Chimpanzee mini-brains hint at secrets of human evolution

At some point during human evolution, a handful of genetic changes triggered a dramatic threefold expansion of the brain's neocortex, the wrinkly outermost layer of brain tissue responsible for everything from language to self-awareness to abstract thought.

Identifying what drove this evolutionary shift is fundamental to understanding what makes us human, but has been particularly challenging for scientists because of ethical prohibitions against studying the developing brains of our closest living relative, the chimpanzee, in the lab.

"By birth, the human cortex is already twice as large as in the chimpanzee, so we need to go back much earlier into embryonic development to understand the events that drive this incredible growth," said Arnold Kriegstein, MD, Ph.D., the John Bowes Distinguished Professor in Stem Cell and Tissue Biology, founding director of the Eli and Edyth Broad Center of Regenerative Medicine and Stem Cell Research at UC San Francisco.

In a study published February 7, 2019, in Cell, Kriegstein and collaborators have gotten around this impasse by creating chimpanzee brain "organoids" – small clusters of brain cells grown from stem cells in a laboratory dish that mimic the development and organization of full-size brains.

"Our ability to take skin cells from an adult chimpanzee, turn them into iPSCs, and then study their development in laboratory dishes is astounding," said Kriegstein. "It's a 'science fiction' experiment that couldn't have happened 10 years ago."