It is the oldest and one of the most extreme case of interbreeding found so far in the tangled tale of human origins. Around 700,000 years ago, a group of ancient humans that would later give rise to both the Neanderthals and Denisovans mated with an unidentified group of hominins.
This mystery group had evolved separately from their new sexual partners for over a million years. Despite this, the two groups were able to interbreed 鈥� although the hybrids may have suffered poor health.
The Neanderthals and Denisovans belong on a distinct branch of the human family tree that split away from our 鈥榤odern human鈥� branch within the last million years. Neanderthals lived in Europe and west Asia, while Denisovans lived in east Asia. Previous studies revealed both species interbred with modern humans and with each other. They also suggest that Denisovans interbred with an unknown population of 鈥渟uper archaic鈥� hominins, the identity of which remains unknown.
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Once more, with feeling
To find out if any other inter-group sex went on, Alan Rogers of the University of Utah in Salt Lake City and his colleagues compiled DNA from four sources: ancient Neanderthals and Denisovans, modern Europeans, and a modern African group called the Yoruba.
The team first asked if the known instances of interbreeding were enough to explain how various genes were distributed among the four populations. They found that many anomalous patterns remained. However, adding an extra episode of interbreeding was enough to get a good fit.
The additional bout of interbreeding took place between the shared ancestors of Neanderthals and Denisovans, after they had split from our branch. Their sexual partners came from the same mystery population of 鈥渟uper-archaic鈥� hominins that the Denisovans later interbred with.
鈥淭his is the only model I鈥檝e come up with that fits well, and no one else has come up with a model that explains these data this well,鈥� says Rogers.
However, Serena Tucci of Princeton University in New Jersey says the finding needs to be confirmed using multiple methods.
X, meet Y
Most anthropologists agree that hominins evolved in Africa within the last 13 million years. The first species to migrate to other continents was probably Homo erectus, which was present in Africa about 2 million years ago, and reached Dmanisi in Georgia by 1.8 million years ago. H. erectus even reached Indonesia, .
鈥淢y results are consistent with the view that these super-archaics are descendants of that original out-of-Africa migration,鈥� says Rogers. In that case, the super-archacis may have been H. erectus. Rogers鈥� analysis shows the super-archaics split from other hominins 2 million years ago, about when H. erectus emerged.
However, it is also possible that the super-archaics are a group not yet identified in the fossil record, says Tucci.
鈥淣别补苍诲别谤蝉辞惫补苍蝉鈥�
Meanwhile, the shared ancestors of Neanderthals and Denisovans split from the ancestors of modern humans perhaps 800,000 years ago. Because this split probably occurred in Africa, it implies that the ancestors of Neanderthals and Denisovans then made their own way out of Africa at some point. When they reached Eurasia, they met the super-archaics 鈥� possibly H. erectus 鈥� and mated with them.
Anthropologists used to think Homo heidelbergensis, known from Africa and Europe, were the immediate ancestors of Neanderthals. However, more recent studies indicate that many of the remains attributed to H. heidelbergensis are actually early Neanderthals, from just after they had split from the Denisovans. That means the shared ancestors of Neanderthals and Denisovans remain unknown in the fossil record. Rogers simply calls them 鈥淣别补苍诲别谤蝉辞惫补苍蝉鈥�.
Only within the last 200,000 years did modern humans migrate beyond Africa, whereupon they encountered Neanderthals and Denisovans. They probably never met any Eurasian H. erectus, who were extinct by the time they arrived.
Extreme interbreeding
When humans and Neanderthals mated, they had been evolving separately for at most 750,000 years. In contrast, the Neandersovans and super-archaics had been evolving separately for perhaps 1.3 million years. That makes them the most distinct hominin groups known to have interbred.
The hybrids born from these matings may have had health problems, says Rogers. 鈥淭here鈥檚 good evidence that hybrids between either Neanderthals and Denisovans and modern humans seem to have been less healthy,鈥� he says, as evolution has destroyed many of the introduced genes. 鈥淪ince the super-archaics had been separated even longer from Neandersovans, you might expect that that would have been a greater problem for them.鈥� However, for now this is only a hypothesis.
The finding also implies that DNA from the super-archaic population still exists in preserved Neanderthal or Denisovan genomes, or even in modern humans. However, Rogers says finding it will be difficult because the passage of time will have broken it into small chunks, dispersed throughout the genome. 鈥淚鈥檓 not going to say it鈥檚 impossible,鈥� he says, but it will be harder than finding the Neanderthal DNA many of us carry.
bioRxiv
