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A Neanderthal's DNA tale

Scientists say the oldest decipherable DNA from a Neanderthal confirms the view that there was little if any hanky-panky between that long-vanished species and modern humans - but they also say their findings show that the Neanderthals were more genetically diverse than previously thought. If anything, the results deepen the mysteries surrounding our ancient, heavy-browed cousins.

G. Focant / Current Biology
Current Biology's cover highlights a
100,000-year-old Neanderthal specimen
from Belgium's Scladina Cave.

These are the lessons drawn from a snippet of mitochondrial DNA recovered from the 100,000-year-old molar of a Neanderthal pre-teen, found in Scladinia Cave in Belgium's Meuse Valley. An international group of researchers, led by Catherine Hänni of France's École Normale Supérieure de Lyon, reported their findings in today's issue of the journal Current Biology.

Neanderthals were the dominant European representatives of the hominid family tree for most of the past 300,000 years, but they vanished from the scene soon after the arrival of modern humans on the continent about 30,000 years ago. Traces of mitochondrial DNA have been recovered from fossilized bones, and over the past few years, nine Neanderthal samples dating back between 29,000 and 42,000 years have been analyzed.

The genetic signatures of those nine samples fell outside the range of present-day humans - which has led some researchers to conclude that there was no significant genetic mixing with the Cro-Magnon invaders. Another conclusion is that we modern humans basically pushed the Neanderthals into extinction - a theory known as "rapid replacement."

These views are widely debated, of course, and so Hänni's group was interested in looking at older Neanderthal DNA that unquestionably predated contact with modern humans. The researchers recovered 123 DNA base pairs from the Scladina Cave specimen, then matched them up with other Neanderthal samples as well as the DNA of modern humans and chimpanzees.

They found that the Scladina sequence had less overlap with humans than the previously known, more recent Neanderthal samples.

"While the diversity of the more recent Neanderthals is similar to that of modern humans worldwide, the sequence from Scladina reveals that more divergent Neanderthal haplotypes existed before 42,000 years ago," Hänni and her colleagues wrote.

So what does that mean? The researchers speculate that a widely divergent Neanderthal species may have gone through "demographic bottlenecks" that reduced that diversity - bringing them closer to the central branches of the hominid family tree.

"This could explain the shift towards modern human pairwise distributions observed between 100,000 and 40,000 years ago," they write. "Whether this shift should be related to cohabitation, climatic changes, or any subdivision of populations, the Scladina sequence has revealed that the genetic diversity of Neandertals has been underestimated."

Did bad things happen to the quirkier Neanderthals before humans arrived in Europe? What sorts of bad things? Some have suggested that Neanderthals were less able than humans to adapt to changing climatic conditions - or more prone to disease.

But let's not get ahead of ourselves. One sample of 123 DNA base pairs is precious little to base a theory upon, and even Hänni's group acknowledges that more Neanderthal genetic sequences will be needed "to fully understand the extent of the past diversity of Neanderthals."

Svante Pääbo of Germany's Max Planck Institute for Evolutionary Anthropology, one of the pioneers of Neanderthal DNA analysis, is of the same mind about the latest findings. In an e-mail, he told me that "there is not disagreement with what we have found."

"However, I think that there is too little data to say something [about] how diversity changed over time in Neanderthals," he wrote. "There is a single individual that is old ... and only four younger ones for which reasonable lengths of sequence are available, and for the latter ones, the datings are not very secure."

For more discussion of the latest study, check out John Hawks' observations in his Anthropology Weblog and Razib Khan's view in Gene Expression. And for an overview of the past and future hominid family tree, check out our "Before and After Humans" interactive.

In this item, I've changed quoted references to "Neandertal" to our American spelling for foolish consistency's sake.