More than 500,000 years ago, the ancestors of Neanderthals and modern humans were migrating around the world when a genetic mutation caused some of their brains to suddenly improve. This mutation, researchers report dramatically increased the number of brain cells in the hominins that preceded modern humans, probably giving them a cognitive advantage over their Neanderthal cousins.

Due to the power of innovative techniques like whole genome sequencing, researchers were able to sequence a Neanderthal genome in 2014. From that data they identified 96 amino acids (building blocks that make up proteins) that differ between Neanderthals and modern humans in addition to a number of other genetic tweaks. Scientists have been studying this list to learn which of these helped modern humans to outcompete Neanderthals and other hominins.

One gene has been found to stand out. The gene, TKTL1, encodes a protein that is made when a fetus’s brain is first developing. A single genetic mutation in the human version of TKTL1 changed one amino acid, resulting in a protein that is different from those found in hominin ancestors, Neanderthals, and non-human primates.

The team suspected that this protein could be driving neural progenitor cells — which develop into neurons, to proliferate as the brain develops, specifically in an area called the neocortex, which is involved in cognitive function. That, they reasoned, could be a contributor to modern humans’ cognitive advantage over human ancestors.