Researchers use 'mini-brains' to find out why ours grow so large

Researchers studying unique cultures of brain cells from humans, gorillas, and chimpanzees have uncovered why human brains grow so much larger than our primate cousins.

Many scientists believe that it's the size of our brain that gives humans an evolutionary edge over other animals.

"Our total brain size is around three times bigger than that of chimpanzees or gorillas. And what that means is that we have around 80 to 100 billion neurons and other apes have around 30 billion neurons," said developmental biologist Madeline Lancaster. "Those numbers are just really mind boggling."

In their study, Lancaster and her team used brain organoids, which are 3D tissues grown from stem cells, that model the essential functions of whatever organ they're programmed to replicate. Sure enough, very early on, Lancaster said the human brain organoid grew to be visibly larger than the other organoids.

The team found that at a very early stage of development, as the brain is developing from what are called neural progenitors — which are the cells that will eventually make neurons — human tissue cultures spend more time making more of these progenitor cells, thanks to a gene called ZEB2. 

"They're like the mother cells. They're the mothers that are going to make all of these neurons. But they haven't started doing that yet. They're still just making more and more of themselves," said Lancaster. 

She adds that even though the human brain spends just two extra days making progenitor cells, "because all of this is happening during this exponential growth phase, it has dramatic consequences for the numbers of neurons that these cells can make."

Lancaster is a developmental biologist at the Medical Research Council Laboratory of Molecular Biology in Cambridge, UK. You can hear her conversation with Bob McDonald at the link above.

Written and produced by Amanda Buckiewicz