Doubling our genes gave humans bigger brains

Illustration for article titled Doubling our genes gave humans bigger brains

There are many things that set us apart from our primate cousins, but the biggest (literally) is our brains. Our oversized minds have let us carve out a unique evolutionary path, and it's all thanks to a single replicated gene.

Advertisement

That's the finding of University of Washington researchers, who have discovered the special role that a particular gene called SRGAP2 has played in our evolutionary history. At least twice in the last four million years, SRGAP2 has been duplicated in our genome. More importantly, the gene has not been replicated in the other primates, one of only 23 known genes to be uniquely duplicated.

Because duplicated genes can have a far more influential effect on the genome, those 23 genes are thought to hold a big part of the key to our evolutionary divergence from chimpanzees and other primates. It appears SRGAP2 may be the most important of them all.

The researchers found that the protein created by SRGAP2 interferes with structures in the brain called filopodia. These are supposed to allow cells to move around in the brain, but the heightened presence of SRGAP2 forces the cells into more streamlined, large-scale movements. The researchers think this could have allowed us to build a much thicker brain cortex, which helps explain our enhanced cognitive abilities.

There's another reason to think that SRGAP2 is particularly important. The researchers found that the gene was partially duplicated on chromosome 1 about 3.4 million years ago, and then that partial copy was duplicated about 2.4 million years ago. Genetic research shows that the more ancient duplication is missing from some people's genomes, but the more recent copy is fixed in the human population - everyone has that in their genes.

Generally speaking, you don't see a gene that's that common unless it's absolutely vital to our evolutionary history. What's more, 2.4 million years is a relatively short space of time for a gene to be duplicated and fixed in the genome, again indicating that whatever purpose SRGAP2 serves, it's very important to our evolutionary history. It looks like we really do have this one duplicated gene to thank for our big brains.

The International Congress of Human Genetics via Science News. Image by Guido Vrola, via Shutterstock.

Advertisement

DISCUSSION

rolf-swart-old
Rolf Swart

The title is a bit misleading, because it suggests humans grew more brain because the genome increased, when in fact it didn't. All other great apes have 24 pairs of chromosomes, humans have only 23. So the genome actually decreased in size, but 1 particular gene got duplicated, which is not the same as "doubling of genes". The genes (plural) didn't double, only 1 gene got duplicated and the actual genome decreased in size, mainly due to the fusion of chromosome nr.2.

The trouble with a title like that is that it gives the impression that the bigger a genome, the smarter an organism is, when the truth is that the opposite seems to be true. The Amoeba Dubya has a genome that is 300 times larger than that of a human being and we can quite safely say that amoeba's aren't smart.

Furthermore, there is no evidence that gene SRGAP2 has anything to do with the brain. It features twice in our genome, but that doesn't mean it gave us a bigger brain. It just means it's unique to our species. Well, so is pubic hair and religion.