Darwin never wrote much about the brain, but Darwin’s nemesis, Richard Owen, tried in 1861 to protect humans from Darwin’s threatening ideas by arguing that human brains differ fundamentally from those of other apes. This argument provoked a spirited attack by Darwin’s “bulldog,” T. H. Huxley. Darwin did not comment publicly on this controversy, but for the second edition of his Descent of Man, Darwin asked Huxley to write an essay “on the resemblances and differences in the structure and the development of the brain in man and apes.” This essay was a forceful attack on Owen’s argument and showed convincingly that human brains are like fairly typical ape brains, although larger. Thus the Darwinians began to contemplate evolving brains. This colloquium surveyed what has been learned about brain evolution in the 150 years since the Owen-Huxley debate, bringing together leading scientists whose work illuminates the twin questions of how and why complex nervous systems evolved.