Brain growth slowed down about 300,000 years ago due to energetic and climate pressures, according to a study published in Brain & Cognition.
One of the puzzles of human evolution is why Homo sapiens is the only surviving species within the Homo lineage. Larger brains have often been seen as a key advantage, enabling fire use, tool-making, and symbolic communication. However, big brains also come at a cost—they consume around 20% of our resting energy and produce considerable heat, which can be a liability in warmer climates.
Study author Jeffrey M. Stibel examined this evolutionary trade-off. Earlier Homo species experienced strong selection for larger brains, which likely helped them navigate shifting environments and complex social worlds. But fossil evidence suggests that in the past 100,000 years, brain size began to plateau or even shrink, raising the possibility that survival depended not just on biology but also on cultural innovations.
“I am drawn to broad questions about human evolution and cognition—how we became who we are and what forces shaped that path. The slowdown in brain growth around 100,000 years ago was a striking pivot,” said Stibel, a trustee of the Natural History Museum and Tufts University.
“It raised a puzzle: if larger brains were no longer being strongly selected for, how did we manage to expand our cognitive reach? That question sits right at the crossroads of biology, culture, and survival.”
Stibel analyzed 800 cranial capacity measurements from across the Homo genus. These included specimens of H. erectus, H. heidelbergensis, H. neanderthalensis, and H. sapiens, among others, totaling 690 modern humans and 99 non-modern Homo individuals. Juvenile or deformed skulls were excluded to ensure reliable adult estimates.
The brain size estimates were derived using a regression formula validated across 27 primate species. Fossils were grouped into 100,000-year bins, with additional analyses based on major climate phases (glacial vs. interglacial) determined by global isotope records.
“Around 100,000 years ago, the evolutionary pressure for bigger brains eased in humans. Groups that didn’t adapt cognitively may have disappeared—, possibly explaining why some species went extinct. In contrast, those that survived found new ways to keep improving thinking power despite smaller brains,” explained Stibel.
“The primary adaptation appears to be cognitive offloading—shifting mental effort into tools, language, and shared cultural systems. These tools expanded our minds beyond our brains, making us extraordinarily capable but also deeply dependent on the systems we’ve built.”
Brain size increased significantly during the Early and Middle Pleistocene, but growth slowed after about 300,000 years ago. The peak in brain mass occurred roughly 100,000 years ago, with little evidence of further directional growth afterward. Instead, the data suggest a shift toward stabilizing selection, where maintaining brain size, rather than expanding it, became advantageous.
Climate played a critical role. Significant differences between brain size during glacial and interglacial periods only emerged in the last 100,000 years. Brains were larger during glacial phases and smaller during interglacials, suggesting that warmer climates amplified the metabolic and thermoregulatory costs of sustaining large brains. This shift may have heightened extinction risks for some Homo populations while favoring adaptations such as increased brain efficiency and reliance on cultural “cognitive offloading.”
“We still need to understand the fine-grained dynamics of this cognitive shift,” Stibel said. “Did cognitive offloading make certain populations more resilient to environmental shocks? How did these adaptations spread among early humans? And today, are we crossing similar thresholds with digital technology, where our survival hinges less on individual brainpower and more on the stability of vast cultural and technological networks?”
“This isn’t just ancient history—it’s an ongoing evolutionary story. We are the only species capable of anticipating long-term consequences and acting to change them. By understanding the moment when our minds began living outside of our skulls, we can better prepare for the next chapters in human evolution, including the rise of artificial intelligence.”
The study, “Did increasing brain size place early humans at risk of extinction?” was authored by Jeffrey M. Stibel.
