Read more: “10 biggest puzzles of human evolution“
COMPARING modern human DNA with ancient hominin sequences has revealed that between 1 and 4 per cent of the genome of everyone of non-African descent is inherited from Neanderthals (). Melanesians also have 7 per cent derived from Denisovans (). “This is an unequivocal signal that humans mated with these other populations,” says Richard Green at the University of California, Santa Cruz.
These studies suggest that mating between modern humans and our cousins was relatively infrequent, and possibly confined to a single time and place for each species. In the case of Neanderthals it probably happened more than 50,000 years ago in the Middle East.
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Not everyone is convinced, however. “As humans spread across Europe within the past 45,000 years, they would have met Neanderthals on every street corner,” says Mellars. “Yet there’s no evidence that interbreeding took place here.” Why? Green counters that this could just be a numbers game: if there were many more humans than Neanderthals, the DNA signal of mating in Europe would have been weak or lost entirely from the modern human genome.
But there is an alternative explanation for the presence of Neanderthal DNA in the human genome. Imagine that ancient hominin populations in Africa, each with a slightly different genetic makeup, were separated from one another. If one group gave rise to all the hominins that lived outside Africa, while other populations became the ancestors of all Africans, then even without subsequent interbreeding the non-African and Neanderthal populations would share some DNA that African populations would lack. This possibility was mentioned by Green and colleagues in their original paper and has been explored further by Andrea Manica at the University of Cambridge, who believes that it could explain the distribution pattern of Neanderthal genes found today.
“40% Maximum Neanderthal genes in modern humans”
But even if we accept that some interbreeding occurred – and most people do – does that make us hybrids? Martin Richards at Huddersfield University in the UK notes that the species concept is “very fuzzy”, making it difficult to draw neat lines between groups. One definition of species is a group that cannot mate and produce viable offspring with other species, so the genetic analysis calls into question whether Neanderthals and Denisovans were different species to humans at all. Indeed, Neanderthals are sometimes considered a subspecies of Homo sapiens.
The species issue is a distraction, according to Green. “We can define our genetic relationships with Neanderthals and Denisovans in exquisite detail without putting the label of species on these groups.” At a more visceral level, though, the question of whether or not our ancestors mated with other species is central to the way we think about ourselves.
