A team of researchers led by the University of Cambridge in the United Kingdom and the University of Tübingen in Germany gathered measurements of brain and body size for more than 300 fossils from the Homo genus or family, to which modern day humans -- Homo sapiens -- belong.
The team used this data, combined with a reconstruction of the Earth's regional climates from the last million years, and calculated the climate that would have been experienced by each fossil when it was a living human.
Researchers found that climate -- particularly temperature -- has been the main driver of changes in body size for the past million years. Colder, harsher climates were linked to larger bodies, while warmer climates were linked to smaller bodies, the team found.
"Larger bodies can buffer individuals from cold temperatures -- the larger you are, the smaller your surface compared to your volume, so you conserve heat more efficiently," Andrea Manica, a professor of evolutionary ecology at the University of Cambridge, told CNN via email.
"This is a relationship that is found in many animals, and even among contemporary humans, but we now know that it was a major driver behind the changes in body size over the last million years."
The research was published in the journal Nature Communications.
Climate links to brain size are less clear
Researchers also looked at the impact of environmental factors on brain size.
Homo sapiens emerged around 300,000 years ago in Africa, but the Homo genus, which includes Neanderthals and other extinct, related species such as Homo habilis and Homo erectus, has existed for much longer.
Experts said that compared to earlier species such as Homo habilis, we, Homo sapiens, are 50% heavier and our brains are three times larger -- but the reasons behind such changes are still debated.
The team found that climate did have a role in brain size, but there was so much variation in brain size that can't be explained by environmental changes.
"Interestingly, brain size changes were completely unrelated to temperature, so body and brain size evolved under distinct pressures," Manica explained.
"For brain size, we found that larger brains were found in stable environments -- brains are expensive, so you can't sustain them if you lack resources."
He added: "For early Homo, there was also a tendency to develop bigger brains in more open habitats, where our ancestors would have been required to hunt large mammals."
"Importantly, however, climate explains changes in brain size much less than it does for body size. This means that other factors such as added cognitive challenges of ever more and complex social lives, more diverse diets, and more sophisticated technology were likely the main drivers of changes in brain size," he added.
Manica thinks it's unlikely that current climate change will dramatically impact our body sizes for now.
"The changes that we described have happened over thousands of years, or rather, tens of thousands of years," he explained. "So a few years of climate change will do little to our bodies or brains.
"If we managed to keep changing the climate without destroying the whole planet for a long time -- we are not doing too well in that regard -- then maybe, but thankfully this is an issue we don't have to worry about for quite a while," he said.
Warming temperatures are affecting animals
But still, climate change may already shrinking some of the planet's animals.
North American migratory birds have been getting smaller over the past four decades, and their wingspan wider. The changes appear to be a response to a warming climate, according to a 2019 study.
In an analysis of 70,716 dead birds representing 52 species logged between 1978 and 2016, researchers found that 49 saw statistically significant declines in body size.
The authors suggested that the shrinking body sizes are a response to climate warming, with temperatures at the birds' summer breeding grounds north of Chicago increasing roughly 1 degree Celsius (1.8 degrees Fahrenheit) over the course of the study.
And a 2011 study in the journal Nature Climate Change found that ectotherms -- cold-blooded animals like toads, turtles, and snakes that rely on environmental heat sources -- "are already changing a lot" as temperatures rise.
Both aquatic and terrestrial ectotherms have been shrinking, according to the study, with common toads' size and condition decreasing as temperatures rose 1.5 degrees Celsius over a 22-year period.
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