"When Charles Darwin entered the world 200 years ago … all men were men and brothers because all were descended from Adam. By the time Darwin had reached adulthood, however, opinions around him were growing more equivocal. … By the mid-19th century, many influential voices denied that the enslaved African was a brother, and it was broadly taken for granted that as a man, he was of an inferior sort to his white master. … Evolutionary thinking enabled [Darwin] to rescue the idea of human unity, taking it over from a religion that no longer provided it with adequate support, and put the idea of common descent on a rational foundation."