Scientific Method / Science & Exploration

Are our brains a gift of the Neanderthals? (Part II)

Evidence that one allele of a gene involved with brain development took a …

With an understanding of selective sweeps out of the way, we can move onto the results presented in a new, open access article about a potential selective sweep. The change involved is in the middle of a gene that's critical for brain development, microcephalin. Loss of this gene reduces the size of the human brain by three- to four-fold, and a specific form of the gene (we'll call it allele D) appears to do something useful for modern brains, as it's present in 70 percent of the modern human populations, with even higher frequencies in some geographic regions (more on that later). Based on the size of the region that's been in on the selective sweep, the D allele seems to have appeared in the human lineage about 37,000 years ago, well after the origin of modern humans 100,000-200,000 years ago.

But the authors note that, if the D allele had been generated by a mutation on a normal chromosome at that time, its sequence should resemble at least some alleles still present in the remaining 30 percent of chromosomes. They don't. After sequencing 30 kilobases from about 200 chromosomes, they found that the typical variation among non-D sequences was about 20-30 bases across this region. But comparing these to the D allele showed that the variation shot up to 70-80 bases. The D allele was so different, it almost looked like it had arrived from outer space—it certainly doesn't appear to have been the product of a traditional selective sweep.

Of course, claiming "aliens did it!" is not a good way to get a paper accepted by PNAS. Since the sequences looked as if they had come from different species, the authors treated them that way, and looked to see if they had a common ancestor. They did, but the big surprise was when: the best estimate was 1.1 million years ago, well before modern humans existed. The authors looked at a series of ways in which two alleles of a gene could have avoided recombining with each other for that sort of stretch of time, but found that none of them fit the data as well as the simplest one: the D allele was hiding out in a separate species entirely, and only found its way back into modern humans thanks to a rare (perhaps singular) case of interbreeding. Its reappearance in modern humans is the product of what's termed introgression; the introgression was then followed by a selective sweep.

Modern humans originated in Africa, but by 37,000 years ago, were spreading around the globe, where they undoubtedly came into contact with the earlier species that had preceded them. Which of these predecessor species did we pick up microcephalin D from? This is where the geography came in. The D allele should be present at the highest rates where it's been around the longest—this happens to be Europe. And 37,000 years ago in Europe would mean the last days of the Neanderthals, a separate species that split from modern humans over 500,000 years ago.

The authors note that people have looked for signs of Neanderthal genes before, but haven't found any. But they point out that, in the absence of extensive interbreeding, small amounts of Neanderthal DNA would likely have been lost to genetic drift over the millenia. It is only the strong advantage provided by microcephalin D that appears to keep it visible. They also note that we shouldn't be surprised that the Neanderthals might have some very useful genes; after all, they were occupying some environments for hundreds of thousands of years before modern humans got there, and had plenty of time to adapt to them.

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