What causes this increase in size?
The researchers discovered that a gene called ZEB2 is activated much earlier in gorillas and chimpanzees than in humans, something that determines changes in the shape of cell development and optimizes the production of a greater number of neurons in the brain. hence our human brain is considerably different in size from other primates.
The key moment is during the early stages of brain development, when neurons are produced by stem cells called neural progenitors. These cells initially have a cylindrical shape that facilitates their division into identical ‘daughter’ cells with the same shape. The more times the neural progenitor cells multiply at this stage, the more neurons there will be later. As cells mature and slow down their multiplication, they elongate, forming a shape like a stretched ice cream cone. Namely, they need to maintain that cylindrical shape for as long as possible to reproduce much better and therefore have a greater number of neurons.
According to the experiment, in the brain organoids of gorillas and chimpanzees, the process took about five days (in rodents, just a few hours) and in humans, the cycle lasted up to about seven days. Hence, this special configuration is the one that favors the multiplication of the cells responsible for the production of neurons. They have more time to multiply. This could be largely responsible for the roughly three times the number of neurons in human brains compared to gorilla or chimpanzee brains.
“We have found that a delayed change in the shape of cells in the early brain is enough to change the course of development, helping to determine how many neurons are made. It is remarkable that a relatively simple evolutionary change in shape cellular can have such important consequences in the evolution of the brain. I feel like we’ve really learned something fundamental about the questions that have interested me for as long as I can remember: what do we humans do? “Lancaster clarifies.
The researchers note that organoids represent only one model and therefore do not fully replicate real brains, especially mature brain function. But for fundamental questions about our evolution, these brain tissues offer an unprecedented view of key stages of brain development that would be impossible to study otherwise.