Human intelligence: it’s how your brain is wired rather than size that matters

Our brains don’t look that special when looking at their relative size compared to our closest animal relatives. To understand human intelligence, scientists are now looking deeper.

Arthur Keith was one of those misbegotten researchers who have turned out to be wrong in many of the things they said. A prominent anatomist and anthropologist in the early 20th Century, he was a proponent of scientific racism and opposed racial mixing. At least partly because of his racial views, he was convinced humans originated in Europe, not Africa as is now universally accepted. And he was a strong supporter of . They were just 1m (3ft) tall and lived on the island of Flores in Indonesia within the last few hundred thousand years. They died out , depending on how they are expressed. One gene expressed in developing humans but not chimps controls an entire network of other genes, which are thought to be involved in human brain development.

Some changes in gene expression are tantalising. In a 2017 study, Sestan’s team compared gene expression in human, chimp and monkey brains. They found that some neurons in one region of the human brain expressed genes involved in making dopamine, a brain chemical involved in feelings of reward. The equivalent cells in the chimps and monkeys didn’t express these genes. “We cultured these neurons.” says Sestan. “They can make dopamine in vitro.”

“If this holds true in a real brain, we humans could make dopamine internally in the cortex,” says Sestan. He has an intriguing speculation about what this might mean. Humans can feel pleasure just from thinking and solving problems, which may well be unique. If we have cortical neurons that make dopamine, they could be “a reward system for just thinking”. He emphasises, however, that for now this is speculation.

We have come a long way from simply comparing the sizes of different primates’ brains. Scientists are now looking at changes in genome sequences, changes in gene expression, changes in cell shape and behaviour, and changes in the brain’s wiring diagram. What we are missing is “understanding how all of these elements, as an interplay, become a system and this system shapes our behaviour”, says van den Heuvel.

Treutlein and her colleagues took a big step in this direction in 2019, publishing an “atlas” of every cell in a human brain at an early stage of development. In 2023, a team of 500 researchers from across Europe announced the completion of the Human Brain Project, a 10-year project to delve into the complex structure and function of the brain.

A huge ongoing project called the Human Cell Atlas aims to build on the insights gained so far. Its members aim to map every cell type in the human body: its position, shape, gene expression and more. “There are so many cell types in the brain,” says Treutlein. The challenge will be to make sense of the vast dataset.

While that project will take decades, it’s already possible to draw some conclusions about brain size. “I think it’s just one of many factors,” says Silver.

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