How early human brains expanded over time

Encephalization — the increase in brain size relative to the body — is a key feature in human evolution, but exactly how this process occurred is still unclear. Looking at studies comparing the cranial capacity of early modern humans to other hominin species, some researchers have concluded that brain size grew gradually over time. Others have argued that the human brain grew in spurts separated by long periods. 

To complicate matters, there is confusion over how evolutionary processes, such as the branching of lineages and the splitting of an ancestral species into two distinct species, contributed to the diversification of brain size seen in early humans and other hominins.  

Now, new research provides fresh insights into the patterns of brain expansion within and between the hominins. The study, published in the Proceedings of the National Academy of Sciences, reveals that brain size increased within lineages comprising a single species, with the fastest expansion happening in the most recent human ancestors.

Brain expansion

Evolutionary anthropologist Thomas Püschel and his team at the University of Oxford studied fossils from early human ancestors (hominins) dating back 7 million years, creating the most complete dataset on brain evolution so far. Since fossils are incomplete, they used advanced computer models to estimate missing details.

Their analysis showed that while larger-bodied species often had bigger brains, brain size and body mass didn’t always increase together. Instead, brain size tended to grow gradually within individual species, influenced by a mix of genetic, environmental, and behavioral factors rather than a single cause.

Furthermore, the researchers found that, within individual species, the increases in brain growth accelerated in the more recent lineages.    

“The variation we found suggests different intensities and durations of selective pressures within individual hominin lineages, challenging the idea of a single, uniform driver of brain evolution,” Püschel told Big Think.

“Our findings suggest an accelerating within-species increase in brain size over time, supporting hypotheses that propose a potential co-evolutionary feedback loop involving sociality, culture, technology, and/or language.” 

These processes may have reinforced certain selective pressures, whereby greater cognitive capacities allowed for more complex behaviors, which in turn further selected for larger brains. 

“Genetic mutations may have influenced brain growth potential, but it’s more likely that a combination of genetic, ecological, and behavioral factors interacted to shape encephalization differently across species and time,” Püschel added. 

Püschel and his colleagues are already performing additional analyses of the dataset they have compiled. 

“We’ve begun incorporating environmental and climatic predictors to explore how these factors may have influenced brain evolution,” he says. “Many longstanding hypotheses have linked encephalization to climate, but no clear consensus has been reached. Our approach allows us to test some of these ideas more robustly.” 

In a recent preprint, Püschel and his colleagues published preliminary evidence that colder and more variable temperatures may have driven encephalization within species by promoting adaptations that protected against hypothermia.