Climate change may cause humans to become smaller as the planet continues to warm, according to a new study.
Scientists used more than 300 fossils to track how human bodies have fluctuated in size over thousands of years.
They found a clear link that people living in colder environments were larger, while people in warmer regions were smaller.
This trend continues today, the scientists add, with people in colder parts of the planet tending to be bigger than those in warmer places. For example, the average Dutch man is 6ft tall, whereas the average Indian man is just 5ft 5inches tall.
In the study, the researchers from the Universities of Cambridge and Tubingen write that for every degree of warming, there is a 0.87 per cent decrease in body mass.
For example, a 2°C warming would be associated with a body size decrease of one kilogram (2.2 pounds) for someone weighing 60 kg (9 stone 6 pounds). The same trend works in reverse, so for every degree of cooling, body size increases by 0.87 per cent.
Being bigger means a person’s mass to surface area ratio is greater and they are therefore more heat efficient, and lose less warmth than smaller people.
Professor Andrea Manica, a researcher in the University of Cambridge’s department of zoology, who led the study, said: "Our study indicates that climate – particularly temperature – has been the main driver of changes in body size for the past million years."
He added: "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.”
‘Brains are expensive’
Speaking to The Telegraph, Professor Manica added that this trend will likely continue as climate change sees global temperatures soar.
“Over a long time, yes, we would expect a decrease in body size [due to climate change],” he said.
“But the effects that we describe occurred over thousands of years, so a few years of warming will do little to body size. But if we were to keep changing temperature and keep them there for a long time, then yes, one would expect such changes.”
The study, published in Nature Communications, also found brain size changed over time. However, it did not evolve in tandem with body size, following its own timeline.
Brain size however, was linked to when people were living in habitats with less vegetation, like open steppes and grasslands, but also in ecologically more stable areas.
“Brain size was not affected by temperature, but it was affected by long term climatic stability,” Professor Manica said.
“The likely rationale for that is that brains are expensive, and you need reliable resources to maintain them.”
Dr Manuel Will at the University of Tubingen, Germany, first author of the study, said: "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."