Wednesday, 18 November 2015

Ancient Teeth Show Human Roots

Reconstruction of a Neanderthal woman
Source: PLOS Biology
Nuclear and mitochondrial DNA sequences from two Denisovan individuals | PNAS

If the name Svante Pääbo doesn't strike fear into the heart of creationist frauds they haven't done enough reading.

Svante Pääbo heads up the Department of Evolutionary Genetics, Max Planck Institute for Evolutionary Anthropology, D-04103 Leipzig, Germany which has perfected the art of DNA recovery and is producing masses of DNA data and analysis showing how the modern human species , Homo sapiens, not only evolved but how we interbred with related species as we initially diverged, especially out of Africa. Pääbo's team were the first to extract Neanderthal DNA and to show how non-African H. sapiens have around 4% of Neanderthal DNA and thus must have interbred with them when we migrated out of Africa and found an ancient population of H. neanderthalensis already in Eurasia.

They also isolated DNA from a finger bone found in the Denisova Cave in the Altai Mountains of Siberia and showed that a third species of Homo, related to but distinct from Neanderthals, co-existed in East Asia with both us and Neanderthals and contributed a variable amount of their DNA to the H. sapiens of Southeast Asia and Oceania.

And now they done it again. They've now extracted and analysed both normal DNA and mitochondrial DNA (mtDNA) from two molars from two different individuals found in the Denisova Cave, so trebling the number of individuals from which DNA has been recovered. Of course, as more DNA is recovered the more accurately the amount of genetic diversity can be estimated and so the duration of genetic isolation can be gauged. We can also tell with greater accuracy just how much interbreeding with other hominids there was.

Denisovans are a sister group of Neanderthals that were identified on the basis of a nuclear genome sequence from a bone from Denisova Cave (Siberia). The only other Denisovan specimen described to date is a molar from the same site. We present here nuclear DNA sequences from this molar and a morphological description, as well as mitochondrial and nuclear DNA sequences from another molar from Denisova Cave, thus extending the number of Denisovan individuals known to three. The nuclear DNA sequence diversity among the Denisovans is higher than among Neandertals, but lower than among present-day humans. The mtDNA of one molar has accumulated fewer substitutions than the mtDNAs of the other two specimens, suggesting Denisovans were present in the region over several millennia.

Denisovans, a sister group of Neandertals, have been described on the basis of a nuclear genome sequence from a finger phalanx (Denisova 3) found in Denisova Cave in the Altai Mountains. The only other Denisovan specimen described to date is a molar (Denisova 4) found at the same site. This tooth carries a mtDNA sequence similar to that of Denisova 3. Here we present nuclear DNA sequences from Denisova 4 and a morphological description, as well as mitochondrial and nuclear DNA sequence data, from another molar (Denisova 8) found in Denisova Cave in 2010. This new molar is similar to Denisova 4 in being very large and lacking traits typical of Neandertals and modern humans. Nuclear DNA sequences from the two molars form a clade with Denisova 3. The mtDNA of Denisova 8 is more diverged and has accumulated fewer substitutions than the mtDNAs of the other two specimens, suggesting Denisovans were present in the region over an extended period. The nuclear DNA sequence diversity among the three Denisovans is comparable to that among six Neandertals, but lower than that among present-day humans.

Nuclear and mitochondrial DNA sequences from two Denisovan individuals;
Susanna Sawyera, Gabriel Renauda, Bence Violab, Jean-Jacques Hublin, Marie-Theres Gansauge, Michael V. Shunkov Anatoly P. Derevianko Kay Prüfer, Janet Kelso, and Svante Pääbo;
PNAS November 16, 2015 doi: 10.1073/pnas.1519905112

Analysis of the recovered mtDNA shows that the Denisovans shared a last common ancestor with both us and Neanderthals about a million years ago. The authors now believe that Denisovans and Neanderthals diverged about four times as long ago as the deepest divergence of modern human populations. However, the presence of about 0.5% Neanderthal DNA in Denisovan DNA shows that occasional interbreeding was going on between these two species as it was between them and the later modern species. This interbreeding must have produced fertile offspring for the 'foreign' DNA to have entered the general genepool and to have been diluted to such a low level over time.

Divergence at about 1 million years ago means that it is entirely possible that these archaic hominids are diverged directly from H. erectus, not from H. heidelbergensis, as has been proposed. In that case, Neanderthals and Denisovans are truly cousin species of modern humans, not simply regional variant.

Even though precise differences in age between one of the molars (Denisova 8) can't be calculated with certainty by extrapolating modern human mtDNA mutation rates because this may not have been the same for this species, the degree of difference between Denisova 8 mtDNA and the other two specimens (finger bone and the other molar) there is no doubt that it is considerably older, indicating that Denisovans existed in the area for several millennia.

So, we have increasing evidence, not only of three different species of human being co-existing in Eurasia but also being able to successfully interbreed at least occasionally and over a considerable time. Interbreeding was not frequent enough for the gene pools to merge as it would be expected to if they were the same species, but it was at least occasionally successful in producing fertile offspring. In effect, these three species were acting like a ring species or complex super-species where speciation is incomplete and barriers to hybridization have not been fully established.

Over the whole of the period these three species coexisted, the evolution of modern humans was progressing. The eventual extinction of two of these three species in Eurasia, leaving just one successful population, is probably an example of how archaic forms are often replaced by a more advanced species in the course of evolution.

Despite all this evidence of a rich and fascinating human evolutionary story in which we are on but the most recent page, creationists still like to pretend that they are descended from a pair of individuals magically created out of dirt just a few thousand years ago, who had no ancestors, who were unrelated to any other life forms, and who have progressed little either culturally or physically since.

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