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July 9th, 2021 
Awake Goy A new study of fossils from the genus Homo from all over the world has concluded that human body size is linked to differences in climate and temperature. In essence they have found that cold climates made for larger bodies and warmer climates for smaller bodies. The results are literally “hot off the press” and were published today in the journal Nature Communications
This is the first systematic study of how climate has affected body and brain size in the genus Homo over the last million years. ( Kimberley / Adobe Stock)
Analyzing Over 300 Genus Homo Fossils
The study was conducted by an interdisciplinary research team, led by both the University of Cambridge and Tübingen. The aim was to try to understand the role that climate differences play in human evolution. They based their conclusions on the study of over 300 genus Homo fossils from different locations. During their work they calculated the body and brain size from these fossils, and then merged this information with climate models based on data related to regional climates over the last million years. Through this method they “pinpointed the specific climate experience by each fossil when it was a living human,” explains a University of Cambridge article announcing the study.
They assessed the different climatic elements that would have been experienced by humans living in the different locations. “Our study indicates that climate – particularly temperature – has been the main driver of changes in body size for the past million years,” highlighted the leader of the study, Professor Andrea Manica from the Department of Zoology at the University of Cambridge.
This groundbreaking study took sample fossils from the genus Homo found in different parts of the world in order to understand the effect of climate on body and brain size. In the image, skulls from 430,000, 55,000 and 32,000 years ago, as well as femora bones from 540,000 and 44,000 years ago. (Dr. Manuel Will / Nature)
Evolutionary “Growth” and Fluctuating Body Size of the Genus Homo
Modern humans, Homo sapiens , are the only surviving member of the genus Homo. The genus Homo is thought to have existed just over 2 million years, with the appearance of Homo habilis , while Homo sapiens only appears in the archaeological record about 300,000 years ago. Members of the genus also include Neanderthals (Homo neanderthalensis ) and Homo erectus .
It has long been known that the environment affects visible variations between people today, as well as in different Homo species. “A defining trait of the evolution of our genus is a trend of increasing body and brain size; compared to earlier species,” highlights the University of Cambridge article. Homo sapiens , for example, is larger than Homo habilis , 50% heavier to be precise, while the Homo sapiens brain is three times larger than that of Homo habilis .
“The commonly accepted logic behind Bergmann’s rule is that a large body size buffers individuals against the challenges of cold climates, either in terms of thermoregulation and/or resource storage,” explain the authors in Nature Communications . This isn’t only true in humans. It is also common in other animal species as well, including bears. “Polar bears living in the Arctic, for example, weigh a lot more than brown bears living in comparatively warmer climates,” explains Charlotte Burton in The Guardian .
This newly published study takes this evolutionary growth a step further, beyond the realms of hypothesis, by trying to understand the “why” of the fluctuating body size of the genus Homo. It is in fact the “first systematic attempt to quantitatively test different environmental effects on body and brain size variation for the genus Homo during the past ~1 Ma,” as the Nature Communications article highlights.
A defining trait of the evolution of our genus Homo is a trend of increasing body and brain size over time. ( kotjarko / Adobe Stock)
Does Climate Affect Brain Size?
While the study found a direct correlation between body size and climate, it appears that the same is not true when it comes to brain size. “We found that different factors determine brain size and body size – they’re not under the same evolutionary pressures,” explained Dr. Manuel Will from the German University of Tubingen. “The environment has a much greater influence on our body size than our brain size.”
There was however, an indirect environmental effect on the brain size of members of the genus Homo. Individuals studied who had lived in stable or open areas, such as grasslands or open steppes, had larger brains. These individuals presumably hunted large animals for food, which was, according to the University of Cambridge article, “a complex task that might have driven the evolution of larger brains.”
Therefore, while the research seems to show that climate is not a driving force in the size of a Homo brain, it has uncovered that there were non-environmental elements at play which did play a part. These include cognitive challenges created by complex social structures, diverse diet and even the use of increasingly sophisticated technology. Human brains have been shrinking over the last 12,000 years, and this has led the team to conclude that as we increasingly depend on computers and other technology, our brains may shrink even further.
Top image: Study analyzed genus Homo fossils, some of which are shown in this image. Source: Dr. Manuel Will / Nature
By Cecilia Bogaard
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