Anthropologists have tried to tie the recent history of violent human conflict with observations of violent behavior of chimpanzees today in order to claim that murder and mayhem are innate characteristics of humans. Edward O. Wilson is a recent proponent of this hypothesis (which is particularly popular with male anthropologists). In his book, The Social Conquest of Earth, he makes these claims.
“Our bloody nature, it can now be argued in the context of modern biology, is ingrained because group-versus-group was a principle driving force that made us what we are. In prehistory, group selection lifted the hominids that became territorial carnivores to heights of solidarity, to genius, to enterprise. And to fear. Each tribe knew with justification that if it was not armed and ready, its very existence was imperiled. Throughout history, the escalation of a large part of technology has had combat as its central purpose.”
“It should not be thought that war, often accompanied by genocide, is a cultural artifact of a few societies. Nor has it been an aberration of history, a result of the growing pains of our species’ maturation. Wars and genocide have been universal and eternal, respecting no particular time or culture.”
Making claims about which attributes are “universal and eternal” might seem a bit risky since so much of human history is unknown. But are those millions of years completely unknowable to us? Human male and female bodies evolved in different ways over years than did the bodies of our ape relatives. Perhaps we can derive some information about how humans lived from those evolutionary differences.
Richard O. Prum has produced a fascinating look at the role of evolution in species development in his book The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World - and Us. He reminds us that Darwin eventually wrote two books discussing the topic of evolution. The first, On the Origin of Species by Means of Natural Selection, proposed the view that most describe as “survival of the fittest.” This firmly established the notion that species evolved via natural selection as individual characteristics best adaptive to species survival in a given environment would propagate via sexual transmission and eventually become dominant in successive generations. This established the notion that all evolutionary changes should be determined by improved adaption to the environment. But Darwin clearly recognized that not all evolutionary developments could be explained by this hypothesis, leading him to develop the concept of mate selection as an additional factor in species evolution in The Descent of Man and Selection in Relation to Sex.
Darwin was particularly troubled by the physical development of the peacock, leading him to conclude that the outrageous and functionally useless tailfeathers are not a result of environmental adaptation, but of aesthetic choices made by female peahens. They preferred to mate with males they found attractive. One way or another the female of the species gained control over the mate-selection process and determined which characteristics would propagate. The notion of female dominance in an evolutionary process was not popular with the male scientists of Darwin’s era. Consequently, this latter process was greatly ignored and the “adaptionist” approach dominated evolutionary theorizing. Prum’s book is aimed at resurrecting mate selection and elevating it to a more dominant place in evolutionary research.
In each species, there is a “war between the sexes” to control the process of mate selection. Generally, males are less selective and are willing to mate with any female who is available. Females, generally, prefer a choice as to who they mate with. Given that the females bear the brunt of the effort and responsibility for producing offspring, it is not surprising that they would desire some choice in the matter.
Where males dominate the process, and particularly where they can force sex upon a female if they choose, dominant male characteristics will propagate. Access to females is often a result of male-male competition. Chimpanzees and gorillas are examples of species where males dominate. In fact, male domination often leads to the practice of infanticide. As a new male becomes the dominant male, he wishes to celebrate his status by impregnating females. If none or few are available because they are lactating as they raise their offspring, the newly dominant male will often kill the young apes in order to bring the females back into estrus.
When females control the mating selection process, individual characteristics preferred by them will propagate. In Prum’s specialty, birds, mate selection by females is the dominant process. Consequently, it is the male who must present the physical attributes that will prove attractive to females. Females then have the power to reconstruct males in a manner more to their liking, presumably eliminating male means of forcing sex and making males more aesthetically pleasing. Curiously, Prum points out that the ancestor of our bird species possessed a penis. Somehow, evolution took away the penis of most bird species, making it almost impossible to inseminate a female without her cooperation.
Much of Prum’s book involves illustrating to the reader the power of mate selection in determining male characteristics within various bird species. It becomes clear that this process has little to do with making the species more survivable.
Prum spends the final sections of his book speculating on the role of mate selection in human development. From this perspective, humans are rather unique in the sense that both males and females have developed differing degrees of selectivity in choosing a mate. Prum concludes that many of the characteristics developed over the ages in both male and female humans can be explained as results of humans seeking increased sexual satisfaction.
Can evaluating human evolution in terms of mate selection provide us a different picture of what was important to humans as they evolved? Or are we stuck with the notion that constant physical conflict determined that characteristics necessary for warfare must dominate human evolution? Let us see if Prum might be correct.
One of the major differences between humans and our ape relatives is that we developed larger brains. This fact is often viewed as demonstrative of human exceptionalism. However, Suzana Herculano-Houzel, after developing a method of counting brain cells, concluded that the human brain has the same number of brain cells as expected from scaling neuron numbers of other primates as a function of body mass. Human brains are, in that sense, not exceptional. The brains of gorillas and chimpanzees are exceptional because they fall below the scaling line; their brains are smaller than expected. The explanation for this follows from the fact that brains consume an inordinate amount of energy for their size. The other apes had difficulty consuming enough food to provide sufficient energy for both their large body and a larger brain. Their evolutionary response was to develop a larger digestive system to extract more nourishment from their food supply. Humans learned to control fire and use it to cook food. Cooked food is more easily digestible and can provide up to three times the nutrients of raw food. Humans thus provided themselves the luxury of a brain appropriate to their body mass. It is hard to see where either mate choice or the need to fight played a direct role in this.
Pound for pound, chimps are stronger than humans. If war and conflict are “universal and eternal” occupations of humans, then how is diminished strength to be viewed as a boon to survivability. It can be argued that humans traded brute strength for better control of their muscular capabilities. This would likely be helpful in fabricating and using weapons. But both sexes probably appreciated the availability of slow and steady hands in caressing. Call it a slight advantage to the adaptionists.
Male chimpanzees and gorillas have large and sharp canine teeth that are effective in fighting and useful for intimidating females. Humans evolved away those canines. How is that adaptive to an environment in which violent conflict is common?
Which of these specimens do you expect to females to be more comfortable with encountering on a blind date?
Chalk this one up for mate selection.
The difference in size between human males and females also diminished over time. A relative increase in female size would be consistent with a lesser threat that males could force sex on a female against her will. Females would obviously be more comfortable with less threatening males.
“….there is consistent evidence that females do not prefer the most ‘masculine’ facial features, which have been characterized as prominent square jaws, wide prominent brows, thick eyebrows, and thin cheeks and lips. Numerous studies have shown that women instead prefer intermediate or even what some researchers call ‘feminine’ facial features in men, and one study has shown that females prefer a light stubble over a more masculine full beard. According to a handful of disparate studies cited by Gangestad and Scheyd, these facial preferences seem consistent with the evidence on what women like to see in male bodies. They tend to like lean but somewhat muscular male bodies with broad shoulders and v-shaped torsos the most, and men with larger, more muscle-bound bodies the least.”
Chalk another one up for mate selection.
An obvious characteristic of humans is that they lost most of their body hair. It has been suggested that this would allow humans to dispose of excess body heat more easily while running. That could be an advantage in survivability in hunting or fighting. For females however, the lack of body hair had significant ramifications. Chimp infants instinctively know to grasp their mothers’ hair and hold on. This allows a mother chimp to be with her child essentially all the time, even when foraging for food. Human infants still have that grasping instinct, at least for a while, but the mother lost an important means of caring for her infant while she foraged—or fought for that matter. Human females—and males—would have to agree to collaborate with the mother in caring for her infant while a mother went about her business. This would require a dramatic revolution in social behavior compared to that of other apes.
Humans currently view body hair as something unsightly except in designated ornamental configurations. And there are plenty of hairy animals that are quite survivable in Africa. It is difficult to view loss of body hair as other than a product of mate selection.
Actually, humans then proceeded to add other patches of hair in their underarms and pubic regions. The fact that these displays only emerge at puberty is highly suggestive that they are involved with sexual signaling of some sort. More points for mate selection.
“Regardless of whether the reduction of body hair is an aesthetic trait or not, it is clear that another unique trait—the retention of specialized patches of hair in the armpits, pubic region, scalp and eyebrows—is ornamental. The fact that the retention of these patches of hair is the same in both sexes….strongly implies that it evolved through mutual mate choice, like the bright beaks and plumage of male and female puffins, parrots, and toucans. The hypothesis that underarm and pubic hair are evolved sexual signals is further supported by the observation that these patches of hair do not develop until puberty. These unusual patches of hair likely evolved for the purpose of pheromonal, sexual communication between mates, which is very common in mammals.”
One of the most unique characteristics developed by humans is the existence of permanent breasts on females.
“Among the more than five thousand species of mammals on earth, permanent breast tissue is unique to humans. The mammaries of all other mammals increase in size only during ovulation and lactation, and they are not enlarged at other points in the life cycle. Human females, however, develop enlarged breasts with the onset of sexual maturity, and they retain enlarged breast tissue throughout their lives.”
It would really be a stretch to see permanent breasts as a survivability feature. They would get in the way when foraging or fighting, and be a hindrance if women had to move fast. It would be a long time before the sports bra would be invented. Permanent breasts could only develop with the support of males who signaled their enthusiasm via mate selection.
Human males have also developed rather unique sexual features. A chimpanzee female copulates with many males in order to suggest to as many as possible that they may be the father of her coming infant. This is in hope that a male will not kill one of its own offspring. This means the chimp with the most sperm has the greatest chance of fertilizing the ovum. Chimps therefore have evolved large testes relative to their body size. Humans face no such condition and have developed modest sized testes. However, for some reason, humans have developed a larger scrotum that the chimp even though its testes are smaller.
The human penis has also developed some unique features that appear to be driven by effectiveness in sexual intercourse and/or attractiveness to females.
“By any measure, the human penis requires a lot of explanation. It is substantially larger—both in absolute and relative size—than that of any of the other apes, even though humans are intermediate in body size between gorillas and chimpanzees. The erect gorilla penis is only an inch and a half long. The chimpanzee penis is three inches long when erect, very thin, smooth, and finely pointed at the tip. The human penis is both longer—averaging about six inches when erect—and wider than the penis of other apes. The human penis is also characterized by a distinctly bulbous glans and coronal ridge at its tip. Similar structures have evolved in other primates, but they are not present in African apes.”
There is another puzzling feature to the human penis. Humans are one of only two primates that have evolved away their penis bone, the baculum.
“The existence of a baculum in the other primates means that an erection is guaranteed by the presence of an ossified bone within the penis….we do know that aside from producing erections, the baculum functions in retracting the penis between erections. What its other functions might be is still not clear.”
It seems that male animals who do spend their days attacking or being attacked have mechanisms whereby their genitals are tucked away in a safe place, but the supposedly superior human male leaves his permanently exposed—and oh how vulnerable they are! And now consider the extent of the sexual display when humans became bipedal and began standing upright and the females’ breasts became more clearly visible to the males, and the males’ large penis and large scrotum readily available for evaluation by females. Do naked men and women look in any way ready to do battle? Chalk one up for mate selection!
Mating is driven by the pleasure involved in the act. For most species it is a one-off event. Mating season comes or a female signals that it is in estrus and male and female both go into action. And then it is over. It seems a really clever animal would notice that sex was pleasurable and devise a way to have sex whenever possible without the added concern of creating offspring. With modern contraceptives humans have reached that state. But the first step in the process was taken long ago when women lost any signaling of when they were in estrus. It is not clear how this happened, but mate selection would be a prime suspect. This meant that mating generally required multiple sexual encounters in order to generate offspring. As a result, both sexes could evaluate potential mates for both aesthetic characteristics and for their effectiveness as a sexual partner. This extended mating process is probably responsible for selecting males who could copulate much longer than chimps or gorillas before ejaculation, and females who could experience an orgasm when sufficiently excited—something not observed in brief ape sex. It is amazing how much of human evolution seemed devoted to maximizing sexual pleasure.
This extended period of sexual activity before pregnancy also gave both males and females time to evaluate partners on other than sexual performance. Human males are variable in their levels of support for their offspring, but they are much more engaged than chimps or gorillas—another probable result of mate selection.
This discussion should convince most that mate selection has played a significant role in human evolution, and that this role did not result in making humans better warriors.
There is another conclusion that can be drawn from this discussion. It appears that females of the past had greater sexual autonomy than females of today. Prum provides this perspective.
“….I think that the advances in female sexual autonomy that occurred over millions of years since our common ancestry with the chimpanzees….have been challenged by two relatively recent cultural innovations—agriculture and the market economy that developed along with agriculture….These twin inventions came into being a scant six hundred human generations ago and created the first opportunity for wealth and the differential distribution of wealth. When males gained cultural control over these material resources, new opportunities were created for the cultural consolidation of male social power. The independent and parallel invention of patriarchy in many of the world’s cultures has functioned to impose male control over nearly all aspects of female life, indeed human life. Thus the cultural evolution of patriarchy has prevented modern women from fully consolidating the previous evolutionary gains in sexual autonomy.”
In other words, women are now struggling to get back to where they were a thousand generations ago, before men began convincing them they were the weaker sex, and that sex itself was a male preoccupation—not one women should expect to enjoy.
The interested reader might find the following articles informative:
The Evolution of Beauty by Richard O. Prum
Are Humans Inherently Warlike?
Humanity and Violence
Of Chimps and Men: Mothers and Others
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