Sunday, December 2, 2012

Why you can all stop saying meat eating fueled evolution of larger brains right now

Returning from hunt
Hadza returning from hunt in Tanzania. Credit Andy Lederer.
In William Shakespeare's comedy Twelfth Night, Sir Andrew, who was worried that a joke may have been made at his expense, reasons out loud that maybe his diet had something to do with his lack of intelligence, saying, "But I am a great eater of beef, and I believe that does harm to my wit" (Act I, Scene III). Dialogue like that was how Shakespeare famously poked fun at what he considered "foolery" in his time; it was a common belief of the Elizabethan Age that eating too much meat made you a meat-head. Now, it appears the tables have turned. Vegetarians are getting a taste of similar medicine from comedians of our time.

On November 15th's episode of The Colbert Report, Stephen Colbert interviewed one of the world's foremost paleoanthropologists, Chris Stringer of the Natural History Museum, about his newly published book. During their conversation, Stringer sums up nicely why meat eating may have been the primary force that drove evolution of a brain-gut tradeoff, where a shrinking gut allowed for more energy input into the brain. Here is Stringer's explanation at about minute 18:30 in the episode:
Chris Stringer: "There's a thing called 'expensive tissue hypothesis'. And this says we evolved our large brains by changing our diets. Our ancestors had great big guts because they were vegetarian. They never had enough spare energy because their guts were using 20 percent of their energy; they never had enough spare energy to evolve a large brain. When we started eating meat, a much more concentrated sort of food, it freed up energy and we could start to run a bigger brain."
Stephen Colbert: "That's why vegetarianism seems so stupid to me."

There's little use challenging the evidence that meat, whether eaten raw or cooked, was "brain food" for our ancestors. The fossil record shows a clear correlation between the appearance of hominin meat eating some two million years ago -- as evidenced by stone tools, cutmarked bones, and changes in hominin teeth structure -- and the drastic increase in cranial size that continued until the agricultural revolution. Studies in genetics and in primatology add more support that meat helped enable our ancestors to attain the nutrients required for the development of all our extra neurons. In my last post, for example, I discuss Greg Wray's research on how changes in differences of fat metabolism and neuronal signaling that may have developed as a result of more meat in the diet and probably helped produce our fatter brains.

However, while meat eating may have played a role, the arguments espoused often by writers, "paleo" and Atkins enthusiasts, and meat-lovers of all kinds are often flawed from a nutritional standpoint. The problem is, it just doesn't make a lot of sense that meat eating could have fueled evolution of our larger brains. It's more likely that carbs did.

Neurons run on glucose, not meat

Neurons, which use twice the energy as any other cell type in the body, run almost exclusively on glucose. They don't run on protein and fat.* Moreover, because neurons aren't able to store glucose as glycogen as other cells in the body do, they must receive glucose in constant supply. That's glucose that must be received from the bloodstream 24 hours a day, seven days a week, even while you're asleep. That's glucose for some 86 billion neurons, more than any other primate; by comparison, gorillas contain about 33 billion and chimpanzees only 28 billion neurons. That's glucose in amounts that could not possibly be supplied by any abundance of meat eating.
A human brain is ~3x larger than a chimp's.

To get an expert's standpoint on the topic, I wrote to Peter Ungar, distinguished professor and chair of anthropology at University of Arkansas, who was recently named a fellow of the American Association for the Advancement of Science, what his take was on the role of meat eating versus carbs in human brain evolution. He wrote back,
Even the staunchest meat advocates recognize that protein and fat cannot power the brain – and we lose much of our gluconeogenesis capabilities at weaning. The argument is that meat eating provided the calories needed to power other parts of the body, freeing available carbohydrates to focus on the brain… Even in that case, it’s carbs, not meat that powers the brain (even though meat facilitates the process).
So, let me repeat in my own way: Why was meat important for evolution of larger brains? Although meat does provide some valuable micronutrients and essential fats, there may not be anything incredibly special about meat nutritionally except that it freed up carbohydrate calories for feeding brains that were roughly three times larger than chimps without the use of gluconeogenesis (synthesis of glucose).

According to Wray's evidence from regulatory genes, humans also developed double the glucose transporters in the brain in comparison to chimps. Chimps, on the other hand, have double the glucose transporters in their muscles. That translates to about four times more glucose going to our brains versus our muscles in comparison to chimps. When I asked Ungar what he thought regarding the role of other types foods (e.g. tubers), as well as preparation techniques (e.g. cooking) in also freeing carbs for the brain, he responded,
I believe the hominin lifestyle is more about a broadened niche than meat per se… Lots of people live in lots of places because they can find something to nourish themselves in whatever environment they find themselves in. Western Australian aboriginals did quite well without lots of meat. 
On Wray's research, Ungar also added,
I’m not an expert on fueling the brain, but it makes sense to me that we’re more efficient at getting glucose across (we need it). One could argue that this allows LESS carb intake for a given brain size… Since our brains are roughly 4x the size of Chimps, does that mean we need about the same number of carbs to power them? 
As for evidence of increased starchy food intake – I think the amylase gene copies didn’t take off until relatively recently. Still, evidence of a broadened subsistence base in Homo could certainly have included starchy foods, meat, or anything else they could get their hands on. 
I’m pretty conservative in terms of single-cause, magic bullet explanations without solid evidence from the fossils themselves… Maybe it’s my time chasing broad-diet primates in the rainforest, but I’m hesitant to invoke one food type to explain brain expansion.
Lastly, he wrote,
...the problem now is coming up with ways of testing those hypotheses (meat, cooking, flexibility, fish, etc). In the end, I woudn’t be surprised if there were no magic bullet…  but after all the new and exciting findings coming from isotopes, microwear, etc. not much would surprise me re: the evolution of human diet.
So there you have it -- it's more appropriate to say that a "broadened niche" is what helped fuel evolution of larger brains. Meat provided micronutrients, but more of it more often probably did nothing more than provide more calories so that we could use carbs to fuel our brains. It's certainly possible that other foods like cooking, shore-based foods, and other foods could've played the same role, too.

Cooking Was More Important Than Meat for Brains

In a recent live chat with science writer Ann Gibbons over at ScienceLive, biological anthropologist Richard Wrangham at Harvard and comparative neuroanatomist Suzana Herculano-Houzel had a few interesting things to say on this topic while discussing their recent study published in Proceedings of the National Academy of Sciences. Their study showed that humans would have to spend more than nine hours a day eating raw foods to feed hungry brains (at 2,000 kcal a day) despite whether raw meat was included or not.

In the chat, Wrangham said, as he's argued in his book,
Cooking increases the net energy gained from food from 2 main routes. (1) It increases digestibility (the proportion of food that gets digested and absorbed). For instance cooking is estimated to increase the digestibility of starch in grains by around 30%. It also increases the digestibility of meat, by denaturing the protein. (2) It reduces the physiological costs of digesting our food - because cooking softens food, so it is easier to digest.
A commenter, Fred, questioned the role of meat eating versus cooking in leading to larger brains, to which Wrangham responded,
The meat vs. veg question is totally unresolved! Recent hunter-gatherers ate least meat near the equator, about 35% of diet, rising to ~100% near the poles. But recent hunter-gatherers are not necessarily good models for our ancestors. They have technology (bows and arrows, and poisons) that made hunting easier; but there again, there may have been more animals in the past. Also, the truly great places for hunter-gatherers to live in may have been those where farmers took over in masses, such as the Nile delta, with terrific plant supplies. So the 'paleo-diet' idea that our ancestors were all heavy meat-eaters doesn't acknowledge the likely large variation in % meat / % plant. Anyway, nowadays vegetarians have about the same body mass index as meat-eaters. So meat doesn't seem to do much to affect energy gain. (But raw-dieters, whether vegetarian or meat-eaters, are much thinner on average than cookivores, whether vegetarian or meat-eaters.) Bottom line: meat is less important than cooking when it comes to energy gain.
Is Wrangham suggesting redemption for raw dieting and vegetarianism as a healthy dietary approach in a modern world? Some people might think so. Although, as a nutritionist, I wouldn't recommend them a raw or vegetarian diet. After all, animal protein is valuable for maintaining or building muscle because of its higher quality amino acid profile; specific animal fats (DHA and EPA) are valuable for cardiovascular and brain health; and animal foods generally do provide necessary micronutrients such as zinc that are important for brain health.

Yet, the point here is there was probably "no magic bullet" that led to evolution of human brains and that meat is not necessarily a brain food. Whatever the combination of factors -- cooking, broadened niche, even food sharing -- that led to larger brains, carbs are what really fuel your neurons, and that's not to say you should overconsume carbs either; most of us already do too much of that. So, there's no sense in using evolution of larger brains as an argument for gorging on steak. Too much beef (and too little glucose), as The Bard would've believed, really might do "harm to your wit."

Update: I'm thrilled to share that this blog post was blogged about by Barbara J. King of NPR over at 13.7 -- check it out here. Enjoy!

*12-7 note: Many of you have pointed out my statement "They don't run on protein and fat" is not entirely accurate. Amino acids can be used to synthesize glucose, although this is a metabolically costly process in terms of ATP. As Ungar said, gluconeogenesis is limited in humans. Also, it is possible for the brain to use products of fat catabolism called ketones as fuel (to spare muscle amino acids; in fact, muscle will use ketones first); however, ketones are only use in the brain in periods of prolonged starvation (or on a diet extremely low in calories with little to no carbohydrates) and may lead to adverse health consequences.