Over the course of the Pleistocene epoch, between 2.6 million years ago and 11,700 years ago, the brains of humans and their relatives grew. Now, Tel Aviv University scientists have a new hypothesis for why: As the largest animals from the landscape disappeared, the scientists propose, human brains it had to grow to allow hunting smaller and faster prey.
This hypothesis holds that early humans specialized in taking down larger animals, such as elephants, which would have provided hearty greasy meals. When the number of these animals declined, humans with larger brains, who presumably had more brainpower, adapted better and captured smaller prey, leading to better brain survival.
Ultimately, adult human brains expanded from an average of 40 cubic inches (650 cubic centimeters) 2 million years ago to about 92 cubic inches (1,500 cubic cm) at the cusp of the agricultural revolution about 10,000 years ago. The hypothesis also explains why the brain size was reduced slightly, to about 80 cubic inches (1,300 cubic cm), after farming began: the additional tissue was no longer necessary to maximize hunting success.
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This new hypothesis contradicts a trend in studies on human origin. Many scholars in the field now argue that human brains grew in response to many small pressures, rather than one big one. But Tel Aviv University archaeologists Miki Ben-Dor and Ran Barkai argue that a major change in the environment would provide a better explanation.
“We view the shrinking of prey as a unifying explanation not only for brain expansion, but for many other transformations in human biology and culture, and we argue that it provides a good incentive for these changes,” Barkai wrote in a email Live Science. . “[Scholars of human origins] we are not used to looking for a single explanation that covers a diversity of adaptations. We think it’s time to think differently. “
Big prey, growing brains
The growth of the human brain is evolutionarily outstanding, because the brain is an expensive organ. the Homo sapiens The brain uses 20% of the body’s oxygen at rest even though it accounts for only 2% of body weight. An average human brain today weighs 2.98 pounds. (1,352 grams), far surpassing the brains of chimpanzees, our closest living relatives, at 0.85 pounds (384 grams).
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Barkai and Ben-Dor’s hypothesis is based on the notion that human ancestors, beginning with Homo habilis and peaking with Homo erectus, he spent the early Pleistocene as expert carnivores, taking down the biggest and slowest prey Africa had to offer. Megaherbivores, researchers argue in an article published March 5 in the journal Yearbook of Physical Anthropology, it would have provided abundant calories and nutrients with less effort than hunting for plants or stalking smaller prey. Modern humans are better at digesting fat than other primates, Barkai and Ben-Dor said, and human physiology, including stomach acidity and intestinal design, indicate adaptations for eating fatty meat.
In another article, published on February 19 in the magazine Quaternary, the researchers argue that the tools and lifestyle of the human species are consistent with a change from large prey to small prey. In Barkai’s fieldwork in Africa, for example, he has found Homo erectus sites covered in elephant bones, which disappear at later sites between 200,000 and 400,000 years ago. Human ancestors at those more recent sites appeared to have been eating mostly fallow deer, Ben-Dor wrote in an email to Live Science.
In general, megaherbivores weighing more than 2,200 pounds. (1,000 kilograms) began to decline in Africa around 4.6 million years ago, with herbivores weighing more than 770 pounds. (350 kg) declining about 1 million years ago, the researchers wrote in their paper. It’s unclear what caused this decline, but it could have been climate change, human hunting, or a combination of the two. As larger, slower, and fatter animals disappeared from the landscape, humans would have been forced to adapt by switching to smaller animals. This change, the researchers argue, would have put evolutionary pressure on human brains to grow because hunting small animals would have been more complicated, given that smaller prey are more difficult to track and catch.
These growing brains would explain many of the behavioral changes throughout the Pleistocene. Hunters of small and floating prey may have needed to develop complex language and social structures to successfully communicate the location of the prey and coordinate its tracking. Better fire control would have allowed human ancestors to extract as many calories as possible from smaller animals, including the fat and oil from their bones. Weapon and tool technology would have had to advance to allow hunters to shoot down and dress small game, according to Barkai and Ben-Dor.
A fuzzy past
However, unique hypotheses for the evolution of the human brain have not held up well in the past, said Richard Potts, a paleoanthropologist and director of the Smithsonian’s Human Origins Program in the Washington, DC, location, who was not involved in the research. And there are debates about many of the arguments for the new hypothesis. For example, Potts told Live Science, it is unclear whether early humans hunted megaherbivores. There are human cut marks on the bones of large mammals at some sites, but no one knows whether the animals were killed or stolen by humans.
Researchers also sometimes use time-period arguments that might not apply to earlier times and places, Potts said. For example, evidence suggests a preference for large prey from Neanderthals that lived in Europe 400,000 years ago, which would have served those human relatives well in winter, when plants were rare. But the same might not have been true a few hundred thousand or a million years earlier in tropical Africa, Potts said.
And when it comes to brains, size isn’t everything. Complicating the picture, brain shape also evolved during the Pleistocene, and some human relatives, such as Homo floresiensis, who lived in what is now Indonesia between 60,000 and 100,000 years ago, had small brains. H. floresiensis it hunted both small elephants and large rodents despite its small brain.
The period during which humans and their relatives experienced this brain expansion is poorly understood, with few fossil records to continue. For example, there are perhaps three or four sites firmly dating to between 300,000 and 400,000 years ago in Africa that are certainly related to humans and their ancestors, said John Hawks, a paleoanthropologist at the University of Wisconsin-Madison who was not involved in the research. and he was skeptical of their conclusions. The human family tree became complicated throughout the Pleistocene, with many branches, and the growth in brain size was not linear. Neither were the declines in large animals, Hawks told Live Science.
“They have sketched a picture where megaherbivores decrease and brains increase, and if you look at that through a telescope, it seems true,” Hawks told Live Science. “But actually, if you look at the details on both sides, the brain size was more complicated, the megaherbivores were more complicated, and it’s not like we could establish a direct relationship between them.”
However, the paper draws attention to the fact that human species may have hunted large mammals during the Pleistocene, Hawks said. There is a natural bias in fossil sites against the preservation of large mammals, because human hunters or scavengers would not have dragged an entire elephant back to camp; instead, they would have cut packages of meat, leaving no evidence of the feast in their places of origin for future paleontologists and archaeologists.
“I’m sure we’re going to talk more and more about what the role of megaherbivores was in human subsistence, and were they important for us to become humans?” Hawks said.
Originally posted on Live Science.