Using laboratory-grown mini-brains, scientists have figured out why humans have bigger brains than monkeys.
About 5 to 8 million years ago people and monkeys they parted from a common ancestor. Some time after that, humans began to evolve to have bigger brains; now human brain are about three times the brain size of chimpanzees, our closest living relatives.
If you ask “what’s special about our brains,” compared to other monkeys, the most obvious answer is size, said lead author Silvia Benito-Kwiecinski, a postdoctoral researcher at the MRC Laboratory for Molecular Biology in the UK. “The choice of the big brain was powerful, so it would seem that our bigger brains have something to do with our unique cognitive abilities.”
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Between 2.6 million and 11,700 years ago, the human brain had a strong growth, doubling in size, Live Science has previously reported. Due to the lack of fossil records dating back to the time of the expansion of the human brain, scientists cannot easily separate what prompted people to grow bigger brains; but with modern tools, we can see it now how our brain grows differently than the monkey’s brain.
Because the brains of monkeys and monkeys increase rapidly on the surface in early development, scientists previously hypothesized that differences could occur very soon after conception, before the cells mature into brain cells, Benito-Kwiecinski told Live Science. But since the early brain tissue of the human and ape fetus is not readily available for research, previous studies have mainly focused on later developmental stages when neurons already form the brain landscape.
But the advent of organoid technology, which are models of organs grown in the laboratory, now allows us to look at these earlier stages. Scientists create these brain organelles from stem cells or cells that can transform into any type of cell in the body and reprogram those cells to grow into brain structures.
Although these are not real brains, they still imitate impressively; previously, scientists created brain organoids that could grow their own blood vessels or create their own brain waves, Live Science has previously reported.
In a new study, Silvia Benito-Kwiecinski bred a “mini-brain” of chimpanzees, gorillas and people in the lab (this is the first time a gorilla brain organoid has ever been made). They started with 3D beads of cells called embryonic bodies that mimic the early stages of brain development – about a month after conception – before the stem cells mature into brain cells. They then placed these cells in gel matrices and allowed them to develop “budding structures” or neural cousin cells, which are stem cells that will eventually turn into brain cells.
“The reason why these stem cells are interesting is that, ultimately, the number of neurons generated depends[s] about the number of progenitor cells formed, “said Benito-Kwiecinski. In other words, the more progenitors divide, the more neurons will eventually form. These progenitor cells are cylindrical in shape, but as they mature, they begin to elongate and become more spindle-shaped.
These elongated cells divide much more slowly than their cylindrical predecessors. Over time, spindle cells become fully developed neurons.
The researchers found that it takes several days longer for human brains to mature neuronal progenitor cells into these slower-divided elongated cells than in the chimpanzee and gorilla brains.
“People seem to be late with the transition” into a spindle shape, Benito-Kwiecinski said. In that extra time before the transition, human progenitor cells divide more than their ape counterparts, creating more cells that will mature into brain cells and thus a larger brain.
To understand why, the researchers looked at genes that were turned on and off during this early stage of brain development in different organoids. They found that the ZEB2 gene was involved in gorilla brain organoids rather than in human organoids. ZEB2 “appears to be the regulator of this cell shape change,” Benito-Kwiecinski said.
Certainly, when the researchers delayed the activation of ZEB2 in gorilla pedigree cells, the transition to elongated cells took longer, making the cells in the gorilla’s organelles grow more similar to the cells in human organoids. When they previously included ZEB2 in human organelles, the opposite happened: cells in human organelles began to grow more like cells in monkey organelles, meaning they transitioned more quickly into elongated cells.
It is not clear how soon after humans separated from apes, the expression of this gene began to change; and it is also unknown which other genes are involved. Benito-Kwiecinski and her team now hope to understand what regulates ZEB2 expression, and thus why that gene is later expressed in humans than in monkeys.
The findings were published in the journal on Wednesday (March 24th) Cell.
Originally posted on Live Science.