Scientists found the first scrap of Neanderthal DNA in a fossil in 1997. Since this discovery, they have recovered genetic material, even entire genomes, from a number of neanderthal bones. These investigations have yielded the remarkable surprise: 1 to 2 percent of the DNA in non-African people comes from Neanderthals.
This genetic legacy is the result of interbreeding about 50,000 years ago between Neanderthals and the common ancestors of Europeans and Asians. Recent studies even suggest that human health today is influenced by Neanderthal genes, contributing to conditions from allergies to depression.
Now, scientists have found that genes have flowed both ways. In a study published on Nature, scientists reported that another instance of interbreeding left Neanderthals in Siberia with chunks of human DNA. This exchange seems to have taken place 100,000 years ago. This is confusing, because evidence indicates the ancestors of todays non-Africans did not expand out of Africa until roughly 55,000 years ago. Thus, it is possible that these Neanderthals acquired DNA from a mysterious early migration of humans.
Humans and Neanderthals split from a common ancestor in Africa around 600,000 years ago. At some point after this, the ancestors of Neanderthals spread to Europe, the Middle East and Central Asia. Along the way, Neanderthals took on the stocky anatomy we are all familiar with. The last Neanderthals appear to have died around 40,000 years ago.
Scientists Dr. Siepel and Dr. Castellano found an unexpected example of so-called gene flow: the Altai Neanderthals shared some mutations with living Africans, but not Europeans or Asians. This pattern suggests that an African lineage of humans interbred with the Altai ancestors, after they split from Neanderthals. They came to the conclusion that when DNA gets passed down generation to generation, it gets shuffled into new arrangements, which can be built into some sort of timeline. Thus, scientists estimated that humans and the ancestors of Altai Neanderthals interbred about 100,000 years ago - way before people were thought to have left Africa.
I believe this information throws a wrench in many previous findings and understandings about Neanderthal interbreeding. But, like many other scientific data, new findings alter beliefs every single day. I always believed that Neanderthals simply interbred with human ancestors to make the humans on Earth today, but I never realized that Neanderthals could possibly have human DNA. It is hard to imagine, because this must have happened before non-Africans expanded out of Africa, therefore the Neanderthals must have acquired human DNA from a mysterious early migration of humans. It is amazing how DNA mutations can serve as a timeline, and solve many genetic mysteries.