by The human brain, more than any other characteristic, sets our species apart. Over the past 7 million years or so, it has grown in size and complexity, enabling us to use language, plan for the future and coordinate with each other on a scale never before seen in the history of life. .
But our brains have come at a disadvantage, according to a study published on Wednesday. The regions that have expanded the most in human evolution have become extremely vulnerable to the ravages of antiquity.
“There is no such thing as a free lunch,” said Sam Vickery, a neuroscientist at the Jülich Research Center in Germany and an author of the study.
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The 86 billion neurons in the human brain cluster into hundreds of distinct regions. For centuries, researchers could identify some regions, such as the brainstem, by hallmarks such as clusters of neurons. But these large regions turned out to be smaller regions, many of which were only revealed with the help of powerful scanners.
As the structure of the human brain came into focus, evolutionary biologists became curious about how the regions evolved from our primate ancestors. (Chimpanzees are not our direct ancestors, but both species descended from a common ancestor about 7 million years ago.)
The human brain is three times as big as the chimpanzees. But that doesn’t mean that all our brain regions have expanded at the same speed, like a map drawn on an inflation balloon. Some regions increased only slightly, while others grew significantly.
Vickery and his colleagues developed a computer program to analyze brain scans from 189 chimpanzees and 480 humans. Their program mapped each brain by identifying clusters of neurons that formed specific regions. Both species had 17 brain regions, the researchers found.
These maps then allow the researchers to calculate how much larger each of the 17 regions in the human brain was. They found some regions that were roughly the same size in both species. But some areas were much larger in people. One of them was the orbital cortex, a region that sits directly behind the eyes and is essential for decision making.
Vickery and his colleagues then looked at what happened to the aging brain. Neuroscientists have known for a long time that by the time people reach their 30s, people begin to lose some of their connecting branches. As a result, their brains begin to shrink. But it’s hard to compare our declining brains to the brains of other animals, because we live much longer than we do. In addition to the normal loss of brain volume, older people may have diseases such as Alzheimer’s and Parkinson’s which can destroy more neurons.
Since chimpanzees rarely live beyond 50, the scientists chose people of comparable age to examine how their brains age. They analyzed human volunteers between 20 and 58 and chimpanzees between 9 and 50. In both species, the researchers found, the brain shrank over the years. But some regions shrink faster than others. In humans, the regions that shrank the most were the orbital cortex and other parts of the brain that expanded the most over the past few million years.
The new study is “the next step in the ladder we’re climbing to understand the aging brain,” said Caleb Finch, an evolutionary biologist at the University of Southern California who was not involved in the work. But he noted that research has yet to show why parts of the brain that have recently expanded are so vulnerable to shrinking as we age. “It’s not clear at all,” he said. “The neurons have no chemical differences.”
One possibility, Vickery said, has to do with the fact that the fastest parts of our brain facilitate our most complex thinking. It is possible that the neurons that carry out this thought quickly wear out, causing the regions to shrink.
Aida Gomez-Robles, an anthropologist at University College London who was not involved in the study, warned that 189 scans of chimpanzees can only provide a vague picture of their aging brains. “Similar studies of aging in humans typically include thousands of people,” she said.
Furthermore, the new study found only a modest link between enlarged regions and rapid aging. “It’s true for some of those regions, but we don’t know if it’s true for all of them,” Gomez-Robles said.
Ironically, it’s our big brains that help us live years longer than chimpanzees. They have enabled our species to secure a more stable food supply, discover the importance of clean water and invent new types of medicine. But in our later years, our brains continue to shrink. And Vickery’s study suggests that the very regions that help us live longer are shrinking the fastest.
In other words, the frustrations of aging — trouble recalling words, for example, or switching from one task to another — may be a legacy of our evolution.
“You have this great brain,” Vickery said, “but it comes at a cost.”
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