For much of their 400,000 year history, Neanderthals were few and far between, a new analysis of genetic material from several of the extinct, ancient humans now suggests. In fact, new genetic evidence from the remains of six Neanderthals suggests the population hovered at an average of 1,500 females of reproductive age in Europe between 38,000 and 70,000 years ago, with the maximum estimate of 3,500 such female Neanderthals.
     "It seems they never really took off in Eurasia in the way modern humans did later," said study researcher Adrian Briggs of the Max-Planck Institute for Evolutionary Anthropology in Germany. The research suggests the small population size of our ancestral cousins may have been a factor in their demise. "Because there never really were millions of them, they probably were more susceptible to some event that made them go extinct, which to me, suspiciously coincides with the emergence of modern humans," Briggs said.
     Ian Tattersall, curator of anthropology at the American Museum of Natural History in New York, who was not involved in the current research, said the study "does support notions that toward the end of last ice age, the Neanderthal population was declining as a result of harsh circumstances." He added, "I don't believe Neanderthals would've gone extinct if it wasn't for this new element, the Homo sapiens competing for the same resources."
     The Neanderthals inhabited the plains of Europe and parts of Asia as far back as 230,000 years ago. They disappeared from the fossil record more than 20,000 years ago, a few thousand years after modern humans appeared on the scene.
Figuring out why Neanderthals died out and what they were like when alive have kept plenty of scientists busy. Now, Briggs and his colleagues have used a new method that targets the genetic material of interest, analyzing so-called mitochondrial DNA from the fossils of six Neanderthals, who lived between 38,000 and 70,000 years ago. That genetic material comes from females and so can be used to trace maternal lineages.
     To get a sense of the genetic diversity, and ultimately population size, the team compared the Neanderthal sequences with one another. Then, the researchers looked at such genetic information from 50 living humans from around the world, asking, "how different are their genes from one another?" The Neanderthals had about three times less genetic diversity than the modern humans. Because of this low diversity, Briggs' team infers that Neanderthal populations must have been relatively small. "Populations with much larger sizes carry more genetic diversity, you have more individuals and more mutations," he says. Briggs suggests the entire population could be roughly estimated by doubling the number of females, which they set at no higher than 3,500. In addition, the sequenced genetic material from the Neanderthals did not support any interbreeding among Neanderthals and modern humans. The entire Neanderthal genome is expected to be reported later this year and could shed more light on the interbreeding question, Briggs added.
     Chris Stringer, a palaeontologist at the Natural History Museum in London, agrees that Neanderthal populations were probably small. "They must have been on the edge of extinction by this time to have so few people scattered in Europe," he says. However during warm spells, their numbers and range probably swelled, only to contract in leaner times, Stringer says. "I think the numbers would have fluctuated. They would have had good times and bad times, and this data reflect that in the last 100,000 years they were having bad times." It's tempting to think that the arrival of modern humans to Europe about 45,000 years ago pushed Neanderthal numbers even lower by competing for increasingly scarce resources. But the invading Homo sapiens would have been relatively rare too, Stringer says. "You've got to consider the possibility that they might not have met each other that often."

Sources: LiveScience (16 July 2009), New Scientist (17 July 2009)