Figure 9.1. Tree of life, depicting the evolutionary relationships among all living organisms. Adapted from “A Simplified Family Tree of Life” in Hotton (1968).
Many evolutionary psychologists believe that the modern human mind evolved during the Pleistocene, a period that ran from about two and a half million years ago until twelve thousand years before the present time. At this time, our hominid ancestors had already split from the ancestors of chimpanzees, and many of them had already gone extinct. The genus Homo made its first appearance during the Pleistocene, and all the fossils that come from this era belong to Homo erectus and Homo sapiens, which acquired the anatomy of modern humans approximately fifty thousand years ago. Thus, many evolutionary psychologists believe that the modern human mind evolved during the period in which humans acquired their current anatomy, in what they call our “environment of evolutionary adaptedness.” Because of this belief, many evolutionary psychologists do not find it useful to try to reconstruct the phylogenetic history of the human mind and behavior by studying other primates, or other animals in general. When Homo sapiens developed its big new brain, their argument goes, all bets were off and humans started playing new games with each other, with entirely new behavioral rules. For instance, Tooby and Cosmides, considered by many the founders of modern evolutionary psychology, have stated many times that studying the phylogenetic history of human behavior by comparing it to the behavior of other animals is not useful, or even possible.13 But others disagree.
I will use an analogy between body anatomy and behavior to explain why I think this position is wrong. The body of Homo sapiens might have modified its shape and acquired its current anatomical features during the Pleistocene, but we can nevertheless clearly see two things. First, there are still very strong similarities between the anatomy of modern Homo sapiens and that of great apes and other primates; second, from modern human bodies we can still discern elements of a very ancient evolutionary history of human anatomy, which—as nicely illustrated by paleontologist Neil Shubin in his Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body—goes all the way back to primitive fish.14 The same applies to brains and behavior. The mind of Homo sapiens might have been modified by natural selection in some important ways during the Pleistocene and acquired all the characteristics of the modern human mind. As we’ll see later, it is likely that new cognitive abilities emerged during this period, such as speaking languages, thinking about other people’s minds, and making moral judgments. The social problems faced by Homo sapiens during the Pleistocene, however, were probably the same problems that its ancestors and other primates had already been dealing with for millions of years. Sure, at some point early humans started talking about these problems over dinner—an evolutionarily novel twist—but I doubt that these dinner conversations led to completely new solutions. We deal with the same problems today, and we still use the same behavioral solutions we inherited from our ancestors.
Behavior has a phylogenetic history, which means that simple behavior patterns as well as complex social strategies are inherited from ancestor species and maintained in new species, sometimes with only minor modifications. Evolutionary biologists call the tendency for descendant species to retain the characteristics of their immediate ancestor phylogenetic inertia. When morphological, physiological, behavioral, or psychological traits are similar in different species owing to inheritance from a common ancestor, these traits are called homologous. As it turns out, when it comes to social behavior, humans still resemble not only the great apes and other primates but other animals as well, so that in our behavior we can still see phylogenetic traces of our “inner fish” or even our “inner insect.” For both anatomy and behavior, the traits that are most likely to be preserved across many branches of the phylogenetic tree are traits that evolved by natural selection and that play an important role in organisms’ survival and reproduction; therefore, many adaptations (for example, the emotion of fear) are likely to be homologous across species. Phylogenetic inertia doesn’t necessarily hold back the action of natural selection; some of the traits with high inertia are likely to be adaptive.
Some of the reasons why evolutionary psychologists are skeptical about studying the phylogenetic history of human behavior are shared by evolutionary biologists, and this skepticism applies not only to human behavior but to behavior in general. Although the skeptics recognize that behavior evolves by natural selection just as the body does, they maintain that we can’t study the phylogenetic history of behavior because behavior is “special” and qualitatively different from the body. One way in which behavior is special is that it is too labile, too variable, too susceptible to influences of the environment to be the subject of phylogenetic analyses. Our body too is affected by the environment in which we live, but not as much as our behavior is. Another difference between bones (or any other part of the body) and behavior is that bones are considered a “structure” while behavior is considered the “functional” product of a structure, the brain. In other words, the brain is a structure equivalent to a bone, and behavior is what the brain does, which would be equivalent to the way a bone moves.
This distinction between structure and function matters because, according to some evolutionary biologists, we can study homologies for structural traits but not for functional ones; for example, we can look at homologies between the legs and brains of animals of different species, but not homologies between the ways these animals walk or behave. A related, but less radical, objection is that two functional traits can be considered homologous only if they are produced by the same structure. For example, rhesus monkey sleep and human sleep could be considered homologous only if sleep is produced and controlled by the same regions of the brain in the two species.
In a rebuttal to these objections, two American primatologists, Drew Rendall and Anthony Di Fiore, have convincingly argued that behavior is not too labile or variable to preclude phylogenetic analyses; that homologies can be established for both functional and structural traits; and that behavioral traits can be homologous even if they originate from different areas of the brain.15 They thus conclude that there is nothing “special” about behavior from an evolutionary standpoint. And there are data to back up this conclusion: research has shown that similar adaptive behavioral traits shown by different species are as likely to be homologous as morphological traits are. For example, in the early 1990s biologists Alan de Queiroz and Peter Wimberger conducted phylogenetic analyses of morphological and behavioral characteristics in a wide range of animals, including insects, fish, frogs, reptiles, and birds.16 The morphological characteristics included body size, the size and shape of bones, and other physical characteristics of the animals. The behavioral characteristics ranged from simple stereotyped movements to complex social behaviors (for example, courtship behavior, territoriality, or parental care). They concluded that behavioral traits are as likely to be homologous across different organisms as morphological characters are—as some biologists have known, or suspected, for a long time.
The study of the phylogeny of behavior started in the first half of the twentieth century, gained momentum in the middle of the century, then was virtually abandoned. Now, in the twenty-first century, it is coming back with a vengeance: phylogenetic studies are one of the fastest-growing areas of animal behavioral research. European biologists Konrad Lorenz, Niko Tinbergen, and Karl von Fritsch were awarded the Nobel Prize in Medicine in 1973 for their contribution to the birth of a new scientific discipline, ethology, often described as the study of the biological bases of behavior. A major focus of early ethological research was the phylogenetic history of behavior.
Trained in comparative anatomy and systematics, Lorenz and other European ethologists believed that certain sequences of animal movements are as reliable in identifying species as closely or distantly related as any of the morphological characteristics used in comparative anatomy. Two of their predecessors, biologists Charles Whitman (1842–1910) a
nd Oscar Heinroth (1871–1945), had suggested half a century before Lorenz and Tinbergen that the concept of homology was equally applicable to morphological characteristics and behavior. Whitman and Heinroth focused on one particular aspect of behavior: motor patterns. For example, Heinroth argued that the movements involved in yawning and self-scratching are probably homologous among many vertebrate animals. Lorenz and Tinbergen later expanded this type of research by showing that movements in the courtship displays with which male ducks and gulls attract females are very similar in closely related species as a result of inheritance of behavior from common ancestors.17
The approach of comparing behavior in closely and distantly related species to find out whether behavior shows evidence of genetic transmission from ancestor to descendant species can be applied to the study of not only movement patterns but also the complex social behaviors involved in mating, parenting, attachment, cooperation, aggression, and submission and defense. These complex behaviors are unlikely to originate from scratch in any single species. In fact, more and more studies encompassing insects, lizards, frogs, birds, and mammals are discovering phylogenetic continuities across different species in a wide range of complex social behaviors. An early example comes from a comparative study of rodents conducted by American biologist John Eisenberg in the 1960s.18 Eisenberg studied the social systems of kangaroo rats, pocket mice, and related species of rodents and found that each species’ social system was better explained by the phylogenetic history of the species—that is, the type of social system possessed by its ancestors and its closest relatives—than by the characteristics of the environment in which the species lived. Studies of social organization in different species of iguana lizards have found that the phylogenetic tree of these lizards explains social traits, such as the presence or absence of male territoriality and male dominance hierarchies, better than it explains morphological traits (for example, differences in body size between males and females).19
Similar discoveries have been made in nonhuman primates. In the mid-1970s, two biologists, John Spuhler and Lynn Jorde, classified twenty-one different primate species on the basis of nineteen behavioral characteristics and found that the occurrence of particular behaviors in the various species was explained equally by the characteristics of the environment in which they lived and by the species’ position on the primate phylogenetic tree: in other words, a species had inherited particular behaviors from its ancestor.20 Twenty years later, Di Fiore and Rendall conducted a similar study: they classified sixty-five primate species on the basis of thirty-four aspects of their social organization, such as migration tendencies, grouping, community structure, mating patterns, social relations within and between the sexes, and reproductive investment.21 They discovered that many aspects of female social behavior, such as the tendency to form dominance hierarchies and aggressive coalitions and the tendency to groom their female relatives, were extremely uniform among Old World monkeys. Essentially, most species of macaques, baboons, langurs, and other Old World monkeys have similar social systems and general patterns of social behavior because these patterns were inherited as a “package” from their ancestors and maintained relatively unmodified during the subsequent evolution and diversification of these monkey species. This finding demonstrates that even very general patterns of behavior associated with social organization can be conserved over large evolutionary time scales. Since many species of Old World monkeys are now quite different from each other in size, physical appearance, and other aspects of their biology, these studies also suggest that behavioral traits may be even more resistant than morphological traits to the evolutionary changes that occur along with species diversification and ecological adaptation to new environments. Another implication is that lacking a knowledge of a species’ phylogenetic history may make it difficult for researchers to understand why the species exhibits a particular social system or a particular pattern of complex social behavior. By the same token, it is difficult to understand the evolutionary basis of human social behavior without some knowledge of the social behavior of other primates, and particularly the social behavior of the species closely related to us.
Many aspects of human behavior and cognition were probably inherited from our mammalian ancestors. The probability that two species share similar behaviors owing to common descent is higher the closer the phylogenetic relationship between the species is. The great and lesser apes are, along with the Old World monkeys, the animals that are phylogenetically closest to humans. Therefore, human behavior is more likely to be homologous to the behavior of these primates than to the behavior of other animals. Let’s look at the example of the smile, a universal human facial expression reported in all cultures around the globe. In his 1872 book The Expression of the Emotions in Man and Animals, Darwin suggested that many human facial expressions evolved from those of our primate ancestors, and primatologists have long believed that the primate “bared-teeth display” and the human smile are homologous, which means that rhesus macaques, chimpanzees, and human beings all inherited this behavior pattern from the ancestor these three species had in common about 25 million years ago.22
As mentioned in Chapters 1 and 2, in rhesus macaques and chimpanzees the bared-teeth display is mainly used as a submissive signal. Also known as a “fear grin” or “fear grimace,” it is often displayed by low-ranking individuals who have been attacked, threatened, or sometimes simply approached by a high-ranking individual. The signal reflects a combination of fear and a plea for mercy (“don’t attack me!”). Rhesus macaques and other species of Old World monkeys and apes also use this signal during friendly interactions, such as when an adult male approaches a female and invites her to mate, or when a mother encourages her baby to walk and follow her. Human beings use the smile mainly for friendly purposes, but its submissive component still exists, as when we nervously smile at our boss. The primate bared-teeth display and the human smile are produced by contraction of some of the same facial muscles and have similar submissive and friendly functions. Although it would be difficult to provide conclusive and incontrovertible evidence that they are homologous, this explanation seems more reasonable and more likely to be correct than the alternative one, according to which the human smile is an evolutionary novelty that does not have a long phylogenetic history.
Figure 9.2. Left: The bared-teeth display shown by a rhesus macaque. Center: The bared-teeth display shown by a chimpanzee. Left and center photos courtesy of Dr. Lisa Parr. Right: The human smile. Photo by Dario Maestripieri.
A smile is a relatively simple behavior pattern, and an easy case can be made that other simple behavior patterns, such as yawning, have an even longer evolutionary history, given that they are ubiquitous not just in primates but in all mammals. A good example of a more complex program—one with emotional, cognitive, and behavioral components—that humans probably inherited as a package from their primate ancestors is the infant attachment system I discussed in Chapter 6. Many primate infants are dependent on their mother for nutrition, thermoregulation, and protection; as a consequence, they need to be carried all the time or kept in close proximity to their mother. The problem is that if infants become separated and lost from their caregivers, they starve or freeze to death or are eaten by a predator. To solve this problem, natural selection came up with the infant attachment system. As mentioned in Chapter 6, the infant attachment system has a set goal—the maintenance of contact with or proximity to the mother—and specific activating and terminating conditions. The attachment system is activated when the infant is separated from the mother, and it’s terminated when contact or proximity is achieved. The infant attachment system has three defining features, or what evolutionary psychologists call design features: separation anxiety and fear of strangers; use of the mother as a “safe haven” for protection; and use of the mother as a “secure base” for exploration.
A humanlike infant attachment system with the same set goal, the same activating and terminating conditions, the s
ame design features, and many of the same behaviors, such as infant crying and following, is almost ubiquitous among the Old World monkeys and the apes and absent or rare among the prosimians and the New World monkeys. This suggests that the infant attachment system is an adaptation whose history can be tracked in the evolution of the Primate order. Although there may be slight differences among species in some of the physiological mechanisms or cognitive processes underlying the regulation of attachment processes, the attachment system as a whole shows evidence of homology across primate species.23
The Recent Past
The amazing growth in the size and complexity of the human brain that occurred after our hominid ancestors split from those of the other apes was accompanied by the acquisition of many new “mental powers.” We developed the ability to think about ourselves, about events happening in the future, and about concepts such as life, death, and the immensity of the universe. The evolution of language gave a big boost to both our ability to think and our ability to communicate with others. Many new mental abilities evolved to solve the problems of a complex social life, and these new abilities, in turn, made our social life even more complex. If it is true, as primatologist Marc Hauser has argued in his book Moral Minds, that our ability to think about the moral value of our actions and to make moral judgments about our own actions and those of others evolved by natural selection, this process must have occurred relatively recently.24 Although the social contract view of morality has validity, it is likely that complex new emotions evolved to support morality, such as shame (subjective penalty for norm violation), guilt (subjective penalty for violation of expectations), pride (subjective payoff for norm adherence), moral outrage (anger that occurs when norm violations by others are experienced as if they were transgressions against the self), and contempt (long-lasting condemnation of others who have transgressed norms or violated expectations). These emotions appear to be evolutionarily recent and unique to humans. Consistent with Noë’s suggestion (mentioned in Chapter 8) that team-playing may have played an important role in human social evolution,25 evolutionary psychologists Fessley and Haley also suggest that “corporate” emotions such as admiration and elevation—which reward individuals who are team players, who behave altruistically, and whose actions benefit others or society as a whole—may also have recently evolved in the human lineage.26 Finally, pressure to form and maintain heterosexual pair-bonds for joint child-rearing and to invest in our children for the rest of our lives probably favored the evolution of romantic love and parental love to a degree that is not observed in other animals.
Games Primates Play: An Undercover Investigation of the Evolution and Economics of Human Relationships Page 30