By Clare Wilson

D. ROBERTS/SCIENCE PHOTO LIBRARY

How did humans get to be so much cleverer than other apes? One counter-intuitive idea is that it was made possible by a slowdown of our brain growth during fetal development.

The suggestion comes from a relatively new approach of growing embryonic-like stem cells in a dish and coaxing them to turn into nervous system cells until they form pea-sized three-dimensional clumps known as organoids.

These mini brains seem to replicate real neuron behaviour more closely than when brain cells are grown in a flat layer. They have nothing like the complexity needed for thought or consciousness, but do develop into different kinds of brain tissue and display patterns of electrical activity that have some similarities with real brains.

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Human brains are certainly bigger than those of our nearest primate relatives, but there are surprisingly few differences in structure. So it is unclear what gives rise to the huge differences in our mental abilities.

Gray Camp at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues used stem cells from humans, chimpanzees and macaque monkeys to make mini brains for each species. After four months, a key difference was that nerve cells in the chimp and monkey organoids were more mature.

Identifying such differences may be a step towards explaining why humans are more intelligent although the team doesnt speculate on exactly how their findings might relate to this puzzle.

Until now it wasnt possible to compare human and chimp organ development, says Camp. The organoids, some made from stem cells that can be generated directly from adult cells, offer a way of making that comparison.

Camp and his team also delved into another long-standing puzzle: why there are so few differences between the protein-coding genes of humans and the other apes, considering the huge disparity in our intellects.

A recent technique for analysing which genes are turned on or off in individual cells, known as single cell RNA sequencing, has suggested the answer might lie in differences in which genes are turned on at different times.

In the latest study, Camps team charted which genes are turned on in different brain cells over four months of mini brain development, comparing the results across humans, chimps and macaques, to make a database other researchers can also use.

Organoids are most useful for society in letting us understand disease, says Camp. But its also very interesting to think about where our species came from and how we became uniquely human.

Journal reference: Nature, DOI: 10.1038/s41586-019-1654-9

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Humans evolved to think faster by slowing down brain development - New Scientist News