RACHEL MARTIN, HOST:
Prehistoric humans didn't always stick to themselves. There's a lot of evidence that they intermingled and had relations with other species of human relatives living alongside them. NPR's Merrit Kennedy reports scientists believe ancient humans in West Africa procreated with an unknown human species about 50,000 years ago.
MERRIT KENNEDY, BYLINE: Researchers have found traces of what they call ghost DNA in the genomes of present-day West Africans - ghost because they think the DNA comes from a human relative unfamiliar to scientists. Here's Sriram Sankararaman, a computational biologist at UCLA.
SRIRAM SANKARARAMAN: It doesn't seem to be particularly closely related to the groups from which we have genome sequences from.
KENNEDY: Over the past decade, we've learned that humans today - all around the world - carry DNA from other species that weren't as successful over the course of evolution. Present-day Europeans and Asians have Neanderthal DNA, and people in Oceania have genes from the Denisovans. Our understanding of the human family tree is growing more complicated, more tangled.
SANKARARAMAN: There are these branches that are splitting off, coming back together and interbreeding. And we're only beginning to piece together small pieces of this bigger puzzle.
KENNEDY: The UCLA researchers combed through genomes from more than 400 people for their paper in the journal Science Advances. Sankararaman says the method...
SANKARARAMAN: Goes along a person's genome and pulls out chunks of DNA, which we think are likely to have come from a population that is not modern human.
KENNEDY: Scientists hope to learn more about these previously unknown relatives. One way to do that would be to find physical evidence, like bones with DNA. That's been a challenge in Africa due to the climate, says Sharon Browning, a biostatistics professor at the University of Washington.
SHARON BROWNING: The bones have to stay intact, and the DNA within those bones has to stay intact to some extent.
KENNEDY: Browning adds the technology that can extract usable DNA from fossils is rapidly improving. That could one day help flush out the mysterious ancestors behind the ghost DNA.
Merrit Kennedy, NPR News.
(SOUNDBITE OF ERODED SKY'S "FUSIONS")
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.