Researchers examine how climate has affected body size during evolution

Skulls: - Left: Amud 1, Neanderthal, 55.000 years ago, ~1750 cm³ - Middle: Cro Magnon, Homo sapiens, 32.000 years ago, ~1570 cm³ - Right: Atapuerca 5, Middle Pleistocene Homo, 430.000 years ago, ~1100 cm³ Femora: - Top: Middle Pleistocene Homo, Trinil, 540.000 years ago, ~50 kg - Bottom: Neanderthal, La Ferrassie 1, 44.000 years ago, ~90 kg. (Credit: Manuel Will)

TORONTO — A group of researchers have found the connection between climate and humans’ fluctuating body size as we’ve evolved over the last million years.

The study, published in Nature Communications on Thursday, found that human size fluctuated significantly over the last million years, and that colder temperatures meant larger body sizes. It also found that brain size has changed dramatically -- but not in conjunction with body size changes or temperatures.

The team from Cambridge University and the University of Tübingen looked at the 300 fossils of the genus Homo from across the globe and regional climates from the last million years. They combined this data to pinpoint the exact climate each fossil would have lived through.

Homo sapiens, our species, have been around for some 300,000 years, but the Homo genus has been around much longer, going back to Neanderthals and other extinct species like Homo erectus.

Increasing size has been a defining feature of evolution. Homo sapiens, according to the study, are 50-per-cent heavier than the extinct Homo habilis, and our brains are three times the size.

"Our study indicates that climate -- particularly temperature -- has been the main driver of changes in body size for the past million years," Prof. Andrea Manica, a researcher in the University of Cambridge's Department of Zoology who led the study, said in a press release.

Colder climates often breed larger bodies; the bigger size is believed to work as a buffer against cold temperatures.

“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,” said Manica. “We now know that the same climatic influences have been at work for the last million years."

The connection between environment and brain size isn’t quite as strong as the correlation between size and temperature, but researchers noted that larger brain sizes were associated with areas with less vegetation, but also in more ecologically stable climates. The researchers believe that this may have been a result of hunting among these groups, a complex task that could have impacted brain size.

"We found that different factors determine brain size and body size - they're not under the same evolutionary pressures. The environment has a much greater influence on our body size than our brain size," Manuel Will at the University of Tubingen and first author of the study, said in a press release.

These environments also bore more sustenance and nutrients needed to aid in brain growth.

"There is an indirect environmental influence on brain size in more stable and open areas: the amount of nutrients gained from the environment had to be sufficient to allow for the maintenance and growth of our large and particularly energy-demanding brains.”

The researchers found that non-environmental factors like complex social lives, diverse diets and sophisticated technology impacted brain size more than climate.

Evidence suggests that our brains and bodies are still evolving, according to researchers -- our brain size has been thought to be decreasing in size for nearly 12,000 years and our continued reliance on technology and computers to complete complex tasks could see further shrinkage. But the experts say that this is still speculation and nothing is set in stone.

"It's fun to speculate about 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," said Manica.