Washington, August 13 : A new DNA study has suggested that Neanderthals and anatomically modern humans did not interbreed.
According to a report in National Geographic News, researchers sequenced the complete mitochondrial genome-genetic information passed down from mothers-of a 38,000-year-old Neanderthal thighbone found in a cave in Croatia, to come up with their findings.
The new sequence contains 16,565 DNA bases, or "letters," representing 13 genes, making it the longest stretch of Neanderthal DNA ever examined.
Mitochondrial DNA (mtDNA) is easier to isolate from ancient bones than conventional or "nuclear" DNA-which is contained in cell nuclei-because there are many mitochondria per cell.
"Also, the mtDNA genome is much smaller than the nuclear genome," said study author Richard Green of the Max Planck Institute for Evolutionary Biology in Germany.
"That's what let us finish this genome well before we finish the nuclear genome," he added.
The new analysis suggests that the last common ancestor of modern humans (Homo sapiens) and Neanderthals lived between 800,000 and 520,000 years ago. This is consistent with previous work on shorter stretches of Neanderthal DNA.
Contrasted with modern humans, Neanderthals exhibited a greater number of letter substitutions due to mutations in their mitochondrial DNA; although they seem to have undergone fewer evolutionary changes overall.
The fact that so many mutations-some of which may have been harmful-persisted in the Neanderthal genome could indicate the species suffered from a limited gene pool.
This might be because the Neanderthal population was smaller than that of Homo sapiens living in Europe at the time.
A small population size can "diminish the power of natural selection to remove slightly deleterious evolutionary changes," said Green.
The researchers estimate the Neanderthal population living in Europe 38,000 years ago never reached more than 10,000 at any one time.
Homo neanderthalis first appeared in Europe about 300,000 years ago but mysteriously vanished about 35,000 years ago, shortly after the arrival of modern humans-Homo sapiens-in Europe.
"If there were only a few, small bands of Neanderthals, barely hanging on, then any change to their way of life could have been enough to drive them to extinction," said Green. "One obvious change would have been the introduction of another large hominid-modern humans," he added.
According to Stephen Schuster, a molecular biologist at Pennsylvania State University, the new study should silence a lot of theories about interbreeding between Neanderthals and modern humans.
"The study shows that at least for the maternal lineage, there are no traceable genetic markers that suggest admixture of Neanderthals and modern humans," he said.