Given Darwin’s reluctance to discuss human evolution in The Origin why did he then decide to publish an account of the topic in 1871? By this time several other scientists, including Thomas Huxley and Ernst Haekel, had expanded Darwin’s theories to the human species. In the meantime Darwin had published on fertilisation of orchids, climbing plants and variation in domestic species. All the while, of course, being limited to a few hours work a day due to ill health; if he had been well The Descent would probably have come out sooner.
But why write it at all? Darwin writes in the introduction to The Descent that writing the book was ‘all the more desirable as I had never applied these views to a species taken so singularly’. But others argue that perhaps Darwin felt previous accounts were not up to scratch. In particular, a major topic of debate still was whether humans were one or more species, a topic Darwin had long had strong views on.
Britain was more settled politically and socially by the late 1860s so, in The Descent, Darwin could finally set his cards out on the issue of human evolution and the races without fear of political reprisals. A major theme in the book then is detailing evidence to support the (correct) conclusion that all humans are a single species, descended from the same ape-like ancestor. Darwin drew his evidence from three main categories; similarities between humans and other primates, similarities in embryological development, and vestigial organs (part of the animal which still develop to some degree but seemingly have no purpose). With this three pronged attack Darwin provided very strong evidence for man’s common ancestry with the living apes, and that all human populations were more closely related to each other than to any living primate; they were the same species.
Darwin’s explanation for the differences between the races was his theory of sexual selection. The book’s full title gives away the plot; The Descent of Man and Selection in Relation to Sex. The book is actually two in one. In the first half Darwin deals with the similarities and differences between humans, apes and other primates and then again between human populations. In the second half Darwin outlines his theory of sexual selection, by which adaptation to increase the chance of mating successfully can act independently or even against adaptation to increase survival (natural selection). Darwin illustrates this new theory with many examples from the animal kingdom, but he also uses it to explain racial differences. The reason native people from different continents look different is due to differences in mating preferences.
One major omission from The Descent is a discussion on human fossils. By this time fossils of Homo neanderthalensis had been found in Germany but there was no mention of them. Why leave these out? Surely they are pretty good evidence for human descent from another species?
Despite considering himself a geologist, Darwin was very sceptical that the fossil record would provide conclusive evidence to support his theory. The incompleteness of the fossil record as he saw it received two whole chapters of discussion in The Origin. In addition, in the 1860s a great hunt was underway for ancient human remains, mostly being paid for by rich individuals who would offer big rewards for new finds. As such, counterfeit fossils and tools were common and there are a number of well known hoaxes which fooled some quite qualified scientists. Finally, Thomas Huxley, who was a trained comparative anatomist, had dismissed the Neanderthal finds as just a wild, modern human.
Given Darwin’s lack of faith in the fossil record, the common problem of hoaxes and Huxley’s conclusions, Darwin could skate past the issue of fossil human remains without limiting his evidence. By relying on comparisons with apes he could successfully demonstrate shared ancestry. He had no need to risk basing his argument on a few, as yet unverified, remains.
One of the most amazing of Darwin’s predictions in The Descent is his prediction that humans evolved in Africa:
‘In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probably that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man’s nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere.’
At that time there was no evidence to support this claim, yet by simple observation of the range and distribution of man’s closest relatives and application of his theories, Darwin correctly predicted what we now know is widely supported.