The mystery over why human brains have shrunk over time

The brains of modern humans are around 13% smaller than those of Homo sapiens who lived 100,000 years ago.Exactly why is still puzzling researchers.

Traditionally our "big brain" is thought to be what sets our species apart from other animals. Our capacity for thought and innovation allowed us to create the first art, invent the wheel, and even land on the Moon.

Certainly, when compared with other animals of a similar size, our brains are gigantic. The human brain has nearly quadrupled in size in the six million years since our species last shared a common ancestor with chimpanzees. However, studies show this trend toward larger brains has reversed in Homo sapiens. In our species, average brain sizes have shrunk over the course of the last 100,000 years.

For example in a recent 2023 study, Ian Tattersall, a paleoanthropologist and curator emeritus with the American Museum of Natural History in New York City, tracked the braincase volumes of ancient hominins through time. He started with the oldest known species, and ended with modern humans.

He found that rapid brain expansion occurred independently in different species of hominins, and at different times across Asia, Europe and Africa. Species whose brains grew over time include Australopithecus afarensis, Homo erectus, Homo heidelbergensis, and Homo neanderthalensis.

However, the trend for brain enlargement over time was turned on its head with the arrival of modern humans. The skulls of men and women today are on average 12.7% smaller than that of Homo sapiens who lived during the last ice age.

"We have very peculiarly shaped skulls, so early humans are very easy to recognise – and the very first ones have extremely large brains," says Tattersall.

Tattersall's finding replicates those of others. For example in 1934, Gerhardt Von Bonin, a German-born scientist affiliated with the University of Chicago at Illinois, wrote that "there is a definite indication of a decrease [in the human brain] at least in Europe within the last 10,000 or 20,000 years."

So how can we explain this striking reduction? Tattersall suggests that the shrinkage in brain size began around 100,000 years ago, which corresponds to a period of time in which humans switched from a more intuitive style of thinking to what he terms "symbolic information processing" – or thinking in a more abstract way to better understand your surroundings.

"This was the time when humans began producing symbolic artefacts and engravings with meaningful geometric images," says Tattersall.

Tattersall believes that the catalyst that caused this change in thinking style was the spontaneous invention of language. This led to the neural pathways of the brain being reorganised in a more metabolically efficient way, allowing humans to get more "bang for their buck".

In other words, as smaller and better organised brains were able to perform more complex computations, metabolically expensive larger brains simply became unnecessary.