January 2, 2011
When it comes to brain size, bigger doesn't always mean better. As humans continue to evolve, scientists say our brains are actually getting smaller.
The downsizing of human brains is an evolutionary fact that took science writer Kathleen McAuliffe by surprise.
"I said, 'What? I thought it was getting bigger!'" she tells NPR's Jacki Lyden. That was the story up to 20,000 years ago, she learned. Then, the brains of our ancestors reversed course and started getting smaller — and they've been shrinking ever since.
Cro-Magnon man, who lived in Europe 20,000 to 30,000 years ago, had the biggest brains of any human species. In comparison, today's human brain is about 10 percent smaller. It's a chunk of brain matter "roughly equivalent to a tennis ball in size," McAuliffe says.
The experts aren't sure about the implications of this evolutionary trend. Some think it might be a dumbing-down process. One cognitive scientist, David Geary, argues that as human society grows increasingly complex, individuals don't need to be as intelligent in order to survive and reproduce.
But not all researchers are so pessimistic. Brian Hare, an anthropologist at the Duke University Institute for Brain Sciences, thinks the decrease in brain size is actually an evolutionary advantage.
The Domesticated Brain
"A smaller brain is the signature of selection against aggression," Hare tells Lyden. "Another way to say that is an increase in tolerance."
Hare says when a population selects against aggression, they can be considered to be domesticated. And for a variety of domesticated animals like apes, dogs or turkeys, you can see certain physical characteristics emerge. Among these traits are a lighter and more slender skeleton, a flattened forehead — and decreased brain size.
Hare's studies focus on chimpanzees and bonobos. In evolutionary terms, they are much like humans, but are physically quite different from one another. Bonobos have smaller brains than chimpanzees — and are also much less aggressive.
While both have the cognitive ability to solve a given puzzle, Hare says, chimpanzees are much less likely to accomplish it if it involves teamwork. Not so with bonobos.
"If the food is quite sparse and it's not easy to share, [bonobos] can solve the problem," Hare says. "Chimpanzees, in that same context — where there's not much food and it's not easy to share — they just refuse to work together. They can't solve the problem, even though they know how."
Hare does admit that the shrinking human brain could signal an evolutionary dumbing-down, but more important is what the phenomenon tells us about ourselves. Comparing our evolution to that of other animals enriches our understanding of the human condition.
"The nice thing about studying animals and human nature," Hare says, "is that it helps us design or think of some strategies that deal with our darker sides."
Recent research shows that Tibetans, who have lived isolated in these high altitudes for thousands of years, enjoy a genetic variation that keeps their hemoglobin levels in a normal range. A variation of EPAS1, a gene that is sometimes associated with increased athleticism, causes an enzymatic change in the way oxygen binds to blood and is transported around the body. Compared to lowland Chinese, Tibetans thrive in high altitude—they do not suffer from chronic altitude sickness and their children are born with normal weight.
"It makes them super athletes at altitude, without a doubt," says Ken Kamler, a surgeon, author of Surviving the Extremes and an editorial advisor to Popular Mechanics. "I've been on climbs with these guys, and I'm maybe a foot taller than some of them, and they carry loads on their backs that I can't even lift off the ground, and they will carry them way faster than I'm climbing with a much lighter load."
Even with rigorous training, a person from sea level will almost always lag behind the Sherpas at altitude, Kamler says. "They climb steadily and rapidly to the point where they are getting, and then they wait," he says. "These guys are obviously different from us."
The EPAS1 found in Tibetans is the fastest example of human adaptation ever recorded—in 3000 years, the frequency of the gene grew from 10 percent to 90 percent in Tibetans, says Rasmus Nielsen, an evolutionary biologist at University of California–Berkeley. To put that time frame in perspective, lactose tolerance in humans, a trait that about 80 percent of Europeans have, developed over a 7500-year period. "The interesting thing to think about is that a lot of people would have had to die in that period for the allele frequency to change like that," Nielsen says—making it one of the clearest examples of natural selection in humans.
The argument was that its a selection mechanism. As cooperation and specialization takes over the needs of the population, the resukts has been a stedy measurabke decrease of brain size.
Im wornering whethere these arent merely some manifestation of neotony where the traits of the immature species is retained through life. Maybe brain shrinkeage is one of those associated traits.
They will need to devise an experiment to differentiate between this effect and some form of selection.
AS A SPECIES, WE ARE EVOLVING SMALLER BRAINS