V-ati futut cu neandertalii si maimucile, aaaaaaaa ??
- From: † Prof. Dr. Ing. IPS Raspopitul <raspopitul@xxxxxxxxx>
- Date: Wed, 03 Oct 2007 22:58:44 -0000
DNA probe finds hints of human
Rex Dalton (Nature)
Contamination of ancient samples may have led to claims that humans
and Neanderthals interbred.
A groundbreaking analysis of Neanderthal DNA that suggested they
interbred with humans was based on samples contaminated with human
DNA, a new study suggests.
The study1, published on 28 August in PLoS Genetics, reanalysed about
one million base pairs of fossilized Neanderthal DNA that had been
analysed in a paper published last November in Nature2. The Nature
paper and a paper in Science3 published the same week on 65,000 base
pairs were the first reports on Neanderthal nuclear DNA.
But around 80% of the sequences in the Nature paper are modern human
DNA, not Neanderthal, claims Jeffrey Wall, an evolutionary geneticist
at the University of California, San Francisco, who led the PLoS
Genetics study. This indicates that human genetic material was somehow
introduced into the samples. This known risk is increased by the
closeness of the two species — the 3-billion-base-pair genomes of a
human and a Neanderthal differ by less than 0.5%.
The results in the Nature paper suggested that there was interbreeding
among Neanderthals and humans in their common European home ground
before Neanderthals became extinct 30,000 years ago. The Science
article found no genetic evidence of interbreeding.
DNA probe finds hints of human
J. TRUEBA/MSF/SPL
Neanderthal DNA has been reanalysed, leading to suggestions of human
DNA contamination.
Svante Pääbo, senior author of the Nature paper, concedes that his
group at the Max Planck Institute for Evolutionary Anthropology in
Leipzig, Germany, had problems with contamination. These prompted him
to change laboratory procedures and to add controls late in 2006,
after the paper was published. "I agree with [Wall's] analysis," Pääbo
says. "Their observations are formally correct."
Pääbo's co-author Michael Egholm, who is research vice-president at
454 Life Sciences in Branford, Connecticut, adds: "There is no denying
contamination. It was one of the dangers of doing this." But ongoing
analysis indicates that human contamination in their study was just
30%, Egholm adds.
There had been intense debate over the contrasting results in the
Nature and Science papers, which analysed the same 38,000-year-old
Neanderthal bones from Croatia using different sequencing methods.
Pääbo's group used 454's rapid 'direct sequencing' approach, whereas
the Science team, led by Edward Rubin of the Joint Genome Institute in
Walnut Creek, California, used a traditional method using cloned DNA
and bacteria to generate the base pairs.
The studies gave different estimates for the time Neanderthals
diverged from humans — the Science article pegged it at 706,000 years
ago, whereas the Nature paper set it at 516,000 years ago. Wall's
study confirms the 706,000-year divergence date. The probable human
DNA contamination led to the more recent date and may have led to the
suggestion of later interbreeding, Wall says.
The Nature paper also found more similarities between genetic
variations called SNPs (single nucleotide polymorphisms) in
Neanderthal and human DNA than the Science paper, even after allowing
for the Nature group's larger number of base pairs. Pääbo's team
reported that about 30% of the SNPs in the Neanderthal DNA are derived
— that is, the mutations occurred — in today's humans.
Pääbo acknowledges there is "a potential problem" with the presence of
these human SNPs in the Neanderthal sequence. These same discrepancies
were noted by Rubin's group. "We had concerns," says Rubin. "We
suspected some of the issues raised by Wall."
Both Pääbo's and Rubin's groups expect to publish further Neanderthal
sequences from other specimens that each group is studying. Pääbo and
Egholm say their analysis will address the anomalies in their Nature
paper.
.
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