Earlier this year I wrote a post about the Sima de los Huesos remains and its possible linkeage to Homo erectus. I was commenting on a paper that had sequenced the mtDNA of the remains from this Spanish site and found it "to be closer to Denisovan mtDNA than to Neanderthal ⁄ Human mtDNA. This is quite hard to explain: how did Denisovans get more of this mtDNA than the Neanderthals did considering that Denisovans are located in Altai while Neandertals are located in between Spain and Altai?".
Well it appears that the mtDNA was closer to Denisovans, but today I read another article (DNA from Neandertal relative may shake up human family tree, by Ann Gibbons, Science 11.Sep. 2015) which says that:
After 2 years of intense effort, paleogeneticist Matthias Meyer of the Max Planck Institute for Evolutionary Anthropology has finally sequenced enough nuclear DNA from fossils of a tooth and a leg bone from the pit to solve the mystery. The task was especially challenging because the ancient DNA was degraded to short fragments, made up of as few as 25 to 40 single nucleotides. (Nucleotides—also known as base pairs—are the building blocks of DNA.) Although he and his colleagues did not sequence the entire genomes of the fossils, Meyer reported at the meeting that they did get 1 million to 2 million base pairs of ancient nuclear DNA.
They scanned this DNA for unique markers found only in Neandertals or Denisovans or modern humans, and found that the two Sima fossils shared far more alleles—different nucleotides at the same address in the genome—with Neandertals than Denisovans or modern humans. “Indeed, the Sima de los Huesos specimens are early Neandertals or related to early Neandertals,” suggesting that the split of Denisovans and Neandertals should be moved back in time, Meyer reported at the meeting...
This is very interesting since it pushes the ancestors of Denisovans and Humans split from the linage leading to Sima de los Huesos earlier than expected. But also, (because that explains how the mtDNA that is closer to Denisovans was found in Sima de los Huesos remains) they did interbreed after they split apart.
The allegory of an evolutionary tree is not the best way to describe our origins. It is more like a tree with criss-crossing branches that split from a common trunk much earlier than expected.
The early splitting is no problem from my point of view, in fact I have written some posts about an earlier origin of humans, but it is a problem for those who base their "molecular clocks" and adjust them based on dates which, are in fact quite wrong. The process can be summarized as follows:
We look at populations (say Amerindians) and jot down their haplogroup markers, and we assume that they mutated when they reached America and then, we guess the date they entered America (say 15 kya). With this we calibrate our clock. We again look at the humans closest to Africa and jot down their haplogroups markers, and once again guess the date these people's ancestors left Africa (say 70 kya), and again calibrate our clock. We take another look at the oldest fossils of AMH in Africa (195 kya) and jot down the most divergent African haplogroups' markers, we recalibrate our clock again. But, as you can see, there are many assumptions in all of this (the dates and, above all, the assumption that these haplogroups are specifically human and mutated recently < 200 ky!).
But what if the origin of H. sapiens was not 200 kya but 300 or 350 kya? this alters the mutation rates, so America could have been peopled much earlier...
More findings will help us understand this better.
A closing thought
The common mtDNA shared by Denisovans and Sima de los Huesos but not the nuclear DNA could be explained as follows:
- Archaic population lives in Eurasia (H. erectus?) with its specific DNA and mtDNA they are the ancestors of Denisovans and a pre-Sima de los Huesos people.
- A later wave of proto-Neandertal reach Europe, they are more successful and breed with the native women of pre-Sima de los Huesos stock. The original mtDNA is preserved in their offspring, but the nuclear DNA is now admixed with proto-Neandertal. After many generations we have a European lineage with mtDNA similar to the original stock and therefore shared with Denisovans, but a DNA which will be more like that of Neanderthals.
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