From Fred Colbourne:
Re: Neanderthal DNA partially sequenced (Nov. 16): After recent fossil discoveries in Spain and now again as a result of studies of Neandertal DNA, speculation surged about the possibility that Neandertals and modern humans interbred and that we now bear Neandertal genes. A hypothesis to this effect might be based on elements of the modern Y-chromosome having been found in Neandertal DNA.
I would feel more confident about this hypothesis if modern genetic material had been found in Neandertal mitochondrial DNA (mtDNA) rather than nuclear DNA. Because mtDNA is transmitted maternally, finding modern human mtDNA in Neandertals would suggest intermarriage and thus strengthen the argument that mixing of the two populations produced at least some fertile offspring.
As it stands, the results obtained so far can be explained away by saying that modern hunters may have casually bred with Neandertal women who bore mixed children in Neandertal family groups. We do not know if these children were fertile or infertile, though we may suspect that a separation of a few hundred thousand years resulted in hybrid dysgenesis, infertility in the offspring. If offspring were infertile, the size of family groups would tend to fall.
Many years ago, Ezra Zubrow (SUNY) developed a demographic model of Neandertal populations and their extinction (The Human Revolution, Mellars and Stringer, eds). He showed that not much would have been needed to push Neandertals to extinction in one millennium. Perhaps hybrid dysgenesis accelerated the process. In my opinion, if hybrid dysgenesis were factored into the Zubrow model, extinction might have occurred in less than one millennium.
By contrast, hybrid dysgenesis would not have prevented modern humans from replacing Neandertals. There was a plentiful supply of new arrivals from the Near East and northern Africa.
2 Comments:
Fred Colbourne's comment on Neandertal DNA contains factual errors and erors of logic. Recent fossil discoveries of modern/neandertal "hybrids" were not made in Spain, but in Romania (Pestera Muierii)and Portugal (Lagar Velho). Mitochondrial DNA, may contain paternal contribution through recombination (See Awadalla et al. in Science 24 Dec 1999)hence it is not the exclusive record of maternal lineage.
How can he know that there was a plenty supply of "moderns" (no fossil proof of population size). Hybrid dysgenesis is unlikely if there was so much DNA similarity, plus hybridisation, if it occured, could occur throughout the entire time of neandertal existence, thus limiting isolation. More importantly, the recent two papers, though they estimate the modern/neandertal split at some 500 ka, give wide margins of error and state that DNA differences between modern human individuals can show splits of up to some 490 ka. Moreover calculation of the split time is based on assumptions of neutrality whereas nuclear DNA is subject to selection. There may not have been a split at all or it could occur much later, or earlier than the figure quoted. Thus any stories about dysfunctional hybrids dying out etc. are a pure speculation, as is the theoretical assumption that neandertals and moderns were separate species. There is no proof of any of this.
It is depressing to see how technically brilliant characterisation of a minute portion of the genome of ONE INDIVIDUAL mushrooms into polygenist speculations of separate creation. Good technology, poor knowledge.
Thanks to Maciej Henneberg for his correction. The paper I had in mind described the finds in Lagar Veho, which is in Portugal, not Spain as I implied, (Trinkhaus, PNAS, 1999).
In June 2000, the editors of Science published three dissenting letters on mitochondria, representing the consensus view that no mitochondrial DNA is transmitted from males, all letters rebutting Awadalla’s paper. Nevertheless, Adawalla’s paper and his defence are intriguing and may some day win out.
In my opinion, what confounds the issue of gene flow between Neandertals and modern humans is the existence of at least three null hypotheses:
1.Humans and Neandertals never separated, but remained one species. The evidence from DNA studies provides a range of dates for separation, but does not attest to speciation
2.No offspring were produced from Neandertal and modern human unions. Fossil evidence is probably strong enough to falsify this hypothesis
3.Gene flow from Neandertals to modern humans has not persisted into the present. DNA studies seem to support this hypothesis.
One hypothesis that would explain the apparent inconsistency among these outcomes states that offspring were produced from Neandertal-modern human unions, but because of the long separation in the lineages of parents, the offspring were not fertile, a well-known phenomenon known as hybrid dysgenesis.
In its favour, this hypothesis can claim only parsimony: it’s the simplest way to explain the conflicting evidence.
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