During the Pleistocene, between 2.6 million years ago and 11,700 years ago, the brains of humans and their relatives grew. Now scientists at Tel Aviv University have a new hypothesis as to why: When the largest animals in the landscape disappeared, the scientists propose: human brain had to grow to allow the hunt for smaller, faster prey.
This hypothesis holds that early humans specialized in knocking down the largest animals, such as elephants, which would have provided enough fatty meals. As the numbers of these animals decreased, people with larger brains, who presumably had more brainpower, were better at adapting and catching smaller prey, leading to better survival for the brainiacs.
Ultimately, the brains of adult humans grew from an average of 40 cubic inches (650 cubic centimeters) 2 million years ago to about 92 cubic inches (1,500 cubic centimeters) at the start of the agricultural revolution, about 10,000 years ago. The hypothesis also explains why brain size shrank slightly, to about 80 cubic inches (1,300 cubic cm), after farming began: The extra tissue was no longer needed to maximize hunting success.
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This new hypothesis is a trend in studies of human origin. Many scientists in the field now claim that the human brain grew in response to a lot of small pressure, 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 see the decrease in prey size as a unifying explanation not only for brain expansion, but also for many other transformations in human biology and culture, and argue that it is a good driver for these changes,” he wrote. Barkai in an email to Live Science.[Scholars of human origins] are not used to looking for a single explanation that covers a variety of adaptations. We think it is time to think differently. “
Big prey, growing brains
The growth of the human brain is excellent from an evolutionary point of view, because the brain is a precious organ. The homo sapiens The brain uses 20% of the body’s oxygen at rest, despite making up only 2% of body weight. An average human brain today weighs 2.98 pounds. (1,352 grams), at 384 grams much more than the brains of chimpanzees, our closest relatives.
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The Barkai and Ben-Dor hypothesis depends on the idea of human ancestors, starting with Homo and with a highlight Standing man, brought the early Pleistocene as expert carnivores, shooting down the largest, slowest prey Africa had to offer. Megaherbivores, the researchers say in a paper published March 5 in the journal Yearbook Physical Anthropology, would have provided adequate calories and nutrients with less effort than foraging plants or stalking smaller prey. Modern humans are better at digesting fat than other primates, Barkai and Ben-Dor said, and the physiology of humans, including stomach acid and gut design, indicates adaptations for eating fatty meat.
In another article, published in the journal Feb. 19 Quaternary, the researchers argue that the tools and lifestyle of the human species are consistent with a shift from large prey to small prey. For example, he found in Barkai’s fieldwork in Africa Standing man sites dotted with elephant bones, which disappear at later sites between 200,000 and 400,000 years ago. The human ancestors on those more recent sites seemed to have mainly eaten fallow deer, Ben-Dor wrote to Live Science in an email.
Generally, megaherbivores weigh in at over 2,200 lbs. (1,000 kilograms) began to decline in Africa about 4.6 million years ago, with herbivores weighing over 770 lbs. (350 kg) about 1 million years ago, the researchers wrote in their paper. It’s not clear what caused this decline, but it could have been climate change, human hunting, or a combination of both. As the largest, slowest, and fattest animals disappeared from the landscape, humans would have been forced to adapt by switching to smaller animals. This switch, the researchers argue, would have put evolutionary pressure on the human brain to get bigger, as hunting small animals would have been more complicated, as smaller prey are more difficult to track and catch.
This growing brain would then explain many of the behavioral changes in the Pleistocene. Hunters of small prey from the fleet may have had to develop language and complex social structures to successfully communicate the location of the prey and coordinate its tracking. Better control of fire would have allowed human ancestors to get as many calories as possible from smaller animals, including fat and oil from their bones. According to Barkai and Ben-Dor, tool and weapon technology should be developed to enable hunters to take down and dress small game.
A hazy past
However, some hypotheses for the evolution of the human brain have not held up well in the past, said Richard Potts, a paleoanthropologist and head of the Smithsonian’s Human Origins Program in Washington, DC, who was not involved in the study. And there are debates about many of the arguments in the new hypothesis. For example, Potts told Live Science, it’s not clear whether early humans hunted megaherbivores at all. There are human cuts on the bones of large mammals in places, but no one knows whether the humans killed the animals or captured them.
The researchers also sometimes use arguments from a period of time that may not apply to earlier times and places, Potts said. For example, the evidence suggests a preference for large prey by Neanderthals inhabiting Europe 400,000 years ago, which would have served those human relatives well in the winter when plants were scarce. But the same may 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. Making the picture more complicated, brain shape also evolved over 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 a cerebellum. H. floresiensis hunted small elephants as well as large rodents despite its small brain.
The period in which humans and their relatives experienced this brain expansion is poorly understood, with little fossil record to proceed. For example, there may be three or four sites firmly dated 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 study. and was skeptical of the conclusions. The human family tree was complicated during the Pleistocene, with many ramifications, and the growth in brain size was not linear. Neither were the declines in large animals, Hawks told Live Science.
“They outlined a picture where the megaherbivores are decreasing and the brain is increasing, and if you look at that through a telescope, it looks kind of true,” Hawks told Live Science. “But actually, if you look at the details on both sides, the brain size was more complicated, megaherbivores were more complicated, and it’s not like we can draw a clear relationship between them.”
However, the article draws attention to the fact that human species did indeed hunt large mammals during the Pleistocene, Hawks said. There is a natural preference for fossil sites over large mammal conservation, as human hunters or scavengers would not have dragged an entire elephant back to camp; instead, they would have cut packages of meat and left no evidence of the feast at their home sites for future paleontologists and archaeologists.
“I’m sure we’ll be talking more and more about what was the role of megaherbivores in human existence, and whether they were important for us to become human?” Hawks said.
Originally published on Live Science.