In a laboratory in the upper recesses of the American Museum of Natural History, away from the public galleries, Dr. Ian Tattersall, a tall Homo sapiens, stooped and came face to face with a Neanderthal man, short and robust but bearing a family resemblance — until one looked especially closely. A paleoanthropologist who has studied and written about Neanderthals, Dr. Tattersall was getting his first look at a virtually complete skeleton from this famously extinct branch of the hominid family. Nothing quite like it has ever been assembled before, the foot bones connected to the ankle bones and everything else...