In his seminal work, On the Origin of Species (1859), Charles Darwin elucidated the theory of evolution by natural selection, explaining how organisms better adapted to their environment are more likely to survive and pass on their genes. What he didn't explain, however, was human evolution — that was addressed in his second but much less famous book, The Descent of Man, and Selection in Relation to Sex (1871).

The Descent of Man sought to answer a question that plagued his initial theory of natural selection: Why do traits like a peacock's tail or stag's horns exist when they have no obvious benefit and may even be a hindrance to avoiding predators? Darwin theorized that these characteristics helped in landing a mate — that they impacted "sexual selection" as he called it. Like his previous work, this theory was incredibly innovative, but because of its mix of observational science and Victorian social belief, The Descent of Man has managed to be both controversially ...