July 28, 2012
Human Evolution And Neanderthals – Did Interbreeding Play A Role?
Brett Smith for redOrbit.com - Your Universe Online
A recent genetics study has shown evidence that another branch of the human evolutionary tree may have existed alongside Neanderthals, fueling the ongoing feud between paleontologists and geneticists.
According to the study published this week in the journal Cell, a team of geneticists led by Joseph Lachance and Sarah A. Tishkoff of the University of Pennsylvania found DNA evidence of an interbreeding with an unknown archaic species of human in blood samples taken from modern day African tribes.
However, all known African fossils are of modern humans and Richard Klein, a paleoanthropologist at Stanford University, told the New York Times that the new claim “is a further example of the tendency for geneticists to ignore fossil and archaeological evidence, perhaps because they think it can always be molded to fit the genetics after the fact.”
In the latest study, which started a decade ago, the geneticists decoded the entire genome of three isolated hunter-gatherer peoples in Africa: the Hadza and Sandawe of Tanzania and a group of the forest-dwelling pygmies living in Cameroon. At a cost of $10,000 per genome, the researchers completely sequenced DNA of five men from each group, who had all submitted blood samples.
The genomes of all three groups were found to contain small stretches of DNA with very unusual sequences. While mutations over the years had left their mark on these particular genetic fragments, these extremely isolated men all appeared to have diverged from a common ancient ancestor that was neither a modern human predecessor nor a Neanderthal.
Tishkoff´s team interpreted these divergent genomic sequences as the remnants of an interbreeding with an unidentified archaic species. They calculated that the interbreeding would have taken place between 20,000 and 80,000 years ago.
Sequencing of the genomes also revealed a variant in the pygmies´ gene responsible for the development of the pituitary gland, which controls reproduction and growth functions. Researchers cited this anomaly as the possible cause for the pygmies´ famously short stature and as well as their early reproduction window.
In addition to calculating a date range for species interbreeding, the geneticists estimated that the archaic species diverged from modern human´s ancestors around 1.2 million years ago, about the same Neanderthals also split from modern human lineages.
Although they existed at the same time, this ℠sister species´ to the Neanderthals had a much different genome, according to the study´s authors.
"We're calling this a Neanderthal sibling species in Africa," said co-author Joshua Akey of the University of Washington in Seattle.
Akey added that modern Europeans show no evidence of these unique DNA sequences, leading his team to believe this ℠sister species´ was confined to Africa.
Meanwhile, paleontologists like Klein claimed that the geneticists´ methods could be flawed and even Tishkoff, while appearing confident in her team´s findings, admitted there could be other genetic factors at play in the African tribal genomes besides species interbreeding.
Klein posited that any flaws in the geneticists´ work will come to light if subsequent research leads to inconsistent claims.
“Meanwhile, I think it´s important to regard such claims skeptically when they are so clearly at odds with the fossil and archaeological records,” he said.