With the help of dozens of lab-grown chimpanzee “mini brains,” scientists are edging closer to answering one of the most profound questions of our existence: how did the human brain come to be?
At a certain point during our evolution, the human brain experienced an unprecedented growth spurt. Much of this flourishing occurred in the neocortex, the outer wrinkly bit of your brain, equipping us with a boom in brainpower ready to use for language, abstract thought, and other proud hallmarks of super-smart cognition.
But how did this ultimate “level up” actually happen?
Writing in the journal Cell, researchers led by the University of California San Francisco have gained some new insights into how the human brain evolved by comparing the early development of human, macaque, and chimpanzee mini-brains in a petri dish – something the study refers to as ‘‘cellular anthropology” – and identifying several hundred genetic changes unique to humans.
“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,” Arnold Kriegstein, Distinguished Professor in Stem Cell and Tissue Biology at UC San Francisco, said in a statement.
Bear in mind, these “mini brains” are not literally tiny brains in a jar, like one of Dr Frankentein’s lesser-known experiments. They are typically just clusters of different brains cells grown to mimic the brain of an animal, as seen in the image below, not capable of making any “real” thoughts or level of cognition.
The team looked closely at human and chimpanzee brain organoids and observed how specific cell types and gene activity orchestrated the growth of the brains. Overall, they found several hundred unique cellular and genetic changes that were associated with the human brain.
They were particularly intrigued by the role of neural precursor cells, called outer radial glia, and their effect on a key growth signaling network known as the mTOR pathway. The team described this as "particularly exciting to discover a molecular pathway in these cells that appears to have been specifically targeted during evolution and may help explain their specialized role in generating the advanced human cortex.”
The study also demonstrated the ability to “mass-produce” chimp brain mini-organs, which is itself a revolutionary feat. The team started by gathering pluripotent stem cells (iPSCs), skin cells that had been reprogrammed into stem cells that can become any tissue in the body. Using the iPSCs, they were then able to grow 56 organoids using stem cells derived from the skin of 8 chimpanzees and 10 humans.
“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.”