Is it because Darwin proposed that human beings were descended from higher primates? That claim, which Darwin developed in The Descent of Man in 1871, certainly raised plenty of ire at the time and since—and not just among dyed-in-the-cloth evangelicals. The entire foundation of Romantic liberalism, not to mention the Thomist position of Las Casas and others, was that man is part of nature but also above nature, because of the spiritual essence of his soul.
