Tuesday, November 15, 2011
Gene Doubling Could Lead to Bigger Brains
A study by Megan Dennis of the University of Washington in Seattle suggests that a change during human evolution could have led to bigger brains. At least twice in the past 3 million years, a gene called SRGAP2 has been duplicated within the human genome. Dennis’ findings show that extra copies of this gene may account for humans’ thicker brain cortex, the area in the brain where thinking takes place. Before, the gene SRGAP2 was identified as one of the 23 genes duplicated in humans but not in other primates. However, Dennis found an ancient form of the gene, located on human chromosome 1, as having been partially duplicated on the same chromosome about 3.4 million years ago, creating a shortened version of the SRGAP2 protein. About 2.4 million years ago, a copy of the partial copy was created and added to the short arm of chromosome 1. Looking at duplicate genes in more than 150 people revealed that the copy made 3.4 million years ago is missing in some people, while the younger version of the gene has become fixed in the human population. Its rapid assimilation might indicate that the gene is key in human evolution. Researchers have found that the shortened version of the SRGAP2 protein interferes with brain cells’ ability to make projections called filopodia, which the cells use to move around. The less the number of projections, the more the cell is streamlined to mitigate futher in the brain, perhaps to the point of allowing humans to build extra cortex layers.