Tuesday, January 4, 2011

If Modern Humans Are So Smart, Why Are Our Brains Shrinking?

Old big head: A 3D image replica of a 28,000-year-old skull found in France shows it was 20 per cent larger than oursKathleen McAuliffe - John Hawks is in the middle of explaining his research on human evolution when he drops a bombshell. Running down a list of changes that have occurred in our skeleton and skull since the Stone Age, the University of Wisconsin anthropologist nonchalantly adds, “And it’s also clear the brain has been shrinking.”

“Shrinking?” I ask. “I thought it was getting larger.” The whole ascent-of-man thing.

“That was true for 2 million years of our evolution,” Hawks says. “But there has been a reversal.”

He rattles off some dismaying numbers: Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimeters to 1,350 cc, losing a chunk the size of a tennis ball. The female brain has shrunk by about the same proportion. “I’d call that major downsizing in an evolutionary eyeblink,” he says. “This happened in China, Europe, Africa—everywhere we look.” If our brain keeps dwindling at that rate over the next 20,000 years, it will start to approach the size of that found in Homo erectus, a relative that lived half a million years ago and had a brain volume of only 1,100 cc. Possibly owing to said shrinkage, it takes me a while to catch on. “Are you saying we’re getting dumber?” I ask.

Hawks, a bearish man with rounded features and a jovial disposition, looks at me with an amused expression. “It certainly gives you a different perspective on the advantage of a big brain,” he says.

After meeting with Hawks, I call around to other experts to see if they know about our shrinking brain. Geneticists who study the evolution of the human genome seem as surprised as I am (typical response: “No kidding!”), which makes me wonder if I’m the world’s most gullible person. But no, Hawks is not pulling my leg. As I soon discover, only a tight-knit circle of paleontologists seem to be in on the secret, and even they seem a bit muddled about the matter. Their theories as to why the human brain is shrinking are all over the map...

via If Modern Humans Are So Smart, Why Are Our Brains Shrinking? | Human Evolution | DISCOVER Magazine.

Several theories have been advanced to explain the mystery of the shrinking brain. One is that big heads were necessary to survive Upper Paleolithic life, which involved cold, outdoor activities.

A second theory is that skulls developed to cope with a chewy diet of rabbits, reindeer, foxes and horses.

As our food has become easier to eat, so our heads have stopped growing, according to supporters of this theory.

Other experts say that with high infant mortality, only the toughest survived - and the toughest tended to have big heads. Therefore a gradually decreasing infant mortality rate has led to a proportionate decrease in the size of our brains.

A recent study conducted by David Geary and Drew Bailey, cognitive scientists at the University of Missouri, explored how cranial size changed as humans adapted to an increasingly complex social environment between 1.9million and 10,000 years ago.

They found that when population density was low, such as during the majority of our evolution, the cranium increased in size. But when a certain area's population changed from sparse to dense, our cranium size decreased.

They concluded that as increasingly complex societies emerged, the brain grew smaller because people didn't have to be as smart to stay alive.

But Dr Geary warns against stereotyping our ancestors as being more intelligent than us.

He said: 'Practically speaking, our ancestors were not our intellectual or creative equals because they lacked the same kind of cultural support.

via dailymail.co.uk

"We know the brain has been evolving in human populations quite recently," said paleoanthropologist John Hawks at the University of Wisconsin at Madison.

Surprisingly, based on skull measurements, the human brain appears to have been shrinking over the last 5,000 or so years.

"When it comes to recent evolutionary changes, we currently maybe have the least specific details with regard the brain, but we do know from archaeological data that pretty much everywhere we can measure — Europe, China, South Africa, Australia — that brains have shrunk about 150 cubic centimeters, off a mean of about 1,350. That's roughly 10 percent," Hawks said.

"As to why is it shrinking, perhaps in big societies, as opposed to hunter-gatherer lifestyles, we can rely on other people for more things, can specialize our behavior to a greater extent, and maybe not need our brains as much," he added.

via livescience.com

I have to ask... why do people assume that brain size equates with intelligence? If that was the case, we would be the third smartest animal on the planet. Shanna Freeman at  HowStuffWorks says this:
The average adult human brain weighs about 3 pounds (1,361 grams). The dolphin -- a very intelligent animal -- also has a brain that weighs about 3 pounds on average. But a sperm whale, not generally considered to be as intelligent as a dolphin, has a brain that weighs about 17 pounds (7,800 grams). On the small end of the scale, a beagle's brain is about 2.5 ounces (72 grams), and an orangutan's brain is about 13 ounces (370 grams). Both dogs and orangutans are pretty smart animals, but they have small brains. A bird like a sparrow has a brain that weighs less than half an ounce (1 gram).

You may notice something important in all of those comparisons. An average dolphin's body weighs about 350 pounds (158.8 kilograms), while a sperm whale can weigh as much as 13 tons. In general, the larger the animal, the larger the skull, and therefore, the larger the brain. Beagles are fairly small dogs, at about 25 pounds (11.3 kg) maximum, so it stands to reason that their brains would also be smaller. The relationship between brain size and intelligence isn't really about the actual weight of the brain; it's about the ratio of brain weight to the entire body weight. For humans, that ratio is about 1-to-50. For most other mammals, it's 1-to-180, and for birds, it's 1-to-220. The brain takes up more weight in a human than it does in other animals.

I'm not sure why brain to body ratio makes any difference. I guess more brain area is required to map more body surface area. Or perhaps we are just reaching for some way to twist the numbers to fit our expectation that we are the smartest? I found this interesting with regard to brain mass and body mass:
Hominin brain mass as a percentage of body mass

Our brains are shrinking, but if our bodies are also shrinking faster than our brains this would show that we are getting smarter. I suspect only a few of us are getting smarter. You know who you are... remember when the Earth blew up and we moved to this new Earth and they decided not to tell the stupid people?  You don't?  Oooooh... never mind. (Nod to Steve Martin).


Xeno said...

Thanks, I agree. Do you think our brain mass is shrinking faster than our body mass?

Mike said...

I think we should be asking ourselves why we no longer require excess neural architecture. The one theory regarding social division of labor and external support "devices" for memory is intriguing. I do not necessarily memorize many facts, but I know where to find them...

Humans are becoming larger than our ancestors, mostly due to healthcare and diet. There is apparently enough sensory and motor neural architecture to run our slightly larger bodies. The question becomes: how much cognitive architecture do we need to remain deep thinkers?

Xeno said...

The Neanderthal was bigger, stronger, and had a bigger brain. I'd like to see a movie where we genetically revive them and they quickly surpass and replace us.