The Descent of Sex

To mark the 150th anniversary of The Descent of Man, paleoanthropologist Jeremy DeSilva has gathered a team of experts, mostly scientists, to pen reflections and update Darwin’s analysis: one essay each on The Descent of Man’s introduction and its seven chapters about human evolution, one summarizing the eleven chapters Darwin devoted to sexual selection, one on sexual selection in humans, and a conclusion. The essays in A Most Interesting Problem collectively present an image of Darwin as both “remarkably prophetic” with regard to some predictions and “flat out wrong” with others. ³

As several authors point out, scientists now have far better evidence to work with than Darwin did, including thousands of fossilized hominin specimens spanning the past 7 million years, as well as robust genetic analyses of contemporary and historic populations. Darwin made the case for human evolution without any of this information, relying instead on comparative data from skeletons, embryos, brains, and behavior.

He concluded that the major factors defining humanity—large brains, complex moral codes, culture, and an appreciation for beauty—exist in other primates too, although to a lesser extent. The differences between humans and other primates were thus a matter of degree, not of kind. This reasoned assertion of Darwin’s proved correct. So, too, did his intuition that human evolution had taken place largely in Africa. ⁴

Unsurprisingly, the most controversial of Darwin’s claims, in his time and ours, regard race and sex. It is in the essays on racialized differences and sexual selection in humans—written by Agustín Fuentes and Holly Duns­worth, respectively—that his ideas come under direct fire. Fuentes calls out Darwin’s “ethnocenetric, Eurocentric, and anti-African biases.” Dunsworth suggests that for Darwin, “women were wives, but men were so much more than husbands.” ⁵

The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick! CHARLES DARWIN, LETTER TO ASA GRAY, 1860 ⁶

In 1860, Darwin was well recognized for explaining how organisms’ adaptations evolved for survival. But when he gazed at the peacock’s tail, it stared back at him as a stark challenge to this theory of natural selection. It was obvious, at least to Darwin, if not to Wallace, that the peacock’s tail was not an adaptation for survival, more likely hindering survival than promoting it. As the male is signaling his presence to potential mates, he is also being eyed as a potential meal by a variety of predators from tigers to mongooses. His tail makes him more attractive as a mate but more conspicuous as a meal. If the peacock’s tail was a single aberrant glitch in the workings of natural selection it might not have been such a focus of Darwin’s consternation. Darwin acknowledged, however, that these types of traits, those that seem maladaptive for survival, are ubiquitous in the animal kingdom. Long tails, bright colors, elegant courtship dances, and elaborate vocalizations all seem to invite death, not to circumvent it. How could this be explained? ⁷

It was not difficult to resolve the conflict between natural selection and sexual selection. Traits evolve only if they are passed on to the next generation through mating, but animals can mate only if they survive. Thus, a male peacock with an abnormally short tail might survive quite well but never mate, while a male with an abnormally long tail might be quite attractive to females but likely would not live long enough to try out his tail. Thus, in many cases, there is a conflict between natural selection and sexual selection, and the traits that evolve strike some balance between survivorship and mating success. ⁸

Why is it typically the males that have to compete for females and not vice versa?

…

The answer is as fundamental as can be imagined. Many of the differences between males and females, in almost all the species known to reproduce sexually, result from differences in gamete size. In humans and in most other animals, a female’s eggs are the largest cells in her body, while a male’s sperm are the smallest cells in his body. Females invest much more in their gametes than do males, even though males produce many more gametes. In humans, females have a few hundred eggs, while a male will produce a few billion sperm during his life. ⁹

Sperm Sucking Lice
Gamete size is the fundamental character that defines an animal’s sex. Many would think that an individual’s genitals might be diagnostic of its sex, but those many would be wrong. As a case in point, there is a species of lice that lives in caves in Brazil and feeds on bat guano. This is a typical sexually reproducing species, in that there are males and there are females. But atypically, the females have “penises” and the males have “vaginas.” A female inserts her penis into the male’s vagina and sucks up the sperm from the inside of his body into her body, where his sperm fertilize her eggs. It is not a very typical mating strategy, but these lice are quite typical in that the females have the large gametes and the males have the small gametes. ¹⁰

This simple fact has caused many evolutionary biologists to question why males even exist, a conundrum referred to as the “cost of males.” If females reproduced clonally, with no genetic input from males, all of their offspring, all of which would be daughters, would bear offspring. If females in a similar population reproduced sexually, only half of their offspring would be female, only their daughters but not their sons would bear offspring, and the sexual population would grow at a much slower rate than the asexual population. The short answer explaining the existence of males is that sexual reproduction provides a means for creating genetic variation among offspring, even though it slows the rate of population growth. ¹¹

There were two main criticisms of Darwin’s theory of sexual selection by female choice. The first cited the lack of compelling evidence that females choose mates. The second criticism was that even if it were to occur, Darwin could not explain why females have these preferences. ¹²

Similarly, the túngara frog has evolved additional syllables, called chucks, to adorn its basic mating call, a whine, which stimulate an inner ear organ that had yet to be recruited for use, in communication among close relatives. This added stimulation of the ear has two results: it leads to enhanced stimulation of the auditory centers in the brain, and female túngara frogs find whines with chucks more attractive than a simple whine. Sensory biases are key components of the aesthetic preferences of females. ¹³

SUMMARY
Darwin proposed his theory of sexual selection to explain how elaborate, sexually dimorphic traits used in courtship could evolve, despite being maladaptive for survival. ¹⁴

Darwin … suggested that female animals have a “taste for the beautiful,” that they possess sexual aesthetics not all that different from ours. His theory lay dormant for 100 years and was resurrected in the 1970s, primarily by Trivers’s theory of parental investment. Since then, there have been hundreds of studies to demonstrate clearly the efficacy of female mate choice, thus validating Darwin’s primary prediction about sexual selection. Disagreements still exist as to what causes the evolution of female mate choice, echoing the fundamental disagreement between Darwin and Wallace. ¹⁵

Sociobiology’s rise in the 1970s perpetuated old, and inspired new, captivating but biased (and worse) stories about human evolution and “human nature” that continue to grip popular culture. ¹⁶

In the Darwinian context, Western notions of masculinity and patriarchy are justified because gendered behavior is deemed the driving force of evolution—a process that Darwin valued as progress and improvement, and our culture still does. ¹⁷

… to Darwin white men were superior and, therefore, men were the most evolved humans, which in turn increased white male superiority because he valued “higher” states of evolution (which are fictions). ¹⁸

Science is the human creation of knowledge. Prizing the completely objective scientific mind is foolish because such a mind does not exist. This myth is dangerous because it creates opportunity for scientists to exploit humanity. It is also detrimental to science because it prizes those perspectives judged to be objective over others, excluding so many from knowledge production, while elevating the “objective” perspective to fact. As biological anthropologist Robin Nelson writes, “The idealization of an objective and apolitical science built on rational thought and deliberation has a face, and that face is white and male.” An uncritical belief in scientific objectivity does not just continue to dominate the profession, it influences the public perception of what counts as science and what is valued. Too often when these issues are raised, critics, who may even be scientists themselves, are labeled “anti-science” or “science deniers” or considered to be too politically motivated to be taken seriously as scientists. Just going in nontraditional scientific directions in a nontraditional body can result in a person’s rejection from science. On these issues, indigenous-studies scholar Kim Tallbear writes, “Being differently situated is advantageous for producing different insights but has its risks. When one fails to exemplify a white Western often straight and masculinist gaze that is ironically seen to embody ‘objectivity,’ or if one researches too close to home, one gets accused of bias.” The persistent myth of an unblemished science is what tricks us into believing that Darwin’s ideas, like his harmful beliefs about women, can be challenged only with science and, ironically, by those scientists deemed to be objective about such issues: men. ¹⁹

Just after its publication, readers interpreted Descent of Man as a call to action to contribute to the evolutionary process by aiding selection’s improvement of our species. Where there was once God’s plan that we must carry out, now there was selection’s. Talk about stringing the same data together with two different narratives. From this view, cutting-edge science justified limiting the freedom of all but upper-class white men. Women evolved to be wives (and not scientists or scholars) and to carry out evolution’s plan. Natural and sexual selection conveniently favored what society already did. The scientific value of Descent is impossible to untangle from the oppression that it inspired. ²⁰

Perhaps more than any other science, evolutionary science is a “collection of stories about facts.” These stories are difficult to separate from the facts and, indeed, become the facts without as much burden of proof placed on them as some facts. For example, men are on average larger than women (fact) because of large, winning male ancestors (fiction—i.e., hypothesis or possible explanation—which is far more difficult to establish as fact). ²¹