Darwin, The Descent of Man, and Human Evolution

The following is a version of a talk I’m giving for Darwin’s 200th Birthday.

My field of study, Human evolution, owes its very existence to the ideas of Charles Darwin. Obviously, we can’t talk about human evolution without talking about the mechanism of natural selection. However, as important as he is to our field, Darwin was rather reluctant to talk about evolutionary theory as applied to humans. When asked whether he would talk about humans in his On the Origin of Species, he replied, “I think I shall avoid the whole subject, as so surrounded with prejudices, though I fully admit that it is the highest and most interesting problem for the naturalist.” And that is precisely what he did, writing simply, “Light will be thrown on the origin of man and his history.” Twelve years after publishing On the Origin of Species, he published a work called The Descent of Man, and Selection in Relation to Sex, which outlined the origins of humans- their development, their anatomy, their variation, and, most importantly to him, the mystery of their profound intelligence.

Even though Darwin had been careful to avoid mentioning where humans fit into his evolutionary scenario in Origin, the public was quick to catch on to the implication of his work. If Darwin was implying that all living organisms were subject to natural selection, surely humans were as well. The idea that we had descended from the same common ancestor and through the same processes as things like parasites, cockroaches, and rats was offensive to many- and still is! If humans were not part of natural selection, I doubt that we would still be hearing court cases over whether or not it should be taught in schools. Darwin recognized this, and states in the introduction to his work, “During many years I collected notes on the origin or descent of man, without any intention of publishing on the subject, but rather with the determination not to publish, as I thought that I should thus only add to the prejudices against my view.”

Darwin’s theory was antithetical to the prevailing knowledge at the time, which was that there was a Great Chain of Being, and that humans were at the top, right underneath angels and God. Darwin’s ideas rearranged this scenario so that instead of being at the pinnacle of a great chain, we were just another species at the tip of one of the branches on a huge evolutionary bush.

For many years after he published On the Origin of Species, Darwin was content to let Thomas Huxley, a comparative anatomist and Ernst Haeckel, an embryologist, advance his evolutionary ideas into the realm of humans. However, by 1871, Darwin decided that he could contribute something to the argument which he felt was not being said, and set out to write The Descent of Man. He was by no means the first scientist to suggest publicly that humans had evolved from other animals, but he was the first to couch this idea in terms of sexual selection.  Sexual selection is, very simply, the same thing as natural selection, but members of the opposite sex are acting as the selective agent instead of the environment.  He chose this particular framework in response to arguments that the human brain was too big to be the result of selection for mere survival. Even Alfred Russel Wallace, the co-discoverer of evolution, thought that evolution could only go so far. The gap between the intelligence of apes and that of humans was simply too wide to be bridged by natural selection, he said, and the human brain was so much larger and more complicated than what would have been sufficient for survival. Wallace attributed this gap to supernatural causes, but Darwin sought to bridge this gap by explaining both the origins of humans and their diversity in materialistic terms- natural and sexual selection

Darwin set out first to make his argument that humans had evolved from apes.  Darwin studied the comparative anatomy and embryology of our own species in relation to that of gorillas, chimps, and orangutans. He pointed out that humans had rudimentary structures which were really vestiges of our past lives. He pointed out that humans have a tailbone, even though we have no tail. He brought up how wisdom teeth in small-faced Europeans were decreasing in size and structure, and that very often their only “purpose” was to become infected and painful. Some of us can wiggle our ears, and some of us can flex our scalp muscle. He used all of these converging lines of evidence to conclude that humans were most closely related to the African apes- the gorilla and the chimpanzee- and that the common ancestor of all three animals would be found in Africa.

He concludes, rather eloquently, “In a series of forms graduating insensibly from some ape-like creature to man as he now exists, it would be impossible to fix on any definite point when the term “man” ought to be used. But this is a matter of very little importance… man still bears in his bodily frame the indelible stamp of his lowly origin.”

Darwin briefly outlines a few hypotheses about how humans originated.   We adopted a bipedal stance, for example, in order to carry tools and weapons.  We now know that the origin of bipedalism pre-dates the origin of obvious tool use and manufacture by a wide margin, but Darwin’s tool-carrying hypothesis has inspired many other similar hypotheses.  He suggested that we had lost our tail as the result of our upright posture:  We needed more support for our viscera, and so the tail bones were co-opted as muscle attachment sites.  He even suggested that we- and especially females- had become hairless as an ornamental adaptation which made us more attractive to the opposite sex.

However, Darwin spent most of the book explaining not how humans originated, but how they diversified. To do this, he used his theory of sexual selection, which he described in detail with regards to insects, birds, and mammals. He had to establish that sexual selection existed in nature- and it very much does. From there, it was just a short step to conclude that the same mechanisms which work in all other animals were also working in humans, and were the creative force behind the physical and mental diversity in the human species.

Darwin was unusual for his time, in that he thought that all humans were members of the same species, and had a shared common ancestry. He believed that each race had a different ideal of beauty, which had evolved along with each different population. He noted that male birds attracted and won females with their bright plumage or beautiful songs, and that male stags would fight for the privilege of mating. By this same token, women from different parts of the world were wooed by different physical traits, he explained, which resulted in a sort of “self-breeding” into different “varieties,” or races, of human. According to Darwin, human form was shaped by desire.

Darwin thought that the most important result of sexual selection was in human brain size. He made the argument that men are larger, stronger, and more pugnacious than females, because they had to fight for the opportunity to mate. Women were more tender and less selfish, because of their maternal instincts. Human males had to protect their mates and offspring, and because we had given up our bodily strength, this came down to out-thinking both predator and prey, as well as human competitors for the affection of females. Prehistoric men had to be able to observe, reason, and invent in order to keep their families safe and well-fed.

Many of Darwin’s ideas from Descent of Man were seminal in the field of biological anthropology and human evolution.  While Darwin was correct to say that humans were all of the same species, modern human biology takes this idea a step further and asserts that we are so similar as to be of the same race, or variety. There are no subspecies of human. We now know that different skin colors are not the result of sexual selection, but are in fact the result of natural selection. Humans who have lived near the equator benefit from having a protective layer of melanin in their skin which protects against UV radiation and degradation of an important vitamin known as folate.  Humans who lived at higher lattitudes benefit from having lighter skin, which allows sunlight to penetrate the skin and synthesize Vitamin D, which is important in bone health. However, his insights into sexual selection in general hold up very well, and studies of female choice and male-male competition are some of the most exciting research questions in primatology and zoology.

Darwin’s hypothesis that human brain size was the result of both natural and sexual selection for intelligent, creative men has been addressed time and time again. We now know that the differences between male and female cognition are much more nuanced than Darwin thought they were, and it’s probably more of a continuum than it is that there are two distinct categories.  However, that area of research is quite contentious, and since it is outside of my area of expertise, I will leave it simply at that.

Most importantly to me, Darwin’s insight into the similarity between humans and chimps and gorillas proved to be correct, and we now know that humans did, in fact evolve in Africa. The 1968 discovery of a hominid skull in South Africa’s Olduvai Gorge ushered African paleoanthropology to the forefront of human evolutionary studies. When Darwin was writing, the fossil record of human evolution was scant- there were a few Neandertal specimens, and a few Homo erectus fossils that had been found in Asia. We have since discovered so many fossils that the record of human evolution is one of the best-known of any animal. It wasn’t until we found these fossils that we were able to test some of Darwin’s hypotheses, as well as those of other early biological anthropologists. Once we were able to test them with fossil evidence, the field of human evolution really began to evolve.