“Scientists Discover Why the Human Brain Is so Big: Molecular Switch Makes Human Organ Three times Larger Than Great Apes’, Study Finds”, 2021-03-24 ():
It is one of the defining attributes of being human: when compared with our closest primate relatives, we have incredibly large brains.
…Tests on the tiny “brain organoids” reveal a hitherto unknown molecular switch that controls brain growth and makes the human organ three times larger than brains in the great apes. Tinker with the switch and the human brain loses its growth advantage, while the great ape brain can be made to grow more like a human’s.
“What we see is a difference in cellular behavior very, very early on that allows the human brain to grow larger”, said Dr Madeleine Lancaster, a developmental biologist at the Medical Research Council’s Laboratory of Molecular Biology in Cambridge. “We are able to account for almost all of the size difference.”
The healthy human brain typically reaches about 1,500cm3 in adulthood, roughly three times the size of the 500cm3 gorilla brain or the 400cm3 chimp brain. But working out why has been fraught with difficulty, not least because developing human and great ape brains cannot easily be studied.
After several weeks, the human brain organoids were by far the largest of the lot, and close examination revealed why. In human brain tissue, so-called neural progenitor cells—which go on to make all of the cells in the brain—divided more than those in great ape brain tissue.
Lancaster, whose study is published in Cell, added: “You have an increase in the number of those cells, so once they switch to making the different brain cells, including neurons, you have more to start with, so you get an increase in the whole population of brain cells across the entire cortex.”
Mathematical modeling of the process showed that the difference in cell proliferation happens so early in brain development, that it ultimately leads to a near doubling in the number of neurons in the adult human cerebral cortex compared with that in the great apes.