In 1997, scientists found the first scrap of Neanderthal DNA in a fossil. Since then, they have recovered genetic material, even entire genomes, from a number of Neanderthal bones, and their investigations have yielded a remarkable surprise: Today, 1 to 2 percent of the DNA in non-African people comes from Neanderthals.

That genetic legacy is the result of interbreeding roughly 50,000 years ago between Neanderthals and the common ancestors of Europeans and Asians. Recent studies suggest that Neanderthal genes even influence human health today, contributing to conditions from allergies to depression.

In the movie Sliding Doors (1998), a woman named Helen, played by Gwyneth Paltrow, rushes to catch a train on the London Underground, but just misses it, watching helplessly from the platform as the doors slide shut.

For much of the past five million to seven million years over which humans have been evolving, multiple species of our forebears co-existed. But eventually the other lineages went extinct, leaving only our own, Homo sapiens, to rule Earth. Scientists long thought that by 40,000 years ago H. sapiens shared the planet with only one other human species, or hominin: the Neandertals. In recent years, however, evidence of a more happening hominin scene at that time has emerged. Indications that H. erectus might have persisted on the Indonesian island of Java until 25,000 years ago have surfaced. And then there's H. floresiensis—the mini human species commonly referred to as the hobbits—which lived on Flores, another island in the Indonesian archipelago, as recently as 17,000 years ago.

Now researchers writing in the journal Nature report that they have found a fifth kind of hominin that may have overlapped with these species. (Scientific American is part of Nature Publishing Group.) But unl