Brain-body allometries without and with evolution. a, Brain size at age 40 versus body size at age 40 on a log-log scale, developed according to the brain model of 106 randomly sampled genotypes using sapiens scenario parameter values. b, Brain size at age 40 versus body size at age 40 over evolutionary time on a log-log scale for two trajectories. The lower trajectory uses scenario parameter values of afarensis and somewhat Naive2 ancestral genotypic traits. The upper trajectory uses the parameter values of the sapiens scenario and the evolved genotypic traits of the lower trajectory as ancestral genotypic traits. Credit: Nature Human behavior (2024). DOI: 10.1038/s41562-024-01887-8
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Brain-body allometries without and with evolution. a, Brain size at age 40 versus body size at age 40 on a log-log scale, developed according to the brain model of 106 randomly sampled genotypes using sapiens scenario parameter values. b, Brain size at age 40 versus body size at age 40 over evolutionary time on a log-log scale for two trajectories. The bottom trajectory uses afarensis scenario parameter values and somewhat Naive2 ancestral genotypic traits. The upper trajectory uses the parameter values of the sapiens scenario and the evolved genotypic traits of the lower trajectory as ancestral genotypic traits. Credit: Nature Human behavior (2024). DOI: 10.1038/s41562-024-01887-8
Mauricio González-Forero, an evolutionary biologist at the University of St Andrews, UK, is proposing a new theory to explain the massive growth of the human brain throughout its evolutionary history.
In his paper published in the journal Nature Human behaviorhe suggests that the human brain was primed to grow due to a number of factors and only took off when the energy previously used to maintain the ovarian follicles was released, allowing the brain to use more than the previous portion .
Over the years, many theories have been proposed to explain the growth of the human brain compared to that of early contemporaries such as the great apes. Many such theories revolve around humans coming down from the trees to become social foragers.
In more recent times, some have suggested that our brains got bigger while our guts got smaller, due to switching to a fermented diet, thereby freeing up resources.
González-Forero has previously proposed other theories, most of which involve changes in the body that free up resources, resulting in changes in the way the body allocates energy resources. In this new effort, he suggests that one such overlooked change may be the ovaries.
González-Forero began his study by noting that most theories that seek to explain brain growth are not supported by hard numbers. This led him to use mathematical tools to separate theories of evolutionary adaptation based on selection from those based on constraints that may have prevented other adaptations. And that led him to human ovaries, or more specifically, the number of ovarian follicles.
Ovarian follicles are spherical clusters of cells found inside the ovaries – they secrete hormones that control the phases of the menstrual cycle. By the time they reach puberty, most girls have between 200,000 and 300,000 follicles – each with the potential to release an egg cell.
González-Forero noted that as humans evolved, the energy needed to maintain those follicles was reduced. And because people still ate as much as they ever did, it freed up energy to be used as a resource by other parts of the body like the brain.
Doing the math, González-Forero found that the amount of energy released by follicle maintenance was roughly equal to the amount of energy growth needed for the human brain to grow to its current size. According to him, the brain was always ready to grow; it was simply held back by a lack of energy.
More information:
Mauricio González-Forero, Evolutionary-developmental (evo-devo) dynamics of hominin brain size, Nature Human behavior (2024). DOI: 10.1038/s41562-024-01887-8
Magazine Information:
Nature Human behavior
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