Researchers have confirmed that hundreds of thousands of years ago, Neanderthals mated with at least four other contemporary species of ancient humans, or hominids, and the evidence lives on in the genes of modern men and women.
A study published Thursday in the science journal PLOS Genetics shows how researchers from Cornell University analyzed the genomes, the complete genetic “map,” from Neanderthals, a prehistoric human ancestor called Denisovans, and modern humans.
Analysis of the genomes revealed new evidence of gene flow between these species, bolstering earlier theories that the species intermated. The researchers found 3% of the Neanderthal genome came from interbreeding with other ancient humans that lived at the same time.
The new study estimates this intermixing happened between 200,000 and 300,000 years ago — far earlier than previous estimates indicated.
The researchers also found that 1% of the Denisovan genome contained genetic material that came from an "archaic human ancestor" that was neither human, nor Neanderthal, nor Denisovan. They suggested it came from Homo erectus, an early human ancestor believed to be the first to spread to what is now Eastern Europe and parts of Asia.
Homo erectus looked much like we do today, but with elongated legs and shorter arms. They are believed to have outlived contemporary hominids, dying out as recently as 117,000 years ago.
The new study suggests that 15% of the genetic pieces that came from Homo erectus have been passed on to humans today. They suggest it split off from the lineage that would become modern humans about 1 million years ago, which would fit the timeline for intermingling with its contemporary hominid species.
The genome for Homo erectus has not been sequenced so it is difficult to precisely figure out how all the different human ancestors got together. But the researchers theorize that migration habits combined with the fact that all four species did overlap for several thousand years made it likely that they intermingled.