Early humans faced countless challenges as they fanned out of Africa: icy conditions, saber-tooth cats, and, according to a new study of ancient skeletons, an unusually high number of birth defects, both debilitating and relatively inconsequential. It’s unclear why such abnormalities seem to be so common, but scientists say one strong possibility is rampant inbreeding among small hunter-gatherer groups. “This paper represents a valuable compilation,” says Vincenzo Formicola, an anthropologist at the University of Pisa in Italy who wasn’t involved in the new work. “Many cases reported in the list were unknown to me and, I assume, to many people...