Ovarian Follicles May Explain Human Brain Expansion

The human brain may have gradually become larger not because evolution singled out ancestors with bigger brains and rewarded their intelligence, but because energy once used to develop egg-bearing ovarian follicles was redirected to the brain.

Ovarian follicles and human brain expansion

This is the proposal put forward by Mauricio González-Forero, a mathematical evolutionary biologist at the University of St Andrews, who modelled hominin brain growth from australopithecines through to modern humans.

Drawing on earlier research, González-Forero used straightforward mathematics to develop equations describing the growth of brain and body tissues. The equations account for the energy costs of growth and for genetic traits that govern how the body distributes energy at different developmental stages.

His results suggest that reproductive capacity, rather than brain size, had a greater influence on human evolution and development. That is a somewhat humbling conclusion for a species that places such value on its unusually large brain.

There is certainly no lack of explanations for why the human brain almost tripled in size across 4 million years, while the brains of our distant chimpanzee ancestors changed very little.

More recently, researchers have proposed that brain expansion, which occurred as human intestines became smaller, may have been fuelled when our ancestors moved away from a raw diet and began experimenting with fermented foods. As fermented food required less processing by the colon, energy may have been released for brain growth.

González-Forero’s earlier research likewise indicated that the remarkable enlargement of the human brain could have stemmed from a reorganisation of the body’s energy budget.

Energy budgets, food and evolutionary pressures

In 2018, González-Forero and Andy Gardner, another mathematician at the University of St Andrews, concluded that ‘ecological’ difficulties – including locating food and making fires – may have been the central challenges favouring increased brainpower. This contrasts with previous ideas that stressed social forces such as cooperation and competition.

Early humans could only obtain the energy needed to offset the immense metabolic expense that otherwise constrained brain growth by finding food. González-Forero’s previous work suggests that solving problems and exchanging cultural knowledge about hunting, gathering and locating tasty food may also have encouraged brain expansion.

However, González-Forero argues that previous explanations have lacked mathematical methods capable of separating evolutionary selection – in which natural selection actively favours beneficial traits – from constraints that simply limit or channel evolution.

He therefore revisited his model, modifying its framework with equations that, he says, distinguish selection from constraints; measure the energy demands of developing brain, body and reproductive tissues; and incorporate differences in genetic traits that regulate human development from infancy and adolescence to adulthood.

Why follicle count may have mattered more than brain size

The revised method “enables deeper analysis, showing that hominin brain expansion occurs in the model because of direct selection on [ovarian] follicle count rather than on brain size,” González-Forero writes in his published paper.

Put another way, humans may only have evolved larger brains once energy assigned to reproductive-tissue growth – particularly the ovarian follicles that support and release one egg each month – became available at a pivotal point in human development.

González-Forero reasons that brain growth was constrained by the number of ovarian follicles nourished early in life, rather than by evolution eliminating smaller brains in favour of larger ones. Our ancestors therefore remained relatively small-brained until energy was shifted from the ovaries to the brain.

According to González-Forero, these reproductive constraints “divert evolutionary change in a direction that causes brain expansion” and were so significant that “that without those constraints, brain expansion is not favored by selection and does not evolve.”

On X, formerly Twitter, González-Forero said that his model accurately recreates the main patterns of human development from childhood to adulthood, along with a doubling in brain size.

It is worth remembering, however, that human brain growth may not have followed a continuous upward course. Some scientists believe brains may at times have become smaller, perhaps thousands of years ago after the last ice age, possibly in response to climate change.

Yet this ‘brain shrinkage theory’ has faced intense scrutiny. In other words, even with all our collective brainpower, there is still only a vague understanding of how and when the human brain enlarged, and whether it genuinely became smaller in order to operate more efficiently.

The study was published in Nature Human Behaviour.