DNA-duplication errors that upped the number of copies of a gene may have catalysed the evolution of complex brains in early humans.

The gene SRGAP2 is expressed during development of the brain's neocortex — a region involved in cognition. Evan Eichler at the University of Washington in Seattle and his team report that humans have four different versions of SRGAP2, as did Neanderthals, whereas other primates have just one. The group estimates that successive duplications of SRGAP2 occurred between 3.4 million and 1 million years ago, as Homo species evolved.

Meanwhile, Franck Polleux at the Scripps Research Institute in La Jolla, California, and his team show that one of the newer versions of the gene, SRGAP2C, blocks the activity of the ancestral SRGAP2 when it is artificially expressed in the brains of mice. Mouse neurons expressing SRGAP2C develop features of human neurons, such as a denser array of projections called dendritic spines that forge connections with neighbouring neurons. The cells also migrated across the developing brain faster than normal mouse neurons.

The authors suggest that these changes, driven by the emergence of SRGAP2C, could have occurred in early humans, who had much larger brains than their ancestors.

Cell http://dx.doi.org/10.1016/j.cell.2012.03.033; http://dx.doi.org/10.1016/j.cell.2012.03.034 (2012)