F Rosa Rubicondior: Human Evolution - More Evidence of Interbreeding

Sunday 7 April 2019

Human Evolution - More Evidence of Interbreeding

Some of the last Denisovans may have intermingled with modern humans on mountainous New Guinea or nearby islands.
Photo credit: Dozier Marc/Hemis.fr/Getty Images
Moderns said to mate with late-surviving Denisovans | Science.

The human evolutionary story just got a lot more complicated and a lot more interesting than we thought.

Before we discovered that non-African modern humans hybridised with Neanderthals we thought it was quite straightforward. Modern humans evolved in Africa then a small group migrated out and spread all over the world.

Then we discovered the Denisovans - a different species of human that lived in Asia and with which we also interbred, making the non-African humans the result of hybridisation between three related but different species.

That picture just got a new layer added by a team led by population biologist Murray Cox of Massey University in Palmerston North, New Zealand and molecular biologist Herawati Sudoyo of the Eijkman Institute for Molecular Biology in Jakarta. They believe they have found evidence in the DNA of people from Papua-New Guinea that Denisovans were not a single group but were in fact three distinct populations as different from one another as they are from Neanderthals. They presented their findings to the American Association of Physical Anthropologists last week.

This would mean that at least some non-African humans are the result of interbreeding between five distinct human populations, even five distinct species which had evolved in Eurasia from an initial earlier dispersal.

The team also believe they have evidence that the last such interbreeding occurred as recently as 15-30,000 years ago, meaning that moderns and Denisovans co-existed for some considerable time.

If confirmed, this would suggest that the archaic human ancestor, Homo erectus, diversified into several sister species of Homo, one of which may have been the Neanderthals, one the Denisovans (D0) and the others so far only identified as D1 and D2 from this study. Meanwhile H. sapiens was evolving from H. erectus in Africa.

Analysis of the DNA suggests that D1 and D2 diversified some 283,000 years ago and that D0 and D2 diversified some 300,000 years ago. The DNA of D1 is found only in Papua-New Guinea, and still exists in large chunks in the chromosomes, suggesting it hasn't had much time to mix and disperse within the genome, the team suggest that this is evidence that an isolated population of D1 survived isolated in the remote mountains of New Guinea or on nearby islands.

There are other interpretations of this data, however, they all involve interbreeding between different species or subspecies of Homo, so one thing is becoming clear - as humans diversified and spread geographically in successive waves, there was frequent interbreeding with varying degrees of success, just as you would expect of a ring species, or a genus at a point in its evolutionary diversification into separate species.

Other studies have suggested that a similar process was going on in Africa as humans spread, evolved in isolation, then merged again when they came back into contact, hence the discovery of 300,000 year-old remains of anatomically modern humans in Morocco recently.

However you look at the evidence, the notion of a single ancestral species for H. sapiens is now untenable, let alone the nonsensical notion of a single ancestral couple. As this reality sinks in, it will be interesting to see how Christianity and Islam, with their dependence on the notion of a single ancestral couple, survive and adapt, if they ever do.

It's hard to see how the notion of original sin can be presented as some sort of metaphor, or how they explain how and where H. sapiens became the special creation of a deity and ceased to be the result of a biological process indistinguishable from the processes that produced all other species.

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