and the rest of their these adaptations reduced the reliance of reptiles on watery ­conspecifics (members of the same species): Only the suit- habitats. A r­ eptile does not have to spend the first stage of able exchange of displays between a courting couple will its life in the watery environment of a pond or lake; instead, lead to reproduction. it spends the first stage of its life in the watery environment of a shell-covered egg. And once hatched, a reptile can live far from water because its dry scales greatly reduce water Course of Human Evolution loss through its water-permeable skin. LO 2.5 Summarize the pathway of evolution from EVOLUTION OF MAMMALS. About 225 million years single-cell organisms to humans. ago, during the height of the age of dinosaurs, a new class By studying fossil records and comparing current species, of vertebrates evolved from one line of small reptiles. The we humans have looked back in time and pieced together the females of this new class fed their young with secretions evolutionary history of our species—although some of the from special glands called mammary glands, and the mem- details are still controversial. The course of human evolution, bers of the class are called mammals after these glands. as it is currently understood, is summarized in this section. Eventually, mammals stopped laying eggs; instead, the females nurtured their young in the watery environment EVOLUTION OF VERTEBRATES. Complex multicellular of their bodies until the young were mature enough to be water-dwelling organisms first appeared on earth about 800 born. The duck-billed platypus is one surviving mamma- million years ago. About 250 million years later, the first lian species that lays eggs. chordates evolved (Satoh, 2016). Chordates (pronounced Spending the first stage of life inside one’s mother “KOR-dates”) are animals with dorsal nerve cords (large proved to have considerable survival value; it provided the nerves that run along the center of the back, or dorsum); they long-term security and environmental stability necessary are 1 of the 40 or so large categories, or phyla (pronounced for complex programs of development to unfold. Today, “FY-la”), into which zoologists group animal species most classification systems recognize about 26 different (Zhang, 2013). The first chordates with spinal bones to pro- orders of mammals. The order to which we belong is the tect their dorsal nerve cords evolved about 25 million years order primates. We humans—in our usual humble way— later. The spinal bones are called vertebrae (pronounced named our order using the Latin term primus, which means “VERT-eh-bray”), and the chordates that possess them are “first” or “foremost.” called vertebrates. The first vertebrates were primitive bony Primates have proven particularly difficult to catego- fishes (Shu et al., 1999). Today, there are seven classes of rize because there is no single characteristic possessed by vertebrates: three classes of fishes, plus amphibians, reptiles, all primates but no other animals. Still, most experts agree birds, and mammals. EVOLUTION OF AMPHIBIANS. About Figure 2.6 A recently discovered fossil of a missing evolutionary link 410 million years ago, the first bony fishes started is shown on the right, and a reconstruction of the creature is shown on to venture out of the water (see Figure 2.6). the left. It had scales, teeth, and gills like a fish and primitive wrist and Fishes that could survive on land for brief finger bones similar to those of land animals. periods of time had two great advantages: They could escape from stagnant pools to nearby fresh water, and they could take advantage of terres- trial food sources. The advantages of life on land were so great that, through the process of natural selection, the fins and gills of bony fishes trans- formed into legs and lungs, respectively, and so it was that the first amphibians evolved about 370 million years ago. Amphibians (e.g., frogs, toads, and salamanders) in their larval form must live in the water; only adult amphibians can survive on land. EVOLUTION OF REPTILES. About 315 ­million years ago, reptiles (e.g., lizards, snakes, and turtles) evolved from a branch of amphib- ians. Reptiles were the first vertebrates to lay Beth Rooney Photography M02_PINE1933_11_GE_C02.indd 52 22/01/2021 10:38  Evolution, Genetics, and Experience 53 there are about 16 groups of primates. Species from five of Figure 2.9). Hominins include six sub-groups including them appear in Figure 2.7. Australopithecus and Homo. Based on the fossil record, Apes (gibbons, orangutans, gorillas, and chimpanzees) Homo is thought to be composed of at least eight species are thought to have evolved from a line of Old World mon- (see Wiedemann, 2014; Gibbons, 2015a); seven of which are keys. Like Old World monkeys, apes have long arms and now extinct. Perhaps you have heard of the Neanderthals grasping hind feet that are specialized for arboreal (treetop) (Homo Neanderthalensis)? They are one of those extinct Homo travel, and they have opposable thumbs that are not long species. And we humans (Homo Sapiens) are the only one enough to be of much use for precise manipulation (see still kicking around. Figure 2.8). Unlike Old World monkeys, though, apes have It is difficult to reconstruct the events of human evolu- no tails and can walk upright for short distances. Chim- tion because the evidence is so sparse. Only a few partial panzees are the closest living relatives of humans; almost hominin fossils dating from the critical period have been 99 percent of genes are identical in the two species (see discovered. However, three important hominin fossil dis- Rogers & Gibbs, 2014; but see Cohen, 2007); however, the coveries have been particularly enlightening (see Harmon, actual ape ancestor of humans is likely long extinct (Jaeger 2013): & Marivaux, 2005). An uncommonly complete fossil of a 3-year-old early EMERGENCE OF HUMANKIND. Primates of the same Australopithecus girl in Ethiopia (see Figure 2.10; group that includes humans are known as hominins (see ­Gibbons, 2009; Suwa et al., 2009; White et al., 2009). F  ossils indicating that a popula- tion of tiny hominins inhabited Figure 2.7 Species from five different groups of primates. the Indonesian island of Flores APE as recently as 18,000 years ago Silver-Backed Lowland (see Callaway, 2014; Stringer, Gorilla 2014). Several early Australopithecine fossils with combinations of human and nonhuman charac- teristics in a pit in South Africa (Pickering et al., 2011; Wong, 2012). Many experts believe that the Australopithecines evolved about 4 million years ago in Africa (see ­Krubitzer & Stolzenberg, 2014; Skinner et al., 2015; Wood, 2010) from a line of apes (australo means “southern,” and pithecus means “ape”). Several species of ­A ustralopithecus are thought to have roamed the African plains for about 2 million years before becom- ing extinct. ­A ustralopithecines were only about 1.3 meters (4 feet) TARSIER tall, and they had small brains, Tarsier OLD WORLD Monkey but analysis of their pelvis and leg MONKEY bones indicates that their posture Hussar Monkey was upright. Any doubts about their upright posture were erased by the discovery of the fossilized CEBID HOMININ footprints pictured in F ­ igure 2.11 Squirrel Monkey Human (see Raichlen et al., 2010). Clockwise from right corner: Kevin Schafer/Photolibrary/Getty Images (APE); Vladimir Sazonov/Shutterstock The first Homo species are (HOMININ); Daniel Frauchiger, Switzerland/Moment/Getty Images (TARSIER); Michael Krabs/Alamy Stock Photo (CEBID); Anatoliy Lukich/Shutterstock (OLD WORLD MONKEY) thought to have evolved from one M02_PINE1933_11_GE_C02.indd 53 22/01/2021 10:38  54 Chapter 2 Figure 2.8 A comparison of the feet and hands of a Figure 2.10 The remarkably complete skull of a 3-year- human and a chimpanzee. old Australopithecus girl; the fossil is 3.3 million years old. Lealisa Westerhoff/AFP/Getty Images species of Australopithecus about 2 to 2.8 million years ago (see Antón, Potts, & Aiello, 2014; Dimaggio et al., 2015; Schroeder et al., 2014; Villmoare et al., 2015; but see ­Wiedemann, 2014; Wood, 2014). One distinctive feature of the early Homo species was the size of their brain cav- ity, larger than that of Australopithecus but smaller than that of modern humans. The early Homo species used fire and tools (see Orban & Caruana, 2014; Schwartz & ­Tattersall, 2015) and coexisted in Africa with various spe- cies of ­Australopithecus for about a half-million years, until Human Chimpanzee the australopithecines died out. Early Homo species also lived outside of Africa for about 1.85 million years (see Lordkipanidze et al., 2013; Wood, 2011). Then, about 275,000 Figure 2.9 A taxonomy of the human species. years ago (see Adler et al., 2014), early Homo species were gradually replaced in the fossil record by modern humans (Homo sapiens). KINGDOM Animal Paradoxically, although the big three human ­attributes—large brain, upright posture, and free hands PHYLUM Chordate with an opposable thumb—have been evident for hundreds of thousands of years, most human accomplishments are of recent origin. Artistic products (e.g., wall paintings and CLASS Mammal carvings) did not appear until about 40,000 years ago (see Krubitzer & Stolzenberg, 2014; Pringle, 2013), ranching and farming were not established until about 10,000 years ago ORDER Primate (see Larson et al., 2014), and writing was not used until about 7,500 years ago. FAMILY Hominin Thinking about Human Evolution GENUS Homo LO 2.6 Describe nine commonly misunderstood points about evolution. SPECIES Sapiens Figure 2.12 illustrates the main branches of vertebrate evolution. As you examine it, consider the following com- Ryan McVay/Getty Images monly misunderstood points about evolution. They should M02_PINE1933_11_GE_C02.indd 54 22/01/2021 10:38  Evolution, Genetics, and Experience 55 in the environment or by adaptive genetic muta- Figure 2.11 Fossilized footprints of Australopithecine hom- inins who strode across African volcanic ash about 3.6 million tions. Whether human evolution occurred gradu- years ago, leaving a 70-meter trail. There were two adults and ally or suddenly is still a matter of intense debate a child; the child often walked in the footsteps of the adults. among paleontologists (those who scientifically study fossils). Few products of evolution have survived to the present day—only the tips of the branches of the evolutionary bush have survived. Fewer than 1 percent of all known species are still in existence. Evolution does not progress to preordained ­perfection—evolution is a tinkerer, not an architect. Increases in adaptation occur through changes to exist- ing programs of development; and, although the results are improvements in their particular environmental context, they are never perfect designs. For example, the fact that mammalian sperm do not develop effectively at body temperature led to the evolution of the scrotum— hardly a perfect solution to any design problem. Not all existing behaviors or structures are adap- tive. Evolution often occurs through changes in developmental programs that lead to several related ­characteristics, only one of which might be a­ daptive— the incidental nonadaptive evolutionary by-products are called spandrels. One example of a spandrel is the human belly button—it is a nonfunctional by-product of the umbilical cord. Also, behaviors or structures that were once adaptive might become nonadaptive, or even maladaptive, if the environment changes. Journal Prompt 2.2 What might be an example of a behavior or structure that is currently adaptive but that might become non- adaptive, or even maladaptive, if our current environ- ment were to change? John Reader/Science Source Not all existing adaptive characteristics evolved to per- form their current function. Some characteristics, called exaptations, evolved to serve one function and were provide you with a new perspective from which to consider later co-opted to serve another. For example, bird wings your own origins. are exaptations—they are limbs that initially evolved for the purpose of walking. Evolution does not proceed in a single line. Although it Similarities among species do not necessarily mean is common to think of an evolutionary ladder or scale, a that the species have common evolutionary origins. far better metaphor for evolution is a dense bush. Structures that are similar because they have a com- We humans have little reason to claim evolutionary mon evolutionary origin are termed homologous; supremacy. We are the last surviving species of a group structures that are similar but do not have a com- (i.e., hominins) that has existed for only a blip of evolu- mon evolutionary origin are termed analogous. The tionary time. similarities between analogous structures result from Evolution does not always proceed slowly and convergent evolution, the evolution in unrelated gradually. Rapid evolutionary changes (i.e., in a few species of similar solutions to the same environmen- generations) can be triggered by sudden changes tal demands (see Stern, 2013). Deciding whether a M02_PINE1933_11_GE_C02.indd 55 22/01/2021 10:38  56 Chapter 2 Figure 2.12 Hominin evolution. Fishes without Amphibians Reptiles backbones Million Years Ago Mammals Bony fishes 6 HOMININS EMERGE 225 MAMMALS EMERGE 315 REPTILES EMERGE 410 AMPHIBIANS EMERGE 525 VERTEBRATES EMERGE 550 CHORDATES EMERGE FIRST COMPLEX MULTI- 800 CELLULAR ORGANISMS structural similarity is analogous or homologous Evolution of the Human Brain requires careful analysis of the similarity. For exam- ple, a bird’s wing and a human’s arm have a basic LO 2.7 Describe how research on the evolution of the underlying commonality of skeletal structure that human brain has changed over time. suggests a common ancestor; in contrast, a bird’s Early research on the evolution of the human brain focused wing and a bee’s wing have few structural similari- on size. This research was stimulated by the assumption ties, but they both evolved because of the common that brain size and intellectual capacity are closely related— advantage of flight. an assumption that quickly ran into two problems. First, There is now considerable evidence that Homo sapiens it was shown that modern humans, whom we humans mated with other Homo species (e.g., ­Neanderthals) believe to be the most intelligent of all creatures, do not they encountered (see Dannemann & Racimo, 2018; have the biggest brains. With brains weighing about 1,350 Gibbons, 2014; Wong, 2015). The discovery of this grams, humans rank far behind whales and elephants, pattern of mating changes the way we should whose brains weigh between 5,000 and 8,000 grams (Man- view our origins: We are not the product of a single ger, 2013; Patzke et al., 2014). Second, the sizes of the brains ancestral Homo population; rather, we are the of acclaimed intellectuals (e.g., Albert E­ instein) were found combined offspring of many Homo populations that to be unremarkable, certainly no match for their gigan- once coexisted and interacted. tic intellects. It is now clear that, although healthy adult M02_PINE1933_11_GE_C02.indd 56 22/01/2021 10:38  Evolution, Genetics, and Experience 57 human brains vary greatly in size—between about 1,000 Figure 2.13 The brains of animals of different and 2,000 grams—there is no clear relationship between ­ volutionary ages—cerebrums are shown in pink; brain e overall human brain size and intelligence. stems are shown in orange. One obvious problem in relating brain size to intel- ligence is the fact that larger animals tend to have larger brains, presumably because larger bodies require more brain tissue to control and regulate them. Thus, the facts that large men tend to have larger brains than small men, that men tend to have larger brains than women, and that elephants have larger brains than humans do not suggest anything about the relative intelligence of these popula- tions. This problem led to the proposal that brain weight expressed as a percentage of total body weight might be a better measure of intellectual capacity. This mea- sure allows humans (2.33 percent) to take their right- ful place ahead of elephants (0.20 percent), but it also allows both humans and elephants to be surpassed by that intellectual giant of the animal kingdom, the shrew (3.33 percent). Human A more reasonable approach to the study of brain evo- lution has been to compare the evolution of different brain regions. For example, it has been informative to consider the evolution of the brain stem separately from the evolu- tion of the cerebrum (cerebral hemispheres). In general, the brain stem regulates reflex activities that are critical for sur- vival (e.g., heart rate, respiration, and blood glucose level), whereas the cerebrum is involved in more complex adap- tive processes such as learning, perception, and motivation. Figure 2.13 is a schematic representation of the rela- tive size of the brain stems and cerebrums of several species that are living descendants of species from which humans Chimpanzee evolved. This figure makes three important points about the evolution of the human brain: The brain has increased in size during evolution. Most of the increase in size has occurred in the cerebrum. An increase in the number of convolutions—folds on the cerebral surface—has greatly increased the surface Cat area of the cerebral cortex, the outermost layer of cerebral tissue (see Geschwind & Rakic, 2013; Zilles, Palermo- Gallagher, & Amunts, 2013). Although the brains of related species differ, there are fundamental similarities: All brains are constructed of many Rat neurons, and the neural structures in the brains of one spe- cies can usually be found in the same locations in the brains of related species (see Goulas et al., 2014). For example, the brains of humans, monkeys, rats, and mice contain the same major structures connected in similar ways, and similar structures tend to perform similar functions (see Cole et al., 2009). The human brain appears to have evolved from the brains of our closest primate relatives (see Hofman, 2014; Bass Matsuzawa, 2013). M02_PINE1933_11_GE_C02.indd 57 22/01/2021 10:38  58 Chapter 2 Scan Your Brain This is a good place to pause and scan your brain to check 6. One distinctive feature of early Homo species was that they your knowledge. Do you remember what you have learned had brains _______ than Australopithecus but _______ than about evolution so far? Fill in the following blanks with the modern humans. most appropriate terms from the first two modules. The correct 7. Incidental nonadaptive evolutionary by-products such answers are provided at the end of the exercise. Before pro- as the belly button are called _______. ceeding, review material related to your errors and omissions. 8. During the course of the vertebrate evolution, birds emerged approximately _______ years ago. 1. There has been a long-standing debate on whether 9. The overall human brain size does not predict _______. humans and other animals inherit their behavioral responses or acquire them through learning. This is 10. Over millions of years, there has been a remarkable called the _______ debate. increase in the surface area of the _______, the outmost layer of the cerebral tissue in humans. 2. The condition that can result from damage to the right parietal lobe and typically involves a lack of awareness of 11. Evolutionary psychologists suggest that male–female one’s own body parts (most commonly on the left side) _______ during copulation ensures that the offspring will is known as _______. survive, reproduce, and pass on their genes to the next generation. 3. Darwin proposed that the striking similarities among living species were evidence that they shared a common _______. 12. _______ structures are similar because of convergent evolution. 4. Through selective _______ programs, major changes have been made to domestic animals and plants. (11) bonding, (12) Analogous. 5. In some species, the _______ male is likely to copulate (7) spandrels, (8) 160 million, (9) intelligence, (10) cerebral cortex, more and, therefore, pass on his genes to the future (3) ancestor, (4) breeding, (5) dominant, (6) larger, smaller, generations. Scan Your Brain answers: (1) nature–nurture, (2) asomatognosia, interbred members always produce offspring with the same Fundamental Genetics trait (e.g., brown seeds), generation after generation. In one of his early experiments, Mendel studied the Darwin did not understand two of the key facts on which inheritance of seed color: brown or white. He began by his theory of evolution was based. He did not understand crossbreeding the offspring of a line of pea plants that had why conspecifics differ from one another, and he did not bred true for brown seeds with the offspring of a line of pea understand how anatomical, physiological, and behavioral plants that had bred true for white seeds. The offspring of characteristics are passed from parent to offspring. While this cross all had brown seeds. Then, Mendel bred these Darwin puzzled over these questions, an unread manu- first-generation offspring with one another, and he found script in his files contained the answers. It had been sent that about three-quarters of the resulting second-generation to him by an unknown Augustinian monk, Gregor M ­ endel. offspring had brown seeds and about one-quarter had white ­Unfortunately for Darwin (1809–1882) and for Mendel seeds. Mendel repeated this experiment many times with (1822–1884), the significance of Mendel’s research was not various pairs of dichotomous pea plant traits, and each time recognized until the early part of the 20th century, well after the result was the same: One trait, which Mendel called both of their deaths. the dominant trait, appeared in all of the first-generation offspring; the other trait, which he called the recessive trait, Mendelian Genetics appeared in about one-quarter of the second-generation off- spring. Mendel would have obtained a similar result if he LO 2.8 Explain how Mendel’s work with pea plants had conducted an experiment with true-breeding lines of has informed us about the mechanisms of brown-eyed (dominant) and blue-eyed (recessive) humans. inheritance. The results of Mendel’s experiment challenged the cen- Mendel studied inheritance in pea plants. In designing tral premise on which all previous ideas about inheritance his experiments, he made two wise decisions. He decided had rested: that offspring inherit the traits of their parents. to study dichotomous traits, and he decided to begin his Somehow, the recessive trait (white seeds) was passed on experiments by crossing the offspring of true-breeding lines. to one-quarter of the second-generation pea plants by first- Dichotomous traits occur in one form or the other, never in generation pea plants that did not themselves p ­ ossess it. An combination. For example, seed color is a dichotomous pea organism’s observable traits are referred to as its p ­ henotype; plant trait: Every pea plant has either brown seeds or white the traits that it can pass on to its offspring through its seeds. True-breeding lines are breeding lines in which genetic material are referred to as its genotype. M02_PINE1933_11_GE_C02.indd 58 22/01/2021 10:38  Evolution, Genetics, and Experience 59 Mendel devised a theory to explain his results. It com- accounts for the result of his experiment on the inheritance prised four central ideas. First, Mendel proposed that there of seed color in pea plants. are two kinds of inherited factors for each dichotomous trait—for example, that a brown-seed factor and a white- seed factor control seed color. Today, we call each inherited Chromosomes factor a gene. Second, Mendel proposed that each organism LO 2.9 Understand the structure and function of possesses two genes for each of its dichotomous traits; for chromosomes. example, each pea plant possesses either two brown-seed In this section, you will be presented with current knowl- genes, two white-seed genes, or one of each. The two genes edge related to two key aspects of chromosomal function: that control the same trait are called alleles (pronounced recombination and replication. The section ends with a dis- “a-LEELZ”). Organisms that possess two identical alleles cussion of the sex chromosomes and sex-linked traits. (e.g., two white-seed alleles) are said to be homozygous for that trait; those that possess different alleles (e.g., one white- REPRODUCTION AND RECOMBINATION. It was not seed allele and one black-seed allele) for a trait are said to until the early 20th century that genes were found to be be heterozygous for that trait. located on chromosomes—the threadlike structures in Third, Mendel proposed that one of the two kinds of the nucleus of each cell (see Brenner, 2012). Chromosomes genes for each dichotomous trait dominates the other in occur in matched pairs in virtually all multicellular organ- heterozygous organisms. For example, pea plants with a isms (see Sagi & Benvenisty, 2017), and each species has a brown-seed gene and a white-seed gene always have brown characteristic number of pairs in each of its body cells (but seeds because the brown-seed gene always dominates the see Sagi & Benvenisty, 2017); humans have 23 pairs. The white-seed gene. And fourth, Mendel proposed that for two genes (alleles) that control each trait are situated at the each dichotomous trait, each organism randomly inherits same location, one on each chromosome of a particular pair. one of its “father’s” two factors and one of its “mother’s” The process of cell division that produces gametes two factors. Figure 2.14 illustrates how Mendel’s theory (egg cells and sperm cells) is called meiosis (pronounced Figure 2.14 How Mendel’s theory accounts for the results of his experiment on the inheritance of seed color in pea plants. True-breeding True-breeding Any offspring Any offspring brown-seed white-seed of first cross of first cross pea plants pea plants crossed crossed with with B1 B2 w1 w2 B1 w1 B2 w2 B1 w1 B1 w2 B2 w1 B2 w2 B1 B2 B1 w2 B2 w1 w1 w2 FIRST CROSS SECOND CROSS One parent had two dominant brown-seed genes (B1 Each parent had one brown-seed gene and one white- B2); the other had two recessive white-seed genes (w 1 seed gene. Therefore, 25% of the offspring had two w 2 ). Therefore, all offspring had one brown-seed gene brown-seed genes (B1 B2), 50% had a brown-seed gene and one white-seed gene (B1 w1, B1 w2, B2 w1, or B2 w2). and a white-seed gene (B1 w2 or B2 w1), and 25% had two Because the brown-seed gene is dominant, all had brown white-seed genes (w1 w2). Because the brown-seed seeds. gene is dominant, 75% had brown seeds. M02_PINE1933_11_GE_C02.indd 59 22/01/2021 10:38  60 Chapter 2 “my-OH-sis”). In meiosis, the chromosomes divide, and one This specific bonding pattern has an important consequence: chromosome of each pair goes to each of the two gametes The two strands that compose each chromosome are exact that result from the cell division. As a result, each gamete complements of each other. For example, a sequence of ade- has only half the usual number of chromosomes (23 in nine, guanine, thymine, cytosine, and guanine on one strand humans); and when a sperm cell and an egg cell combine is always attached to a complementary sequence of thy- during fertilization (see Figure 2.15), a zygote (a fertilized mine, cytosine, adenine, guanine, and cytosine on the other. egg cell) with the full complement of chromosomes (23 pairs Figure 2.16 illustrates the structure of DNA. in humans) is produced. Replication is a critical process of the DNA molecule. The random division of the pairs of chromosomes into Without it, mitotic cell division would not be possible. two gametes is not the only way meiosis contributes to Figure 2.17 illustrates how DNA replication is thought to genetic diversity. Let us explain. During the first stage of work. The two strands of DNA start to unwind. Then, the meiosis, the chro