(CN) — Since emerging from Africa around 300,000 years ago, humans have undergone some pretty drastic changes of both body and mind. New research released Thursday may reveal the driving force behind these adaptations — and what changes may yet be in store.
The genus Homo from which humans, known as Homo sapiens, evolved has been around even longer, and includes related species such as Neanderthals, Homo habilis and Homo erectus. Over millions of years the size of the average hominin has fluctuated wildly; compared to Homo habilis, who lived about 2 million years ago, modern-day humans are 50% heavier and have brains three times larger. There must be some evolutionary driver pushing these changes, but what could it be?
The authors of a new study published in the journal Nature Communications believe they found the answer, and it’s likely tied to the weather and environment experienced by hominin species throughout their evolution.
“We found that body size was strongly influenced by cold temperatures, which led to larger bodies,” said Andrea Manica, a professor in the University of Cambridge's Department of Zoology, and author of the study, in an email. “Larger bodies can buffer individuals from cold temperatures (the larger you are, the smaller your surface compares to your volume, so you conserve heat more efficiently). 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 researchers measured over 300 fossils of the genus Homo found worldwide and compared these with a reconstruction of regional climate conditions over the past million years. By combining this data, they were able to determine the specific climate that each species of Homo originally lived in.
Hominins living in colder climes tended to grow larger than their jungle and desert-dwelling cousins, according to the authors, and had little need for extra layers. Long before puffy North Face jackets were a thing, the only real solution for living in a frozen climate was bulking up — more body mass meant better heat retention. Extra layers of fat and muscle were one’s only real defense against freezing to death — thus those who packed on the pounds could stay alive to reproduce.
As far as intelligence, the authors found only a weak correlation between environmental conditions and brain size, but they did notice some interesting overlap. Hominins living in large, open habitats such as steppes and grasslands, and more ecologically stable regions, tended to have larger brains. According to the study, their larger brains may have evolved in response to the complexity of hunting in these areas, where the inhabitants had to devise all sorts of ingenious methods for trapping dangerous animals.
“Brain size changes were completely unrelated to temperature, so body and brain size evolved under distinct pressures,” Manica explained in an email. “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), and, 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 (megafauna is a main source of food in big open plans, but it is challenging to catch).”
Rather than the weather, the authors suggest hominin’s development of larger brains beginning 800,000 years ago likely stemmed from living under a more complex social order that rewarded cognitive ability, from eating a more diverse diet and from their increasing use of technology. Interestingly, evidence now suggests that human brains have been shrinking for the past 10,000 years because, among other reasons, we increasingly rely on computers to offload complex tasks and calculations.
"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," said Manuel Will of the University of Tubingen in Germany, and first author of the study, in a related statement. "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."
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