The average body size of humans over the past million years is strongly linked to temperature, according to an international team of scientists.
Over the course of human evolution, our bodies and brains have generally been increasing in size. But the driving mechanisms behind these changes are not well understood; hypotheses range from environmental factors to social complexity to diets to the development of tool technology.
Now, researchers led by the Universities of Cambridge and Tübingen have found that temperature plays a vital role: larger bodies evolved in colder, harsher climates, while smaller bodies evolved in warmer ones.
In their study, published in Nature Communications, they analysed the body and brain size of more than 300 fossils from the genus Homo – including Homo sapiens and other extinct species such as Neanderthals, Homo habilis and Homo erectus – and compared their measurements with regional climate reconstructions over the past million years. This allowed them to determine what type of climate each species lived in.
“Our study indicates that climate – particularly temperature – has been the main driver of changes in body size for the past million years,” says lead author Andrea Manica from the University of Cambridge.
“We can see from people living today that those in warmer climates tend to be smaller, and those living in colder climates tend to be bigger. We now know that the same climatic influences have been at work for the last million years.”
The researchers suggest that a bulkier build could be an adaptation to survive more extreme temperatures: by reducing surface area compared to weight, less heat is lost through the skin.
These findings are consistent with Bergmann’s rule, proposed by 19th century anatomist Carl Bergmann. He found that in closely related species of warm-blooded animals, the cooler the climate, the larger the body size – although researchers are cautious when extending this rule back through time.
This new study also examined whether brain size has changed in tandem with the climate, but the links were weak.
“We found that different factors determine brain size and body size – they’re not under the same evolutionary pressures,” explains co-author Manuel Will from the University of Tübingen, Germany. “The environment has a much greater influence on our body size than our brain size.”
Although they found that humans with larger brains did tend to live in areas with less vegetation, like grasslands – where perhaps the complex task of hunting large animals for food may have influenced noggin size – the researchers suggest that other factors like diet, tool development and increasingly complex social lives were more important in the evolution of our brains.
Interestingly, our brain and body sizes are still changing today – but in the opposite direction. In general, humans are shorter and lighter than our ancestors, and our brain size appears to have been shrinking since the end of the last ice age, around 11,500 years ago.
“It’s fun to speculate what will happen to body and brain sizes in the future, but we should be careful not to extrapolate too much based on the last million years because so many factors can change,” notes Manica.
To build on this study, the researchers suggest it would be fruitful to look back further in time to up to four million years ago to see if this temperature-body size trend holds for our earlier ancestors such as Ardipithecus, Australopithecus or Paranthropus. But both the fossil record and paleoclimate record of this earlier era are sparser.
“Improved paleoclimate models and new discoveries with good chronometric ages and taxonomic information will eventually allow for such studies,” the researchers conclude in their paper.