A paper published in Nature tells how a team (Prüfer, Racimo et al)sequenced the genome of a Neanderthal from Denisova and came up with some interesting findings.
Our interbreeding ancestors
The most unexpected finding results from a comparison of genomes. We, the modern African Homo Sapiens never left the continent and therefore were never in the territories occupied by Neanderthals or Denisovans. They therefore had no chance to mix with them and as a consequence, their MtDNA does not have traces of Denisovan or Neanderthal MtDNA. Any similarities between the DNA of all three comes from a common ancestor to all which should be equally distant to all. Well, it isn't.
The Denisovans have sequences that are more distant than expected. This can only be explianed if the Denisovans interbred with some other population that had split from their common ancestor with modern African humans about 1 Million years ago.
The only candidate within this time frame is our old friend: Homo erectus who left Africa around that time. And this is what the team suggests: Denisovans mated with H. erectus.
Between 2.7 and 5% of their genome comes from H. erectus. Now, saying that y% of your genome comes from H. erectus means that the percentage of Denisovan's H.erectus parentage is much higher than y% because most of the genome would overlap, and maybe some lineages became extinct so we have no trace of it. How much greater than 2.7-5%? I can't say, but let's take a guess and place it at 10%.
This means that a considerable ammount of inter species intercouse took place (if the MtDNA is erectus, then the female erectus passed on their genome to hybrid offspring. So Denisovan males mated with H. erectus females.
So, did the lice of Homo erectus move from them to us directly (humans mating with erectus) or indirectly: Denisovans mated with H. erectus, got their lice and then we mated with them and caught their lice?.
Neanderthals and Denisovans split from our line about 400 kya. They split between them (Denisovans and Neanderthals) happened about 300 kya.)
So this is the current model:
- Hominids appeared in Africa
- A group left the continent (H. erectus)
- Those that remained in Africa kept on evolving. Some left for Asia (They became Denisovans and Neanderthals)
- In Asia they had sex with Homo erectus and got some if its MtDNA into its genome
- Modern Humans then left Africa and bred with both Denisovans and Neanderthals, getting their MtDNA into our genome
- Modern African Humans did not mix with anyone.
We already knew that Papuans and Australians had a Denisovan influx in their Genes (about 6%), now the paper mentions that Amerindians (0.2% approx.) and Asians also have Denisovan mTDNA. Which means that the modern humans who passed through Altai on their way to Western Asia also had sex with female Denisovans.
From my point of view, the fact that H.erectus survived to intermingle with Denisovans about 40 kya is amazing, and it is likely tht H. erectus or even Denisovans reached America long before the modern humans did. Perhaps a small Denisovan population interbred with H. sapiens paleoindians in the American continent and passed on their mTDNA to them there and not in Asia.
Perhaps the gene influx is even older (how about H.habilis?), see my post The first Asians were not Homo erectus.
Lets wait and see what further gene sequencing tells us.
See Supplementary information 16a on page 139. There is a lot of data if you are interested.
A Merry Christmas an a Happy new Year to all!
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