I have started reading Charles Darwin’s 1871 book Descent of Man and Selection in Relation to Sex. Darwin published this book well after he introduced the idea of evolution by natural selection in his 1859 book On the Origin of Species. Despite persistent claims to the contrary, Darwin initially dealt very little with human evolution, partially to avoid controversy (as he admits in the introduction of Descent), but also because he was concerned with much broader patterns in nature than the origin of humans. He sums this point up very nicely in the introduction of Descent:
When we confine our attention to any one form , we are deprived of the weighty arguments derived from the nature of the affinities which connect together whole groups of organisms…
Darwin’s insightful comment on the utility of comparisons across many species, geographic areas, etc., while a staple of evolutionary research, is still greatly underappreciated, even by scientists. As Darwin presciently realized, the paucity of species closely related to humans, by prohibiting independent comparisons across many species, is the main problem with much evolutionary psychology research today. To take one example, there are many competing hypotheses as to why selection would have favored large brains in the lineage leading to modern humans and not other great apes - greater tool use, need to communicate, bipedalism allowed longer period of childhood development, and so on. The problem is that many of these selective pressures happened concurrently, and there is often no good way to rule out competing hypotheses since there exist no other big-brained species with which to make an independent comparison. Fortunately, biologists are no so impoverished in other areas of research. For example, we can infer that red, tubular flowers represent an adaptation to hummingbird pollination because those traits have evolved together independently many times across angiosperms.
Despite the dearth of data on human evolution in 1871, Darwin marshals several compelling arguments for the evolution of man from common ancestors with chimpanzee, gorilla, and other apes. Although now accepted as a scientific fact backed up by exquisitely detailed fossil and molecular data, even prominent 19th century naturalists like Wallace, who independently proposed natural selection, doubted the evolutionary origin of man. Darwin, often borrowing from contemporaries like Huxley and Häckel, dispatches such ideas by synthesizing disparate information, usually collected for entirely different reasons. For example, it was well known then that humans and apes suffered from more similar diseases (genetic and infectious) than, say, man and mouse or man and bird. Furthermore, diseases tend to pass more easily between man and ape than between man and more distantly related species (e.g. HIV evolved from Simian Immunodeficiency Virus). Darwin points out that this fact is completely explicable from the standpoint of common descent, but utterly mystifying under the view of special creation of man.
Although Darwin strongly affirms his belief in the natural origin humanity, still a controversial topic among the public, he shows characteristic caution often not observed in later students of human evolution. Brain size, often considered the pinnacle of (human) evolutionary progress and an indicator of intelligence by many biologists and anthropologists, has been abused to justify anthropocentrism and, much worse, racism or eugenics. Darwin however states that “no one supposes that the intellect of any two animals or of any two men can be accurately gauged by the cubic contents of their skills” (pg. 42). In contrarian fashion, he even notes:
[T]he wonderfully diversified instincts, mental powers, and affectations of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a small pin’s head. Under this point of view, the brain of an ant is one of the most marvellous atoms of matter in the world, perhaps more so than the brain of man.
All this is not to say that Darwin’s language never comes off as Europocentric or even racist, or that he didn’t appreciate the unique qualities of human intelligence. He did. However, even in the first few chapters, he gets three things right: 1) that human descended from a common ancestor, like all life on earth, is the only explanation that makes sense of all the data; 2) even though human variation can be explained via natural selection, we should use much caution in naively interpreting the meaning of heritable differences between individuals or races; and 3) many “human” qualities such as intelligence (and morality) are not unique to humans. Scientists and nonscientists alike would do well to remember these lessons.