• Greg Morgan

How Human Beings Developed Big Brains

Human beings have bigger brains than our animal cousins, with the size and complexity of our neural network responsible for continual leaps of evolution. While multiple studies have been carried out on the human brain, new research has uncovered the actual biological switch that may be responsible for our collective growth as a species. When 'brain organoids' were grown in the lab from humans, gorillas, and chimpanzees, scientists could influence the amount of growth and complexity. It seems the human brain is more patient during the early stages of growth, which gives it more time and space to develop complexity down the track.

Brain size has increased rapidly over the course of human evolution, with human brains now much larger than those of other primates. From better sight and hearing abilities to the use of language and complex tools, this growth has given us a range of advantages in the modern world. However, despite multiple studies into the human brain, scientists have never known when and how these key differences emerged. While some experts believed the neocortex was responsible, others thought the cerebellum also had a key role to play.

A new study is the first to identify the exact mechanism responsible for the disproportionate growth of human brains. Our brains have three times as many neurons as chimpanzee and gorilla brains, and it may all be down to a simple switch. The study, carried out by the MRC Laboratory of Molecular Biology, identified a key molecular switch that makes all the difference. A simple gene called 'ZEB2' is responsible, with this switch slightly delayed in human organoids and leading to more complex results.

When human brain organoids were grown in the lab, they took longer to develop than gorillas and chimpanzee organodis, keeping their cylinder-like shape for longer, splitting more frequently, and therefore producing more cells. According to Dr. Madeline Lancaster, the lead author of the study, "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."

Like many things in life, a simple alteration repeated time and time again is thought to be responsible for profound evolutionary changes. The earlier on these changes are made, the more potential they have to influence the evolutionary process. According to Dr. Lancaster, "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."

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