A human brain, dissected

In the 6 million years since the ancestors of humans and chimpanzees went their separate ways, the human brain has tripled in size. This spectacular shift, not observed in other great apes, is a defining feature of what makes us human.

What is the biological basis of our big brains? Previous studies indicate that human brain expansion reflects a universal increase in the number of neuron cells compared to our primate cousins, rather than differences in specific cell types. This suggests that the human “growth spurt” likely occurs very early on in our development.

To test this theory, a team of scientists in Cambridge used a novel lab technique to mimic early brain development across primate species for the first time. The team grew 3D “mini brains” from chimp, gorilla and human cells left over from medical procedures. These leftover cells were reprogrammed into stem cells before being grown into clumps of early brain tissue (organoids) a few millimetres wide. The scientists then compared patterns of growth and gene expression (which genes are switched on or off across time) between the organoids.

They observed that the human organoids were already bigger than the primates’ by day two and were twice as large by week five. This confirmed the theory that differences in brain growth between the species occured during very early development.

The research also shed light on a previously unknown “transition” state where early brain cells move from a period of rapid cell division and therefore growth, to the period of neurogenesis (when cells specialise into mature neurons). Whilst in chimps and gorillas this transition occurred at around day five, in humans it was day seven, meaning early human brain cells have more time to multiply before they mature.

This research not only sheds light on one of the most significant adaptations of human evolution, but could also provide insights into rare neurological disorders – many of which are characterised by abnormalities in brain growth.

This article is from the New Humanist summer 2021 edition. Subscribe today.