Researchers reveal that our changing climate has driven the evolution of human body size, and there are some signs it contributed to changing brain size, too.
A well-known pattern in human evolution is an increase in body and brain size. Our species, Homo sapiens, is part of the Homo genus and emerged around 300,000 years ago. We are much bigger than earlier Homo species and have brains three times larger than humans who lived a million years ago.
There has been debate over the factors causing humans to evolve in this way, prompting a research team, led by Cambridge University and Tübingen University in Germany, to combine data on more than 300 human fossils from the Homo genus with climate models to establish the role the climate played in driving evolution.
The team determined what temperature, precipitation, and other climate conditions each of the fossils, spanning the last million years, would have experienced when it was a living human. The study, published today in Nature Communications, found a strong link between temperature and body size, showing that climate was a key driver of body size during that period.
“The colder it gets, the bigger the humans are,” said Dr Manuel Will, Tübingen University researcher and joint first author on the study. “If you’re bigger, you have a bigger body – you are producing more heat but losing relatively less because your surface is not expanding at the same rate.”
This relationship between climate and body mass is consistent with Bergmann’s rule, which predicts a larger bodyweight in colder environments and a smaller bodyweight in warmer environments. This is observed in animal species such as bears – polar bears living in the Arctic, for example, weigh a lot more than brown bears living in comparatively warmer climates.
“It’s not completely surprising, but it’s interesting to see that in this respect, our evolution isn’t that different from other mammals,” said Dr Nick Longrich from the University of Bath Milner Centre for Evolution, not involved in the research. “We face similar problems when it comes to gaining and losing heat, so we seem to have evolved in similar ways.”
The study also found a link between brain size and climate, but the results show that environmental factors have substantially less influence on the brain size than they do on body size.
“This phenomenon shows that body and brain size are under different selective pressures,” said Prof Andrea Manica, another researcher on the study. “This study really manages to detangle the fact that both [brain and body size] are increasing but increasing for very different reasons”, he adds.
The results showed no association of brain size with temperature. Instead, the researchers linked more stable climates with bigger brains. This effect links to the dietary needs of humans living in environments of variable climatic stability.
“The more stable [the climate] is, the larger brains are,” said Will. “You need a lot of energy to maintain a big brain – in stable environments, you find more stable food, so you likely have sufficient nutrition to give you that energy”.
Researchers also saw indications of behavioural changes that influence brain size in response to hunting strategies in more open environments. These more indirect factors reveal the complexity in understanding what factors have driven human evolution.
“There are other factors bedside climate driving things,” said Longrich. Competitive, social, cultural and technological factors are identified by the researchers but not tested in this study. Future models should aim to include these interacting components.
Will points out that evolution is ongoing, but there are different drivers now to a million years ago.
“The past gives us clues about the future; we can learn from it. But we cannot simply extrapolate from it,” said Will. He explained that while we are currently seeing the climate is getting warmer, we cannot simply assume that our bodies will get smaller as a result.