The idea that humans and chimps interbred causes discomfort in some circles, even as science explores the potential benefits of hybrids and the blurring of what was once a bright line between species. In 1997, developmental biologist Stuart Newman did something relatively unusual for a scientist: He submitted a patent application for a technology he hoped never to use. In it, he laid claim to the humanzee, a chimera made by combining the embryonic cells of humans with those of chimpanzees. Though it was hypothetically possible to manufacture such a creature, he vowed to put the patent in a drawer...