January 2013

The internet is abuzz over a recent paper in the October 22{+n}{+d}issue of Proc. Natl, Acad, Sci.US ( PNAS ) by Drs Fonseca-Azevedo and Herculano-Houzel of Brazil. These two ladies have claimed that the enormous increase in the brain size in humans came about because humans began to eat cooked food while our closest cousins, the great apes did not know how to use fire and ate everything raw. They claim that this sudden and large increase in brain size was an important event in human evolutions thanks to the use of fire.

Why do they claim so? We humans have the largest brains in comparison to our body size. Our brain-to-body ratio is far higher than that of the other primates. We have as many as 86 billion brain cells or neurons, compared to just 28 billion in the great apes. This is a giant step in evolution. How and why this sudden expansion came about has been a nagging question. And brain consumes a lot of energy to operate. After skeletal muscles and the liver, it is the brain that consumes most of our metabolic energy. Though it is only about 2 per cent of the total body mass, it consumes 20 per cent of the total body metabolic rate. In other primates, the number is just 9 per cent. To maintain such an energy-expensive organ, the amount of food we need to eat, or more precisely the number of calories we need, is huge indeed.

The needs

The greater the need for calories, the greater the time needed for feeding. This means more time spent on foraging for food and greater time needed for ingestion and digestion, plus of course the calorie content of the diet. And the amount of calorie intake depends on the number of hours spent on feeding and digestion. It has been estimated that gorillas spend about 10 hours a day for feeding themselves. And it is estimated that as they do so, their total body mass goes up to 120 kg or so. The metabolic cost of maintaining such a size of body is estimated by what is known as the Kleiber scale as 70 x (body mass){+0}{+.}{+7}{+5}kilocalories per day. Since brain cells consume a lot of energy, this puts an upper limit to the brain size of an ape. Its brain can grow only if it feeds continuously the whole day. It is here that raw versus cooked food argument comes in. Dr. Richard Wranghan of Harvard’s Peabody Museum estimated the energetic consequences of raw food and of cooked in his paper in the November 29, 2011 issue of PNAS (Incidentally both this paper and that of the Brazilian duo are downloadable free). He and his group decided to compare the weight gain of mice fed on raw meat (lean beef) versus cooked meat, and likewise fed on raw tuber (sweet potato) versus pounded, cooked and whole, or cooked and pounded. They found that mice that ate cooked meat gained weight (this gain was not attributable to differences in food intake or activity levels); likewise with processed tuber over raw tuber. Cooked food was seen to be more digestible and also helped in killing pathogens present in the raw samples. He thus writes that adoption of cooking would have helped ancestral humans thrive.

Meat and tubers have been eaten by us for over 2 million years. And when we learnt how to make and tame fire, cooking was born. And cooking increases the nutrient content and energy intake in the consumer. Dr. Wrangham published in 2010 a book entitled: Catching Fire: How Cooking Made us Human. Here he points out that as our hominid ancestors started cooking and eating, their digestive track shrunk and brain enlarged.

Pointed out

The Brazilian ladies build on this point. They point out that the energy intake in cooked food is higher than in raw. And that it costs about 6 kilocalories per day to operate a billion neurons. In a typical daily intake of about 1800 kilocalories, 20 per cent or 360 kilocalories go to operate our brain.

Given these numbers, one can see the value of eating cooked food. In order to get 1800 kilocalories per day on raw food, a human weighing 70 kg would have to spend over 16-18 hours eating! Cooking thus not only would have let early homo erectus gain time away from foraging and eating, but also to think more using the greater brain size he would have gained! Azeredo and Herculano-Houzel thus make a logical case when they say that cooked diet may have been a major positive driving force to the rapid increase in brain size in human evolution.

Comments on the web

As expected, the internet is full of comments and criticism on the above two papers. People who prefer to eat raw food (not just plants, fruits and nuts) have written about how they are perfectly healthy, happy and brainy with raw food; others points out that even this “raw” is processed in one way or the other if it is meat or fish (marinated, fomented).

But such arguments miss the main point, namely, how evolution would have been helped by the use of fire and cooking in providing greater energy and nutritive values, at a crucial time period when several other factors would also have acted to help the emergence of homo erectus, and then on to homo sapiens like us who use our brains to think back in time on how we got our brains this big.

Go Back To Main Page