Human fossils illustrating the variation in brain (skulls) and body size (thigh bones) during the Pleistocene. 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. (Photo credit: Manuel Will)

CAMBRIDGE, United Kingdom — Climate change has become a daily concern of modern life. While most of that focus is on the future, a new study finds Earth’s climate has already changed the size of human bodies and even our brains throughout time.

Researchers from the University of Cambridge say the average body size of humans has “fluctuated significantly” over the last one million years, with a strong link to the temperature. Colder, harsher climates drove the evolution of larger body sizes, while warmer climates led to smaller bodies.

Brain size also changed dramatically, but did not evolve in tandem with body size, according to the findings. The team adds our brains are currently shrinking and may continue to do so due to an over-reliance on computer technology.

Bigger bodies defend against the cold

Study authors from Cambridge and the University of Tübingen in Germany collected body and brain size measurements from more than 300 fossils from the genus Homo found around the world. By combining the figures with a reconstruction of the world’s regional climates over the last one million years, they pinpointed the specific climate endured by each fossil when it was a living human.

Their findings, appearing in the journal Nature Communications, reveal that the average body size of humans has seen noticeable changes over the last million years. Specifically, researchers note bigger bodies may act as a buffer against colder temperatures, as less heat is lost from a body when its mass is large relative to its surface area.

Our species, Homo sapiens, emerged around 300,000 years ago in Africa. However, the genus Homo has existed for much longer, and includes the Neanderthals and other extinct species such as Homo habilis and Homo erectus.

The researchers explain that a “defining trait” of the evolution of our genus is a trend of increasing body and brain size, In comparison to earlier species, such as Homo habilis, modern humans are 50 percent heavier and our brains are three times larger. However, scientists continue to debate what drives such changes.

“Our study indicates that climate – particularly temperature – has been the main driver of changes in body size for the past million years,” says study leader Professor Andrea Manica of Cambridge’s Department of Zoology in a university release.

“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.”

Someone thing else may be changing brain size

The research team also looked at the effect of environmental factors on brain size in the genus Homo, but discovered that the link between the two appears to be weak. Brain size tended to be larger when Homo was living in habitats with less vegetation, such as open steppes and grasslands, but also in more ecologically stable areas.

In combination with archaeological data, the results suggest that people living in those habitats hunted large animals as food; a complex task that might have driven the evolution of larger brains.

“We found that different factors determine brain size and body size – they’re not under the same evolutionary pressures,” explains study first author Dr. Manuel Will from the University of Tübingen.

“The environment has a much greater influence on our body size than our brain size. 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 study also suggests that non-environmental factors were more important for driving larger brains than climate, prime candidates being the added cognitive challenges of increasingly complex social lives, more diverse diets, and more sophisticated technology.

Will technology be the downfall of the human brain?

The researchers say there is “good evidence” that human body and brain size continue to evolve as the human physique is still adapting to different temperatures, with larger-bodied people living in colder climates today. However, brain size in our species appears to have been “shrinking” since the beginning of the Holocene, around 11,650 years ago.

Prof. Manica believes that the increasing dependence on technology, such as an outsourcing of complex tasks to computers, may cause brains to shrink even further over the next few thousand years.

“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,” the researcher concludes.

SWNS writer Stephen Beech contributed to this report.

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