In a study published in the journal Cell, scientists compared brain organelles from human, gorilla, and chimpanzee stem cells.
Like a real brain, the organelles of the human brain have grown much larger than the organelles of other great apes.
In the early stages of brain development, neurons are made up of stem cells – neural precursors. These cells are initially cylindrical, which allows them to easily divide into identical daughter cells. The more often they multiply at this stage, the more neurons there will be in the brain in the future. As the cells mature and their reproduction slows down, they elongate, forming the shape of an extended cone.
Earlier studies in mice showed that their precursor nerve cells mature and become tapered and slow down their reproduction for several hours. Now brain organelles have helped scientists figure out how this process develops in humans, gorillas and chimpanzees.
They found that gorillas and chimpanzees take much longer to make the transition than humans. The entire process takes approximately five days.
The progenitor cells in humans were even more delayed at this stage of growth – about seven days. They retained their cylindrical shape longer than other great apes. The difference in the rate of transition from neural progenitors to neurons means that human cells have more time to reproduce. Scientists suggest that this process is responsible for more neurons in the human brain compared to the brain of a gorilla or chimpanzee.
A relatively simple evolutionary change in the shape of cells could have serious consequences for the evolution of the brain.
To uncover the genetic mechanism that governs these differences, the researchers compared the expression of genes that turn on and off in human brain organoids with other monkeys. They identified differences in a gene called “ZEB2”, which was activated earlier in gorilla brain organoids than in human organelles.