Another remarkable feature in humans is the large size of the penis, of which much was made when Desmond Morris’s book The Naked Ape was published in 1967. In fact, the human penis, when erect, is no longer than that of chimpanzees and bonobos, although all of these are about double the length of the penis in much larger orang and gorilla males. But the human penis is considerably thicker than any of the others and has a much more bulbous end. Explaining how and why these differences evolved has led to much speculation, ranging from the enhancement of pleasure to displacing the sperm of competing males, to providing a very obvious sexual display as a signal to either females or other males. The other obvious external part of the male reproductive organs—the testicles, containing the sperm-bearing testes—is less distinct in humans, intermediate in size between that of chimps (very large) and orangs (small) and gorillas (tiny). It is believed that this is related to both frequency of mating (high in chimps, low in gorillas) and competition between males for impregnation of fertile females (again high in chimps, low in gorillas). Humans thus fall between the extremes, suggesting that we mate (or, more appropriately, our ancestors mated) fairly often, but with only moderate levels of promiscuity compared with chimps.
Darwin had to make extensive use of analogies with other animals, because the fossil and archaeological evidence that he would have valued so much took many more years to blossom. However, accepting our close kinship with the great apes, he recognized similarities between their behavior and intelligence and ours. In 1871 he wrote:
As man possesses the same senses with the lower animals, his fundamental intuitions must be the same … But man, perhaps, has somewhat fewer instincts than those possessed by the animals which come next to him in the series. The orang in the Eastern islands, and the chimpanzee in Africa, build platforms on which they sleep; and, as both species follow the same habit, it might be argued that this was due to instinct, but we cannot feel sure that it is not the result of both animals having similar wants and possessing similar powers of reasoning. These apes, as we may assume, avoid the many poisonous fruits of the tropics, and … we cannot feel sure that the apes do not learn from their own experience or from that of their parents what fruits to select.
Darwin has been criticized for his excessive anthropomorphism in the recognition of “human” behavior in other animals, and given his lack of reliable data on great ape behavior—much of it based on captive animals or the tales of explorers—it is not surprising that he got things wrong at times. But overall he was cautious in his extrapolations. We now know far more about our close evolutionary relationship to our primate kin, and we should not be surprised to find both shared behaviors and shared brain pathways behind them. Thus monkeys and apes can recognize the different elements and expressions that make up faces from simple drawings rather than accurate pictures. The neuroscientist Vilayanur Ramachandran stressed the potential importance of mirror neurons in their brains and ours, nerve cells that are triggered both when an animal performs an action and when an animal observes another animal performing that same action. Such acting out of deeds in the brain is thought to be important in human learning, social interaction, and empathy, giving primates the basic elements of “mind reading,” which, as we will see, is so important in complex societies like ours.
But we do have a major problem when we turn to reconstructing the complexities of past human behavior, since what is left behind as physical evidence in the form of stone tools and butchered bones only represents the end products of chains of thoughts and deeds that are lost to us now, and which we attempt to reconstruct at our peril. Certainly we can turn to living apes to provide models for early human behavior for such activities as simple toolmaking and primitive hunting, but how much like a chimpanzee were, say, the Homo heidelbergensis people of Boxgrove in England 500,000 years ago, who were already living far from their tropical African homeland, making complex tools like handaxes, and acquiring not just small mammals but potentially dangerous big game such as horse, deer, and rhino? Just as significant, H. heidelbergensis already had a big brain, one nearly as large as ours today. To understand the evolution of those large human brains, we need to look at what they might have been used for.
There is now evidence that chimps in the wild do have “cultures,” shared traditions of how to behave—for example, in gathering or processing food with tools—which differ from one group or regional population to another. These cultural norms are learned as the chimp grows up in its group, and female chimps seem to be prominent, both in passing on traditions to new generations and in developing new ones. Yet these cultures are still rudimentary, and chimps are seemingly a long way from the cultural repertoire of even the earliest humans in Africa 2 million years ago. We remain unique in the extent to which we modify the world we live in through the things we create. Beyond that, we create imaginary worlds that are entirely virtual, made up of thoughts and ideas—worlds that live in our minds, from stories and spiritual domains through to theories and mathematical concepts. Chimps possess basic concepts of cause and effect; for example, if they strip a grass stem and lick it, it will then be thin enough and sticky enough to be used as a probe to catch termites. But humans have the ability to imagine a much longer chain of cause and effect, to consider several different outcomes that could result from an action or an alternative action. Through the medium of language, we can communicate these complex concepts to each other, both those relating to the material world, such as how to make a fire, and those relating to imagined worlds, such as what may happen to us after we die.
Instead, should we perhaps turn to modern hunter-gatherers in places like Brazil, Australia, and Namibia to help us reconstruct how the Boxgrove people, or the Neanderthals, or our African ancestors lived? We have to use such data cautiously and always be aware of the assumptions and extrapolations we make, since much has evolved and changed in the intervening millennia. So how could such complexity of behavior, including the ability to create virtual worlds, have evolved? One possibility is that an increase in meat eating in our ancestors not only gave access to more concentrated foods, removing previous constraints on large, energetically demanding brains, but also set in motion far-reaching changes in behavior, enhancing the power of reading the minds not only of our prey but of members of our own social group.
Daily life in primate troops in the wild has been compared with the worst aspects of television soap operas or reality TV shows like Big Brother: bullying and domination by the strongest, fear and abuse for the weakest. Yet primate groups also demonstrate tenderness and affection, strong alliances for the greater good, and social bonds that can last throughout life. This brings us to what is called the Social Brain Hypothesis (SBH), advanced by psychologists and anthropologists such as Nicholas Humphrey, Robin Dunbar, Richard Byrne, and Andrew Whiten. By this hypothesis our large brains have evolved not just in response to human needs for things like foraging and hunting skills, toolmaking, and invention, but also because of the complex societies in which we live. All primates have large brains for their body sizes relative to the average in mammals, particularly the so-called higher primates, the monkeys and apes. Brains are very demanding of energy; in fact, in humans, the demands of the brain are second only to those of the heart. So why would, say, a lemur or bush baby need a relatively larger brain than a hedgehog or a squirrel? One argument has been that the forested environments in which primates generally live require a keener intelligence to cope with problems, while another perspective has to do with the longer growth and development found in primates both before and after birth. Yet these explanations on their own do not seem sufficient, which is why SBH has gathered an increasing number of influential supporters.
Various comparative studies have shown that the relative size of the neocortex of the primate brain is much larger than normal in mammals (and in humans it constitutes a whopping 85 percent of total brain weight). The neocortex is part of the cerebral cortex and is known from brain mapping to be respo
nsible for higher-level cognitive functions, such as learning, memory, and complex thought. That might seem to support the idea that it is large because the environment in which primates have evolved demands a keen intelligence in finding food and escaping predators. Yet plots of neocortex size against environmental complexity seem to explain less of the primate pattern than do plots against variables that reflect social complexity, such as group size, numbers of females in a group, frequency of social alliances, and amounts of social play, manipulation, and learning. Thus while environmental hypotheses tend to assume that animals solve problems individually by trial-and-error learning, without relying on the social groups in which they live, SBH proposes that such problems are solved socially, with the need for a larger neocortex to enhance social comprehension and cohesion. No doubt, in reality, both the social and general environment play a part in generating evolutionary demands, but in the case of humans it is very difficult to argue that general environmental demands have been anywhere near as important as social ones in the development of our extraordinarily large brains.
There is evidence that pair-bonded birds and mammals have relatively larger neocortices, and one possibility is that in higher primates, and particularly in humans, the social and thinking skills used in pair-bonded relationships have been extended many times over for creating and maintaining relationships between individuals who are not partners in reproductive terms. Thus individuals of the same or opposite sex may form bonds as intense and long-lasting as those normally found between pair-bonded mates in other species; in other words, humans have mates beyond just sexual mates. For this to work successfully and in the long term, it requires the extension of high-level social skills of trust, empathy, and synchronization of actions beyond the immediate “family group” and into the larger social community. These links would be valuable in terms of reliable support in times of difficulty, and through an extended web of such relationships crosscutting through the whole group, all could benefit from coordinated action, food sharing, protection from predators, et cetera.
Thus many scientists believe that our substantial brains evolved via selection for life in large groups, and this led to the development of deep social minds in primates, with the ability to “mind-read” (observe and interpret the actions of) others in the group, to learn and pass on “cultural” behavior within the group, and to cooperate not only for mutual benefit but for the benefit of others in the group. Mind reading, or possessing a “theory of mind” about oneself and others, can occur at several levels and for many different social purposes, for example, interpreting what individual A thinks about individual B and then behaving so as to manipulate the behavior of A toward B. (This social “skill” is sometimes known as Machiavellian intelligence, a term introduced by Byrne and Whiten, after the Florentine political philosopher Niccolò Machiavelli.)
Mammals and birds seem to have a first order of intentionality, that is, they are aware of their own behavior and its possible impact on others; as mentioned, some of this may be related to the demands of strong pair-bonding or living in herds or flocks. But by the age of four, human children can operate at two levels of intentionality in their social perceptions, that is, perceiving and interpreting not only their own behaviors but also the behaviors of those immediately around them. Thus children have the ability to recognize that others may have the same, or different, perceptions of the world compared with their own. At this stage such recognition means that they can begin to manipulate, or try to manipulate, those around them, whether these are parents, siblings, their peers, or their nursery school teachers. There is evidence that chimpanzees approach the same level as four-year-old children in their theory of mind, but they never move beyond this, whereas most humans develop further to cope with several higher levels of intentionality. Robin Dunbar illustrated this point with reference to Shakespeare’s play Othello, where the playwright had to simultaneously handle four mind-states on the stage: Iago intends that Othello should believe that Desdemona loves Cassio and Cassio loves her. But Shakespeare moved beyond that because, to be successful, he also had to be able to visualize the audience’s reaction to what he was writing—and so he was working to at least a fifth-order intentionality, right at the limits of human mind-reading abilities. These highest levels, supporters of the SBH argue, are unique to modern humans, and they evolved through the need for our ancestors to map the growing complexities of their social relationships. This in turn raises the question of why such complexities had developed.
SBH perhaps helps to explain something that does differentiate most human hunter-gatherer groups both from our primate relatives and from modern industrialized societies: egalitarianism. Hunter-gatherers usually have little in the way of owned material possessions, since they are difficult to maintain and transport with a nomadic lifestyle, and this social equality is reflected in things like food sharing, lack of formal leadership, and the prevalence of monogamous relationships. This last contrasts with the polygamy that characterizes primates such as baboons and gorillas, as well as many agricultural and pastoralist societies, where a few men may accrue disproportionate wealth, status—and wives. Maintaining social equality often requires positive coordinated efforts by the group to resist those individuals who try to assert excessive dominance. Coordination of activities extends to bands of women who plan ahead to go foraging for plant foods, insects, and small game, and to hunting bands, who must also plan ahead, communicate about tracks and signs, and adopt specific roles in catching and processing prey. In terms of the vital activity of food acquisition, the degree of coordination that a sophisticated social brain can help to deliver means that the group acts more like a food-gathering machine than the host of individual and “selfish” foragers typical of a monkey or ape troop.
But there are real practical limits on the size of a group that can successfully interact and function at a personal level, and this has been enshrined as Dunbar’s number, following Robin Dunbar’s research. In primates this may be the subgroup in a troop who regularly interact by means such as mutual grooming of fur, and can range up to about sixty. For Dunbar, what ultimately limits the size of such groups in different primates is the relative size of the neocortex, which governs how many friendly relationships or meaningful acquaintances can be successfully managed at any one time (although recent research suggests that the small regions known as the amygdala, located near the base of the brain, also play an important role in humans). Dunbar’s number in modern humans seems to fall between about 100 and 220 (average 148), and the figure matches quite well with the optimum size of large hunter-gatherer aggregations, tribal villages, Hutterite settlements, small military units, and even the average number of people in effective social networks on the Web. As we shall see later in this chapter and the next, the relatively large size of human aggregations has had consequences in terms of the need to develop new means of communication (symbolism and language) and the evolution of more complex social structures and fully human culture.
With our large brains, evolved for and geared to interact flexibly with a network of people in our social group, we can exchange information of mutual benefit. But how free are we really in these interactions, and how important is our genetic inheritance in determining what we can and cannot do? In chapter 7 I will discuss our DNA and genes, and their importance in reconstructing the processes of human evolution, but there is no doubt that our DNA does provide a basic template for our behavior. It’s a template that provides both limiting and varying factors in what we can and cannot do (for example, in determining the basic size and shape of our brain, the extent of human dexterity, running speed, acuity of vision and hearing). At the same time, it is obvious that humans can improve elements of performance through learning and practice, and many of these are influenced by differences in both the physical and social environments (for example, diet, health, upbringing, and social norms). So our DNA is more like a flexible container than a mold in the way it determines and sets limits o
n how we behave. Nevertheless, as we shall see, some scientists believe not only that the structure of the modern human brain is quantitatively different from that of earlier humans in its size and the extent of its gray matter, but that genetic changes unique to modern humans also qualitatively rewired our brains about 50,000 years ago, making us behaviorally modern at a stroke. If that was so, then despite their large brains, Neanderthals were fundamentally unlike us in their lack of human behavior, because they followed a separate evolutionary trajectory. That same lack would have applied to the modern humans who occupied Africa before 50,000 years ago, because they lived before the mutations occurred that made us fully modern.
How much of our modern behavior was shared by earlier human species? That is a very difficult question to address, let alone answer, from the data we have. Some researchers produced a sort of checklist of modern behavior, which, it is argued, characterizes humans today and which can then be used to examine the archaeological record for when and where those traits first appeared. Other workers would dispute their usefulness, their universality, and how accurately they can be inferred from the imperfect materials that survive in the ground from ancient times. But the list of behaviors often includes: complex tools, the styles of which may change rapidly through time and space; formal artifacts shaped from bone, ivory, antler, shell, and similar materials; art, including abstract and figurative symbols; structures such as tents or huts for living or working that are organized for different activities (such as toolmaking, food preparation, sleeping, and for hearths); long-distance transport of valued materials such as stone, shells, beads, amber; ceremonies or rituals, which may include art, structures, or complex treatment of the dead; increased cultural “buffering” to adapt to more extreme environments such as deserts or cold steppes; greater complexity of food-gathering and food-processing procedures, such as the use of nets, traps, fishing gear, and complex cooking; and higher population densities approaching those of modern hunter-gatherers.
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