Scientists have long envisioned the human “family tree” as a straight-line progression from the apelike australopithecines to the enigmatic Homo habilis to the famous Neanderthals, culminating in us, Homo sapiens. But this model is unlike the evolutionary patterns known for all other vertebrates—patterns that typically reveal multiple branchings and extinctions. In Extinct Humans, Ian and Jeffrey Schwartz present convincing evidence that many distinct species of humans have existed during the history of the hominid family, often simultaneously. Furthermore, these species may have contributed to one another’s extinction. Who were these different human species? Which are direct ancestors to us? And, the most profound question of all, why is there only a single human species alive on Earth now?