In another fun experiment, several human subjects were "locked up" at a zoo, and fed something like the diet of our closest primate relatives (chimps and gorillas), consisting of raw fruits and vegetables. It was mostly vegetables, if I remember correctly, lots of clips of people gnawing on carrots, raw broccoli, and the like. Long story short, everybody hated it. They spent about half the day doing nothing but chewing, and were starving nonetheless. I believe the average weight loss quoted was something like 10 pounds in two weeks (I meant to watch the show again and take notes, but this is the first "free" time I've gotten, and it's only because I'm at the park with the kids). Subjects apparently spoke of the desire for meat quite a bit (when they weren't bitching about all of the chewing and frequent bathroom visits). It's a fun experiment you can try at home yourself!
One part of the show which rather surprised me: one of the scientists visited a remote tribe in Africa who were still living a fairly primal hunter-gatherer existence. What struck me was that these people looked like crap, nothing like the sort of pictures taken by Weston A. Price, or the fossilized H. habilis jaws shown in the chewing experiment. Their faces showed signs of nutritional stress, with small jaws and crowded teeth. One hint here may be the effort they went through to get meat. In the show, they had a porcupine cornered in its burrow (BTW, this thing was huge, the size of a really big dog). The tribesmen spent the better part of the day digging 6-foot deep holes, until they forced the porcupine into the Hobson's choice of which hole to get speared in. It was a tremendous amount of effort to get some meat, and one of the hunters basically said "porcupine sucks, but at least it's meat". They also discussed how socially important it was for hunters to bring back meat, that it brought them status in the tribe, etc. Clearly, meat is at the top of the menu for these people. Yet that they would go to such lengths to obtain it (particularly when they find porcupine distasteful) makes me wonder if hunting isn't so good in this region anymore. Perhaps game has become scarce, hence the appearance of nutritional stress? Yet they're hanging in there, if nothing else a testament to the tremendous adaptability of humanity, made possible by our big brains (and what do you suppose made those big brains possible?)
Anyway, "How Food Made Us Human" has spawned a couple of trains of thought, which I want to share with you here. The first has to do with a mouse experiment demonstrated on the show; the second with the correlations between diet changes and physiological changes over the course of evolution.
The mouse experiment was very interesting, intended to show the effect of cooking on caloric bioavailability. Take some mice, feed them raw sweet potatoes, and measure the change in body mass as well as activity (based on distance run on the exercise wheel). Now cook the sweet potato and do the same thing. If you're still stuck in the calories-in calories-out (CICO) paradigm, the results should spawn massive cognitive dissonance. The mice who ate the cooked food showed the following differences when compared to those eating the raw food:
- They exercised significantly more, AND
- They were heavier.
Of course this all makes perfect sense when considered from the standpoint of evolution and metabolic regulation. Cooking makes calories more available. Though they didn't explicitly say so in the show, one presumes that the quantity of potato was held constant between the two groups (since not doing so would void the entire point of the experiment). So the only difference (presumably) was raw vs. cooked. Mice didn't evolve eating cooked food. The higher caloric availability likely "fooled" their digestive systems into taking up calories too rapidly, faster than required to support normal metabolic operations. Rate of digestion is regulated by hormonal and nervous feedback mechanisms: when the brain and other internal sensory systems think there's enough energy around, gastrointestinal motility decreases, slowing the rate at which food leaves the stomach to be digested and absorbed in the small intestine; and of course when an energy deficit is detected, food moves more rapidly out of the stomach. When the stomach is empty AND your body senses an energy deficit, you get hungry, and are driven to find more food.
That's how it supposed to work. Like all feedback control systems, if you push outside the "designed" range of stability, it starts failing. I expect this to be particularly the case with biological systems. Biological responses tend to follow "S-curve" shapes. There's nothing deep about this. It simply reflects the fact that biological responses are limited by available resources. At some point you run out of the capacity to make more hormones, neurotransmitters, receptors, etc. Insulin response is a great example. As a function of blood glucose, the secretion of insulin follows a shape much like that seen on the Wikipedia page. At some point you either saturate the ability to detect glucose, or saturate the ability of the pancreas to crank out insulin, or both. The point is that it is possible to exceed your body's ability to effectively control blood sugar levels via the action of insulin, simply by changing the effective "sugar density" of the food you consume.
Back to our mice: when faced with an excess of calories, how can the mouse's body respond? It can either store energy, or burn it off (or both). We know some of it got stored, as the mice got heavier. The show gave one example of "burning it off", in the spontaneous increase in activity. I don't know if they measured it, but I'd wager that the mice also gave off more heat, which I think is a more effective way dumping energy. Muscles are remarkably efficient, and it is surprising how much mechanical work you can get out of a kilocalorie, when compared to the equivalent thermal energy (1 kilocalorie will raise 1 kilogram of water only 1 degree C in temperature, but raise a 1 kg mass over 400 meters against gravity). So the outcome of the mouse experiment is wildly inconsistent with the CICO paradigm, but precisely what one might predict from evolution and metabolic regulation. It would be very interesting to see what would happen if the mice were allowed to continue eating cooked sweet potatoes for a longer time period. I wonder if they would develop mouse metabolic syndrome?
The second line of thought follows the main line of reasoning from the show, which is thus: by incorporating more nutrient dense foods into their diets, our hominid ancestors set in path a major evolutionary shift, where gut size was exchanged for brain size. The argument is elegant: if you eat food with greater bioavailable caloric density, you can spend less energy on digestion. That opens up an evolutionary pathway to increase brain size and energy expenditure at the expense of the gut, because you can extract the same energy from food with a smaller and less-demanding digestive system. And indeed, the fossil record seems to indicate that the major jumps in hominid brain sizes came at two critical nutritional junctures. The first was around the time we started eating meat. The second was when we started cooking food, and cooking is hypothesized to have led to modern humans.
It's interesting to consider the evolutionary advantage brought about by our big brains. What advantage did they bring our hominid ancestors? The scientist visiting the African hunter-gatherers went on a bit about how hunting is a fairly complex behavior, particularly as practiced by humans. But there's a lot more to it than that. The brain remembers - it stores information. And an "advanced" brain not only remembers information, but can extrapolate it to the future, making choices now that create advantages later. For instance, it's good to know that when the rains come, wildebeest are going to show up and a certain place, and that there's an effective method for cutting one out of the herd, how to make the tools you need to kill and butcher it, which parts are good to eat, etc. In the context of hunting and gathering, there's a positive feedback loop between the increase in nutrient density and encephalization, each reinforcing the other.
Of course, there's been another major shift in nutrient density since the advent of cooking: agriculture. The advent of agriculture is an interesting case. At first blush it doesn't seem so hot. The main thrust of human agriculture has been domestication of annual grasses for their seeds, i.e. grains. Across the world, in geographically separate locations, populations growing different crops (wheat, corn, rice) uniformly appear to show significant increases in the chronic "diseases of civilization". But evolution doesn't care if your teeth fall out or you drop dead from cancer at age 40. All evolution cares about is reproductive fitness, and agricultural humans had an undeniable reproductive advantage over hunter-gatherers; otherwise we wouldn't be having this conversation.
We had a hint from the mouse experiment above that an increase in dietary effective energy density could lead to "metabolic overload", exceeding the body's ability to balance and regulate the intake and expenditure of energy. And indeed, the evidence continues to mount that a good chunk of our modern epidemic of chronic diseases may be attributable to such metabolic malfunction. It makes me wonder what happened to our Australopithecus ancestors when they started eating meat: did they suffer metabolic disease as well? It's an academic question, of course, as chronic disease or no, meat-eating proved to increase reproductive success. We might ask a similar question about what occurred as cooking gained popularity. These are hard questions to answer from the fossil record. Agriculture happened much more recently, and further is amenable to archaeology (agriculturists tend to gather in large numbers in one spot, as opposed to wandering all over the place looking for food).
But here's a thought: we noted above that big brains are useful for remembering lots of stuff. This is important when living as a hunter-gatherer, because the dynamics of nature are complex. Maximizing reproductive potential in this context means being able to remember and extrapolate the myriad (and often subtle) cause-and-effect relationships of the natural world, along with whatever technological innovations are required to take advantage of this knowledge. This information does not get passed on genetically, but rather through communication, i.e. parents teaching children. Knowledge is power, in a very tangible sense, when talking about hunter-gatherer survival. Greater knowledge implies greater ability to obtain nutrient-dense food, hence greater reproductive fitness; it also means that it takes longer to get that knowledge into the brains of your offspring. It is often argued that diseases of civilization have little effect on evolution, because they generally kill you after the reproductive years. But that assumes the only information being passed along is genetic. If memories are also required for the reproductive success of your offspring, it pays to live long enough to pass along that information. And if you follow this line of reasoning, it's clear there's a volume of information at which the parent will not be able to effectively communicate the body of knowledge while still performing hunting and gathering activities required for survival. Enter Grandma and Grandpa. If the information volume for reproductive effectiveness is sufficiently large, it pays to live long enough to pass along that information to your offspring, their offspring, and so forth. Correspondingly, adoption of new dietary practices must either preserve this longevity, OR require less information to be effective.
So where does agriculture fit in? Is the adoption of agriculture, which brings with it ever-increasing energy density in food, driving us toward the next phase of "big brain" evolution? Good question - but consider this: how much do you need to know to be an effective agriculturist? I would argue rather little, compared to hunting and gathering. A hunter-gatherer may have thousands of foods in their diet, and they have to know where and when to find them, how to prepare them, etc. Agriculturists have relatively narrow diets, and there's a relatively simple and fixed pattern to the whole business: plow the land, plant the seeds, keep out the weeds, harvest. Lather, rinse, repeat. So I think you can argue that agriculture has a much smaller information burden than hunting/gathering. The tremendous technological increases since the advent of agriculture are a testament to how relatively little brain-power is needed for obtaining food anymore, as we apparently had plenty of spare brain capacity to monkey around with things not directly related to getting fed.
Now it is well known that brain volume decreased dramatically with the advent of agriculture. So did adult lifespan. Yet the agriculturists clearly laid the smack-down on the hunter gatherers, evolutionarily speaking. So neither long-term health nor brain size is a reproductive advantage once you start growing your own food (or at least the foods that our ancestors chose to cultivate). There's no point in fueling a big brain if you've got nothing to put in it. And there's no point in keeping old people around if they are not able to contribute directly to the reproductive fitness of their offspring. If you can't work the fields, don't make babies, and we don't need your accumulated wisdom, then you're pretty much just eating food better used for making more genetic copies. So for an agriculturist, dropping dead at 35 may actually have been an advantage.
It makes me wonder what direction agriculture (and more recently, industrial food processing) is driving our "humanness". Does the ever-growing energy density and general availability of food imply we'll evolve even smaller guts and bigger brains? I'll put my money on "No". After all, look around you: it's not like the smartest people are the most successful reproductively. You can damn near be vegetative, contribute nothing to society at all, yet we will ensure you've got all the Big Macs and Twinkies you can stuff in your face to fuel the generation of lots of babies to do the same thing. In our current environment, evolution favors being chronically ill and stupid. It doesn't really matter how much we wring our hands about ethics, culture, and society: reproductive success always wins. So if humanity wishes to achieve its stated long-term goals of giving people long and healthy lives while living sustainably on Earth, we'd better figure out how to align those goals with Nature's overriding law of reproductive fitness.