In The Origin of Species, Charles Darwin refused to discuss human evolution, believing the subject too "surrounded with prejudices." He had been reworking his notes since the 1830s, but only with trepidation did he finally publish The Descent of Man in 1871. The book notoriously put apes in our family tree and made the races one family, diversified by "sexual selection"— Darwin’s provocative theory that female choice among competing males leads to diverging racial characteristics. Though less well known than The Origin of Species, The Descent of Man continues to shape the way we think about what it is that makes us uniquely human.