An increase in body and brain size is one of the defining characteristics of human evolution. However, it remains widely debated what the primary cause of this increased brain and body size may be—researchers have suggested that changes in diet, behavior, or demography may influence larger bodies and brains, or that additional factors like climate, ecology, technology, or culture may also contribute.
New research jointly led by anthropologists Manuel Will from the University of Tübingen and Mario Krapp from the University of Cambridge tested the environmental influence on changes in body size using a dataset of fossil human ancestors consisting of three main taxonomic groups: early Homo (fossils that predate the Neandertals but are not Homo naledi), Neandertals, and humans who lived from around 2.5 million-11,000 years ago. Their data included brain and body estimates from individual fossils belonging to each of these three groups, which were compared with climate reconstruction estimates based on relevant variables such as mean annual temperature and mean annual precipitation.
The study found that temperature is, uniformly, a major predictor of body size. More specifically, larger bodies tend to be found in climates with lower, colder temperatures. These results are supported by Bergmann’s Rule, an ecogeographical rule first introduced by Carl Bergmann in 1847 stating that populations with larger bodies tend to be found in colder climates whereas those with smaller bodies are more often found in warmer climates.
Brain size did not appear to strongly correlate with temperature, suggesting that brain size tends to be less impacted by environmental variables in human evolution. Brain size did, however, correlate with precipitation, suggesting that larger brains occur in the most stable environments. Human brains are expensive, meaning they need adequate resources to sustain themselves, so environments with stable, reliable rainfall allow for adequate resources, such as food, to prosper. Since climate explains less about brain size changes, other factors like complex social lives, technology, or diet are more likely to be the main driver of brain size evolution.
The study suggests that both body and brain size of humans fluctuated over the last million years, and that this fluctuation is—at least in part—linked to a changing climate. With the current warming climate, it remains to be tested how temperature—and possibly rainfall—will affect human brains and bodies. Other studies have found that many plant and animal species are becoming smaller due to rising temperatures and unreliable rainfall, which could impact human nutrition in the future.