Researchers identified a key molecular switch that can make ape brain organoids grow more like human organoids, and vice versa.
The study compared brain organoids – 3D tissues grown from stem cells which model early brain development – grown from human, gorilla and chimpanzee stem cells.
Researchers at the Medical Research Council (MRC) Laboratory of Molecular Biology in Cambridge found that similar to actual brains, the human brain organoids grew a lot larger than those from other apes.
Dr Madeline Lancaster from the MRC Laboratory of Molecular Biology, who led the study, said: “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.”
During the early stages of brain development, neurons are made by stem cells called neural progenitors.
These progenitor cells initially have a cylindrical shape that makes it easy for them to split 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 the cells mature and slow their multiplication, they elongate, forming a shape like a stretched ice-cream cone.
Previous research in mice had shown that their neural progenitor cells mature into a conical shape and slow their multiplication within hours.
Now, brain organoids have allowed researchers to uncover how this development happens in humans, gorillas and chimpanzees.
Researchers found that in gorillas and chimpanzees the transition occurs over approximately five days.
Human progenitors took around seven days.
The study, published in the journal Cell, found that the human cells maintained their cylinder-like shape for longer than other apes and during this time they split more frequently, producing more cells.
This difference in the speed of transition means human cells have more time to multiply.
This could be largely responsible for the approximately three-fold greater number of neurons in human brains than in gorilla or chimpanzee brains, researchers say.
Dr Lancaster said: “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.
“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.”
However, the researchers noted that organoids are a model and do not to fully replicate real brains, especially mature brain function.
But they said that for fundamental questions about our evolution, these brain tissues in a dish provide an unprecedented view into key stages of brain development that would be impossible to study otherwise.
The study was funded by the Medical Research Council, European Research Council and Cancer Research UK.