TORONTO -- A new study is offering some answers as to why the human brain is so big, compared to that of other great apes.
Researchers in the U.K. looked at lab-grown brain tissue, known as "brain organoids," derived from human, gorilla and chimpanzee stem cells and found that the human brain organoids grew much larger than the organoids of gorillas and chimpanzees.
"This provides some of the first insight into what is different about the developing human brain that sets us apart from our closest living relatives, the other great apes. The most striking difference between us and other apes is just how incredibly big our brains are," said study author Dr. Madeline Lancaster in a news release on Wednesday.
The researchers examined the neural progenitor cells, which are the stem cells responsible for developing neurons. These cells are originally in a cylindrical shape, but gradually transition into an elongated cone shape, which shows their multiplication rate.
In chimpanzees and gorillas, the transition takes place over five days. But the transition is delayed in the development of the human brains, taking place after around seven days.
As a result, the neural progenitor cells in human brains retain their original cylindrical shape for a longer and able to produce more brain cells, resulting in larger 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 determine the numbers of neurons that are made," said Lancaster.
A gene known as ZEB2 was found to be an underlying regulator in this transition. The gene was found to be turned on earlier in ape brains, compared to human brains. Using a molecular switch, the researchers also found that manipulating this gene in gorilla brain tissue resulted in the gorilla brain organoids developing a similar fashion to human brains.
Likewise, turning on the gene earlier in human brain organoids resulted in smaller sizes, similar to the way chimpanzee and gorilla brains develop.
The researchers acknowledge that the brain organoids are not a perfect representation, but were the next best thing given that it would be impossible to perform these experiments on living human or ape embryos.
"It's remarkable that a relatively simple evolutionary change in cell shape could have major consequences in brain evolution. I feel like we've really learnt something fundamental about the questions I've been interested in for as long as I can remember - what makes us human," said Lancaster.​
