Our oldest ape-like ancestors mostly ate fruit, but later species like Australopithecus branched out. As well as eating a wider range of plants, such as grasses, they seem to have eaten a lot more meat, and even butchered it with stone tools. More meat meant more calories, and less time spent chewing.
Compared to other apes, humans can’t bite very hard because we have flimsy jaw muscles. This seems to be largely down to a mutation in the MYH16 gene, which controls the production of muscle. This happened between 5.3 and 2.4 million years ago. The smaller jaws could have freed up space for our brains to grow.
Our hands are unusually dextrous, allowing us to make beautiful stone tools and write words. That might be partly down to a bit of DNA called HACNS1, which has evolved rapidly since our ancestors split from the ancestors of chimps. We don’t know what HACNS1 does, but it is active in our arms and hands as they develop.
After the human line split from the chimp line, two genes mutated. SLC2A1 and SLC2A4 both build proteins that transport glucose in and out of cells. The tweaks may have taken glucose away from muscles and into the early hominins’ brains. The glucose would then have boosted the brains, allowing them to grow bigger.
Our ancestors split from their chimp-like relatives over 7 million years ago. At first, they would have looked similar. But within their cells, change was afoot. After the split, the ASPM and ARHGAP11B genes began changing, as did a region called HAR1. It’s not clear what this did, but HAR1 and ARHGAP11B are involved in the growth of the cerebral cortex.
Humans, chimpanzees and gorillas are all descended from an unknown extinct ape. In this ancestor, a gene called RNF213 began evolving rapidly. This may have boosted the flow of blood to the brain by widening the carotid artery. In humans, RNF213 mutations cause Moyamoya disease, in which the artery is too narrow.
The first primates, the group that includes monkeys and humans, evolved soon after the dinosaurs died out. Many quickly began living in groups. This meant each animal had to navigate a complex web of friendships, hierarchies and rivalries. So group living may have driven a steady increase in brainpower.