It is now widely accepted that the species Homo sapiens originated in Africa and eventually spread throughout the world. But did those early humans interbreed with more ancestral forms of the genus Homo, for example Homo erectus, the "upright walking man," Homo habilis, – the "tool-using man" or Homo neanderthalensis, the first artists of cave-painting fame?
Direct studies of ancient DNA from Neanderthal bones suggest interbreeding did occur after anatomically modern humans had migrated from their evolutionary cradle in Africa to the cooler climates of Eurasia, but what had happened in Africa remained a mystery – until now.
In a paper published in the Proceedings of the National Academy of Sciences, or PNAS, a team led by Michael Hammer, an associate professor and research scientist with the UA's Arizona Research Labs, provides evidence that anatomically modern humans were not so unique that they remained separate.
"We found evidence for hybridization between modern humans and archaic forms in Africa. It looks like our lineage has always exchanged genes with their more morphologically diverged neighbors," said Hammer, who also holds appointments in the UA's department ofecology and evolutionary biology, the school of anthropology, the BIO5 Institute and the Arizona Cancer Center.
Hammer added that recent advances in molecular biology have made it possible to extract DNA from fossils tens of thousands of years old and compare it to that of modern counterparts.
However, "We don't have fossil DNA from Africa to compare with ours," he said. "Neanderthals lived in colder climates, but the climate in more tropical areas make it very tough for DNA to survive that long, so recovering usable samples from fossil specimens is extremely difficult if not impossible."
"Our work is different from the research that led to the breakthroughs in Neanderthal genetics," he explained. "We couldn't look directly for ancient DNA that is 40,000 years old and make a direct comparison."
To get past this hindrance, Hammer's team followed a computational and statistical approach.
"Instead, we looked at DNA from modern humans belonging to African populations and searched for unusual regions in the genome."
Because nobody knows the DNA sequences of those now extinct archaic forms, Hammer's team first had to figure out what features of modern DNA might represent fragments that were brought in from archaic forms.
"What we do know is that the sequences of those forms, even the Neanderthals, are not that different from modern humans," he said. "They have certain characteristics that make them different from modern DNA."
The researchers used simulations to predict what ancient DNA sequences would look like had they survived within the DNA of our own cells.