The new results, published today (Dec. 18) in the Proceedings of the Royal Society B, partly explain why humans are so much brainier than our nearest living relatives. But they also reveal why the first two years of life play such a key role in human development. 

"What's really unique about us is that our brains experience rapid establishment of connectivity in the first two years of life," said Chet Sherwood, an evolutionary neuroscientist at George Washington University, who was not involved in the study. "That probably helps to explain why those first few years of human life are so critical to set us on the course to language acquisition, cultural knowledge and all those things that make us human."

Chimpanzees

While past studies have shown that human brains go through a rapid expansion in connectivity, it wasn't clear that was unique amongst great apes (a group that includes chimps, gorillas, orangutans and humans). To prove it was the signature of humanity's superior intelligence, researchers would need to prove it was different from that in our closest living relatives.