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.