Scientists have discovered two previously unrecorded human ancestor groups by analysing the genomes of modern human populations.
The researchers used DNA analysis to trace our human ancestors as they migrated from Africa to southeast Asia and on to Australasia, interbreeding in 'mixing events' with populations that they encountered along the way. They found that the ancestors of modern humans mixed with at least four different archaic human (or hominin) groups.
'Each of us carry within ourselves the genetic traces of these past mixing events,' said Dr João Teixeira at the University of Adelaide in Australia, and first author of the paper published in PNAS. 'These archaic groups were widespread and genetically diverse, and they survive in each of us. Their story is an integral part of how we came to be.'
Two of the ancestral groups identified by the study - the Neanderthals and the Denisovans are already known to archaeologists, while the others have been detected only as DNA traces in modern populations and remain unnamed.
The Neanderthals were the first group that the ancestors of modern humans mixed with. 'All present-day populations show about two percent of Neanderthal ancestry which means that Neanderthal mixing with the ancestors of modern humans occurred soon after they left Africa,' Dr Teixeira said.
The likely date and location of this first mixing event were predicted by studying a human skeleton found in southern Russia, said Dr Teixeira, writing in The Conversation. It is 45,000 years old and has large portions of Neanderthal DNA. This dated the mixing in the region to no more than 230-430 generations before its existence, or 50-55,000 years ago.
As the human ancestors travelled further east they met and mixed with at least three other groups. 'Island southeast Asia was already a crowded place when what we call modern humans first reached the region,' said Dr Teixeira.
These mixing events were localised to southern and east Asia, the Philippines and Indonesia with the Denisovans and with previously unknown groups that the team labelled Extinct Hominins 1 and 2.
'We knew the story out of Africa wasn't a simple one, but it seems to be far more complex than we have contemplated,' said Dr Teixeira.