In The Field

AAAS: Neanderthal Strip Tease

Scientists have sequenced the genome of modern humans’ closest relatives, the Neanderthals. And so far they have found….

…well, not much, actually. At least not much that Svante Paabo, leader of the Neanderthal sequencing project, was willing to share with us this morning. The highlights, briefly: there’s very little or no evidence of interbreeding between modern humans and Neanderthals. Neanderthals weren’t able to digest milk as adults. They have similar mutations to modern humans in a gene involved in language, FOXP2. And they do not appear to have the version of a gene involved in brain size, microcephalin, that is commonly found in Europeans.

And Paabo does not think it will be possible to clone a Neanderthal from fossil DNA: “I would say [cloning a Neanderthal] starting from the DNA extracted from a fossil is and will remain impossible as far as I can see into the future,” he said. But he left open the possibility that some new technology may one day change that state of affairs.


Paabo spoke to a room full of journalists by video conference from Leipzig, Germany, where he directs a center at the Max Planck Institute for Evolutionary Anthropology. But his presentation raised a lot more questions than it answered for me.

Paabo said that scientists assembled DNA from Neanderthal fossils in Croatia, Spain, Russia, and the Neander Valley in Germany into a rough genome draft. This draft is roughly 63 percent complete; for comparison, the reference human genome sequence is more than 90 percent complete. Each base in the Neanderthal draft was sequenced an average of 1.2 times. Again, for comparison, each base in the genome of Nobel laureate James Watson was sequenced an average of 7.4 times using the same technology as is being used in the Neanderthal project. “We still have a lot of gaps, but we have an overview” of what the Neanderthal genome contains, Paabo said.

red headed neanderthal.jpg

The big hope of Paabo and other scientists sequencing the Neanderthal genome is that they will be able to compare it to the human and chimp genomes to find evidence of “positive selection” in regions of modern human DNA that have changed since we diverged from Neanderthals at least 500,000 years ago. These selected sites might underlie modern human traits such as symbolic language and social learning. “Studying the Neanderthal genome will tell us what makes modern humans and why we are really humans,” said Paabo’s colleague at Leipzig, Jean-Jacques Hublin.

Paabo said that his team has found regions of the modern human genome that seem to have been selected since the divergence with Neanderthals – for instance, on chromosome 7. But he couldn’t say more about what genes might be in those regions, because, he says, he does not want to compromise his ability to publish the results of his work in a peer-reviewed journal, which he hopes will happen later this year.

He also moved rather quickly through his explanation of how the team has tried to analyze potential contamination from human DNA, which has been a problem for this team in the past. Scientists have now collected at least six different samples of Neanderthal mitochondrial DNA, an independent type of DNA passed down from mother to child. As a result, Paabo said the group has now grown confident that it is sequencing authentic Neanderthal DNA with no more than 0.3 percent contamination.

Paabo’s team also used a separate method to estimate contamination: since the group assembled the genome of a female individual, it could estimate the degree of human contamination within it by tallying the places where male-specific DNA was erroneously found. Using this method, the team estimated it had no more than 0.5 percent contamination.

But I don’t know how to interpret these contamination rates. Does this mean that each spot in the genome has a 0.3 to 0.5 percent chance of representing modern human rather than ancient Neanderthal DNA? That seems like a really small number, but if you’re looking for tiny changes that could signal major differences between species, it seems like it could be a real major problem.

I’m hoping that the published version of the Neanderthal genome, when it appears, will shed a lot more light on that question – and reveal all the juicy details that Paabo seemed reluctant to delve into today!

Illustration credit: Knut Finstermeier

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