A chart that sums up who had sex with whom during the PleistoceneS

Call it the Pleistocene hanky panky chart. Now that scientists have sequenced a complete Neanderthal genome, we have more evidence than ever that early Homo sapiens had children with Neanderthals and Denisovans tens of thousands of years ago.

This chart, from Nature, is part of a paper published this week by evolutionary biologist Svante Pääbo's team at the Max Planck Institute in Germany, where they sequenced an entire Neanderthal genome from a well-preserved toe bone. It shows three known early human groups — Homo sapiens, Neanderthal, and Denisovan — as well as an unknown one that might be Homo erectus. We see when these groups diverged, and when they (ahem) came together again.

By examining the new Neanderthal genome, Pääbo's team was able to determine that genes from this early human group had entered the Homo sapiens genome, just as Denisovan genes had. Though previous sequences from both Neanderthals and Denisovans had suggested that genes from these human groups had found their way into Homo sapiens, this latest paper bolsters that evidence. The more we learn about our distant ancestors' genomes, the more it seems that Neanderthals, Denisovans and Homo erectus were our distant human cousins rather than separate species.

Also, this paper's findings suggest that dramatically different human groups likely formed some alliances when they met each other. Even if some of those groups died out, they spent thousands of years sharing cultures and habitats with Homo sapiens. They are part of our heritage.

We've also learned more about the life of the Neanderthal whose DNA we sequenced. Writes Carl Zimmer in the New York Times:

Dr. Paabo and his colleagues can distinguish the origin of each piece of DNA — whether it came from the Neanderthal's mother or father. With an X chromosome from each parent, the toe belonged to a female.

A closer look at these two sets of DNA revealed that the Neanderthal female was extremely inbred. Her two parental sets of genes were identical for long stretches. Such similarity can only come about when close relatives have children.

"We don't know if this is typical of all Neanderthals, or just this population in Siberia," said Dr. Paabo. It will take more high-caliber Neanderthal genomes to settle that question.

Anthropologists have long believed that Neanderthals lived in small, isolated groups in Europe. Inbreeding would have been common under such conditions.

Read the complete scientific study in Nature.