The mystery of what killed off the Neanderthals about 30,000 years ago comes a step closer to being solved with a study suggesting that they formed a tiny population that had been teetering on the brink of extinction.
Neanderthals first appeared in Europe at least 300,000 years ago but they disappeared after the arrival of anatomically modern humans, homo sapiens, who first arrived in Europe 50,000 years ago. This has led to speculation about whether the Neanderthals interbred with the new arrivals to form a hybrid population that became submerged in the human gene pool, or were instead wiped out by them, either through competition for resources or by violence.
The latest evidence, an analysis of DNA recovered from a 38,000-year-old fossilized thighbone, suggests the Neanderthals did not interbreed with modern humans but were eradicated by them.
DNA extracted from an adult Neanderthal man who lived near caves in what is now Croatia also revealed that the Neanderthals in Europe probably never numbered more than 10,000 individuals at any one time.
The new evidence about the demise of the Neanderthals comes from the complete sequence of DNA within tiny cellular structures known as mitochondria. This mitochondrial DNA is maternally inherited and is easier to isolate from ancient bones than the conventional DNA found within the cell nucleus.
The scientists repeatedly decoded the mitochondrial DNA from the 38,000-year-old Neanderthal bone 35 times to make sure that they had the correct genetic sequence, so that they could use it as an accurate comparison against the mitochondrial DNA of modern humans and chimpanzees.
“For the first time, we’ve built a sequence from ancient DNA that is essentially without error,” said Richard Green, who led the investigation at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
“It is still an open question for the future whether this small group of Neanderthals was a general feature, or was this caused by some bottleneck in their population size that happened late in the game,” said Green.
Archaeological evidence shows that Neanderthals and anatomically modern humans, known as Cro-Magnon man, occupied the same habitats and sites at overlapping periods of time but there is no hard evidence that there was any direct contact between the two last species of humans to share living space.
“There’s no proof that they saw each other, only that they inhabited the same place at about the same time but I think it’s likely that they came across one another,” said Adrian Briggs, a researcher at the Max Planck Institute, who was part of the study.
“What we’ve done is confirm that the mitochondrial DNA of Neanderthals and modern humans was so different that it forms powerful evidence that there was very little if any interbreeding between the two species,” said Briggs.
“We have also got tantalizing evidence that the Neanderthals formed a small population and we can only speculate as to what happened to them. Small population sizes are always more prone to extinction and they have a greater chance of something going wrong.”
Speculation about who the Neanderthals were, and what happened to them, has raged ever since the first Neanderthal skull was excavated from the Neander Valley, near Düsseldorf, in 1856.
It is now generally agreed that they were not the direct ancestors of modern humans but a side-branch on man’s extensive family tree. However, some anthropologists have clung to the belief that they must have interbred with humans at some stage in their history, which means that there is a little bit of Neanderthal in us all.
However, a number of DNA studies, including the latest published in the journal Cell, have found little to support that theory. Whenever it has been possible to analyze the sequence of heavily degraded DNA fragments extracted from Neanderthal bone, it shows that the genetic variation lies well outside the variation seen in modern humans.
We may never know what happened when modern humans came to live in the same space inhabited by the Neanderthals. They may simply have avoided one another, with Neanderthals retreating to their last stronghold in Europe — a cave system in Gibraltar where the most recent Neanderthal bones have been found.
Or the two species might have engaged in the sort of brutal conflict that has been the hallmark of human history throughout time.
