Monday, June 18, 2007

Why do humans have big brains?

There are several explanations for this: meat and sociality among them. These are of course not mutually exclusive, by the way. There are a couple of papers by Ann Gibbons (here, here) in the latest Science issue that discuss new evidence and the controversies surrounding the role of cooking meat in allowing for increased energy allocation to brain size.

some excerpts:
To find support for his ideas, Wrangham went to the lab to quantify the nutritional impact of cooking. He found almost nothing in food science literature and began to collaborate with physiologist Stephen Secor of the University of Alabama, Tuscaloosa, who studies digestive physiology and metabolism in amphibians and reptiles. Secor's team fed 24 Burmese pythons one of four diets consisting of the same number of calories of beef: cooked ground beef, cooked intact beef, raw ground beef, or raw intact beef. Then they estimated the energy the snakes consumed before, during, and after they digested the meat, by measuring the declining oxygen content in their metabolic chambers. Pythons fed cooked beef spent 12.7% less energy digesting it and 23.4% less energy if the meat was both cooked and ground. "By eating cooked meat, less energy is expended on digestion; therefore, more energy can be used for other activities and growth," says Secor. Secor also helped Wrangham and graduate student Rachel Carmody design a pilot study in which they found that mice raised on cooked meat gained 29% more weight than mice fed raw meat over 5 weeks. The mice eating cooked food were also 4% longer on average, according to preliminary results. Mice that ate raw chow weighed less even though they consumed more calories than those fed cooked food. "The energetic consequences of eating cooked meat are very high," says Wrangham.

But, there is the issue of when hominins (H. erectus) were first able to use fire. The best evidence for human use of fire happens quite long after the burst in brain size that happened about 2 million years ago.the last paragraph:
Others, such as Carel van Schaik of the University of Zurich, think that cooking may have played an important role early on, along with other adaptations to expand human brainpower. As Aiello observes, the big brain was apparently the lucky accident of several converging factors that accentuate each other in a feedback loop. Critical sources of energy to fuel the brain came from several sources--more meat, reduced guts, cooking, and perhaps more efficient upright walking and running. The order in which our ancestors adopted these energy-saving adaptations is under hot debate, with the timing for cooking hardest to test. Regardless, "it's all beginning to come together," says Aiello.


Anonymous said...

There is no link between intelligence and the increased brain size. Early acievements of humans (stone tools and spreading into Asia) happened before the enlargement of brain. Later acievements happened after Gutenberg. Achievements in the past 30ky happened while our brain actually diminished. OTOH, there is pretty strong correlation of the increase in brain size and spreading of Homo to high latitudes. It is solely thermoregulation thing, nothing else. AFAIK, our brain doesn't have more nerunes, it just has more fat.
OTOH, I don't think we ever ate raw meat (except the meat we are eating raw even today, shellfish). There is no way that someone who is eating raw meat regularly (and we were supposed to do that, to turn to cooked meat eating) would turn to cooked meat eating. Even Aborigines, who are eating pretty raw meat for our standards, cook their meat for one hour. It is simply too much. For sure there would be occasions when you are simply hungry, and you want to eat meat right away. In such occasions we would eat meat raw, and we wouldn't become maladaptive for that. But, we simply cannot eat meat raw. We have to cook it.
Cooking is very dangerous, because it shows your position to all your enemies. It is labour intensive, because you have to collect all that wood (so, it isn't as efficient as those researches show), and it is time consuming. You make fire, you catch attention.
Every animal would avoid all this, only if it can. The problem is that we simply cannot avoid this. I am claiming that we always ate cooked meat, that we never ate raw meat. This is why we are eating cooked meat today.
Other thing is that we are eating salty meat, as well. So, it is not that meat is just cooked, it is also salted. Other animals leak salt. There is no reason why we would stop to do that, and start to salt our meat, and vegetables. I am claiming that we always ate salty food, as well. We are not salting fruits. -- Mario Petrinovich

Anonymous said...

The hardest part of man looking at himself is to avoid the arrogance of superiority. It may not be clear but having a larger brain i.e., higher intelligence does not necessary mean that man's place is having more status or that we've done evolving. Maybe the only real bad evolution is passing on traits that lead to extinction (like a big brain that understand nuclear chemistry?). Something happened during the course of evolution and I think that it is a valid question to ponder why our brains have evolved not just in size but in neural complexity which is exhibited in the number of brain convolutions compared to other mammals.

We exist because of our ancestors adapted and bred. Clearly the bigger brains that we possess help us in relating with each other socially. Whether we used our big brains to hunt together, forage together, etc. is really for anyone to say. What is clear is that our big brains were passed down from generation to generation. Mankind with our big brains exists in almost every ecosystem. But nematodes can claim the same without much of a brain and are well adapted including living on and in us as well!

Maybe we didn't really set out to cook meat at all. Maybe our ancestors were opportunistic feeders and happened upon dead carcasses from forest fires caused by thunderstorms and extreme heat leaving animals bbq'd for our early ancestors to feast upon? The first TV dinner? No fuss and no muss. Anyway thanks for letting me share some thoughts. Geo Kim

Locations of visitors to this page