The frequency of the gene increases because people who carry these genes are more fit and they have more kids. We think each gene conveys some sort of fitness advantage in brain biology. It could be an improved cognitive function or a personality trait. We're not sure. But we know whatever consequence they render is highly favored by selection.

I think a lot of people might consider humans to be at the pinnacle of evolutionary lineage -- that we have achieved an advanced state as a species, and we have basically become the end-game. But what we found indicates that the species -- particularly when it comes to the brain, which is perhaps our most defining feature -- is still evolving.

Our studies indicate that the trend that is the defining characteristic of human evolution - the growth of brain size and complexity - is likely still going on. If our species survives for another million years or so, I would imagine that the brain by then would show significant structural differences from the human brain of today.

It's likely that different populations would have a different makeup of these genes, so it may all come out in the wash.

In another 10,000 or 20,000 years, I think the human brain may acquire a form that is quite different than the human brain today. Not necessarily in its shape, which may remain relatively the same. But the function may be different. It may be, on average, a little smarter. Or it may acquire certain skills that in its current form it isn't well-equipped to handle -- advanced cognitive abilities such as abstract reasoning. We don't really know for sure how -- but we are still evolving.

We want to know how broad a trend these two genes represent. Did we get really lucky and hit on two rare examples of such genes? Or, are they representative of many other such genes throughout the genome. I would bet, though, that we will find evidence of selection in a lot more genes.