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| www.sci-news.com |
Over the past few years a number of studies of ancient and contemporary genomes have reached the same stunning conclusion: early human species interbred, and people today carry DNA from archaic humans, including the Neandertals,
as a result of those interspecies trysts. Now two new analyses of
modern human genomes are providing insights into how the acquisition of
Neandertal DNA affected anatomically modern Homo sapiens tens of thousands of years ago and how it continues to affect people today.
In the first study, Sriram Sankararaman and David Reich of Harvard
University and their colleagues compared a complete Neandertal genome
sequence with 1,004 modern human sequences to see which regions of the
modern genome contain Neandertal DNA. Like other researchers before
them, they observed that Asians and Europeans have DNA from Neandertals,
whereas Africans have little or no Neandertal DNA. The pattern is
consistent with a scenario in which early modern humans mated with
Neandertals they encountered when they migrated out of Africa and into
Eurasia, where Neandertals had lived for hundreds of thousands of years.
Moreover, the team determined that Neandertal DNA is not distributed
evenly across the genome. Some genes have a high proportion of
Neandertal ancestry (which is to say, many people today carry the
Neandertal versions of these genes). Those genes with the highest
Neandertal ancestry are associated with keratin, a protein found in skin
and hair. The Neandertal variants of these genes may well have helped
early modern humans adapt to the new environments they found themselves
in as they spread into Eurasia. But the researchers also found that
people today carry Neandertal genes that are associated with diseases
including Crohn’s, type 2 diabetes and lupus.
Intriguingly, other regions of the modern human genome have no or
very low Neandertal contribution, notably the X chromosome and genes
related to the functioning of the testes. According to Sankararaman,
Reich and their collaborators, the absence of Neandertal genetic
material in these regions suggests that male hybrids who inherited a
Neandertal X chromosome were infertile, and thus unable to pass their
genes along to the next generation. The researchers detail their
findings in a paper published in the December 30 Nature. (Scientific American is part of Nature Publishing Group.)
In the second study, published by Science, Benjamin Vernot
and Joshua M. Akey of the University of Washington screened whole genome
sequences from 665 living Europeans and Asians for telltale signs of
Neandertal contributions. Their results show that although non-Africans
individually inherited between 1 and 3 percent of their genomes from
Neandertals, different people carry different bits of Neandertal genetic
material. Together these sequences represent around 20 percent of the
Neandertal genome.
Like the other team, Vernot and Akey found evidence that Neandertals
passed along beneficial skin genes to modern humans, including some
linked to pigmentation. And they, too, observed genome regions devoid of
Neandertal contributions. One such region contains the gene FOXP2,
which plays an important role in speech.
Vernot and Akey’s work is additionally interesting in that they were
able to use statistical and computational methods to identify the
Neandertal contributions in the genomes of modern-day people without
using a Neandertal genome to guide their search. This work raises the
possibility that simply by analyzing the genomes of people alive today,
scientists will be able to discover and describe extinct human species
that mated with early H. sapiens but that, unlike Neandertals,
are unknown from the fossil record. Previous studies of genomes of
living people have hinted at dalliances between early H. sapiens and unknown archaic humans in Africa. Perhaps this approach will shine a light on these mysterious skeletons in our closet.
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