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The Human Tree

The human family tree is complicated and only getting more complex as researchers like Harvard Medical School's David Reich unearth evidence hidden in the genome of interbreeding — not just between different human populations, but between humans and other hominins, writes Carl Zimmer at Discover magazine.

For a while, researchers traced the spread of people out East Africa to Europe and beyond, supplanting the Neandertals that had lived in Europe. But then they noticed that some stretches of the human genome from certain populations looked a bit like the Neandertal genome. "We were suspicious of the result," Reich tells Zimmer. "We found signals of mixture and then worked very hard to make them go away." But they eventually concluded that about 2.5 percent of the human genome from European and Asian populations is from Neandertals.

Making things more complex was the discovery of a new hominin line, now called the Denisovans. While there appears to be no overlap in the genomes of the Denisovans and Europeans, Africans, or Asians, pieces of the Denisovan genome were found in Australian Aborigines and the Mamanwa people from the Phillipines.

And the picture may become even more complicated, as there could be other hominins out there waiting to be discovered and analyzed. "The world is full of things like Denisova Cave," Reich tells Zimmer. "There must be thousands of other bones out there."