This line of thought was provoked by an article I read talking about the importance of cooking food in the development of modern humans.

I’ve come across a number of articles that talk about the importance of a change in diet in human evolution. The theory is that human diet suddenly improved dramatically, and this marked increase in the availability of calories and nutrients was responsible for the growth of the brain. The improvement in diet is generally tied to either the switch from a primarily herbivorous diet to meat eating, and/or to cooking.

Both make sense in terms of calories and nutrients. Meat is certainly a more densely packed source of nutrients than plants. And cooking increases the bioavailability of nutrients, as it breaks up cell walls and structures that hinder our digestive enzymes from reaching the goodies inside cells.

Of course, this still leaves us without an actual mechanism. Evolution requires heritable changes in the genome. These happen largely by  accident, though their selection depends upon what is advantageous to survival and reproduction. At this point, we don’t know very well exactly which genes are responsible for the differences in our brains, say compared to chimpanzees. We don’t know when these changes first appeared. We don’t know what connection they have to an improved diet.

So one part of evolution, that which is related to the genomic changes responsible for our large brains, is mostly unknown to us. Therefore, I think that ideas such as the change-in-diet leading to big brains scenario, tend to ignore the unknowns and focus only on the natural selection side of evolution. They make certain assumptions, for example, that a large brain will be selected, because it enhances survival and the chance to reproduce. This can be somewhat justified if one thinks about  it (large brains, specially the growth of the forebrain is what allows us to make long range plans, analyze complex problems, etc.), and also there is fossil evidence that shows that in fact it was selected. Then there is the assumption that a large brain requires a nutrient rich diet, which can also be justified on the basis of the caloric expenditure in maintaining a large brain. A commonly offered statistic is that for a person at rest, of the amount of energy required to stay alive, the brain uses 20%, or 1/5th. The brain is obviously much less than a fifth of the body in terms of mass, yet it uses an extraordinary amount of energy, in proportion. If you keep the total mass of an organism constant, but increase the size of the brain in proportion to the rest of its body, then such an organism will require a more nutrient rich diet. In effect, you have increased its energy requirements, but have not given it bigger jaws to chew food, a bigger gut to digest it, bigger claws to hunt with, etc.

In fact, both anatomy and the fossil record show that humans became less capable of acquiring food as their brains grew, if we look solely at such biological markers such as tooth/jaw size. Homo erectus had smaller jaws than his ancestors, which would have made it harder for him to grind foods down and extract the most energy from them. Our gut became smaller, and less capable of extracting energy from plants. Our muscles became weaker, less capable of overpowering other animals through brute strength alone.
One would think that the timing of these changes would have some correlation with our behavior or change in diet, or the control of fire (for cooking). Unfortunately, the timing is much harder to pin down. No one really knows when humans first learned to control fire. Homo erectus, with his small jaws, evolved 2 million years ago, but the evidence for the widespread use of fire by humans at this time is scanty at best. Most anthropologists don’t believe that fire was used by humans this early, at least, not in any regular, controlled manner, such as would be needed for cooking. Soft tissues don’t fossilize well, so while we can study humans and chimps today and recognize that the chimp gut is much more suited for eating raw plant material than the human gut, we don’t really know when we evolved our more carnivorous digestive systems.

This leaves a chicken versus egg conundrum. Which came first, the big brain or the adaptations to the big brain lifestyle? Which was responsible for the other? This may be a silly question on the face of it, because obviously one is useless without the other. What’s the point of having a modern jaw or gut if you don’t also have the bigger brains that give you the means for filling that gut with food? On the other hand, how do you sustain that brain and give it energy without eating a more nutrient-rich diet?

So it seems that speaking in terms of absolute causality, one thing causing the other is somewhat simplistic. They probably both happened together, one reinforcing the other, and happened gradually. We didn’t go suddenly from a chimp-sized brain to a human-sized one, as we know from the fossil record. There are many intermediate stages of the brain growing progressively larger. The change in diet, therefore, and the behavioral changes accompanying both the change in diet and the larger brain, must have happened concomitantly.

It’s interesting at this point to bring in the factor we’ve ignored all along – that there must be genomic changes that produce all the anatomical differences – jaws, teeth, gut and brains. These genomic changes also need to be accounted for, and tied into the selection mechanism. A high nutrient diet is obviously not enough; otherwise large cats such as lions and tigers would be smarter than us. They might not cook, but they eat enough high nutrient food to be able to support bigger brains. They have evolved as long as us, why didn’t they learn to cook,  why didn’t they evolve bigger brains?

This brings us back to selection, and fuzzier areas of anthropology such as social behavior and interactions, etc. We have bred dogs for a few thousand years, for example, and we have breeds of dogs today that look very different from each other. Not only is there is a difference in size and color of the fur, but there are also differences in the brain. Some breeds of dogs are smarter than others. We did this by a fairly simple process of selection – pick dogs that have the traits you want, breed them to produce a new generation, keep selecting for the desirable traits and reinforcing them through successive generations. Even with no knowledge of DNA or even Mendellian genetics, our ancestors were able to do this for dogs. We have also bred cows, pigs, goats, sheep, etc. – the modern domesticated forms of which are quite different from their wild ancestors. Not to mention the similar and parallel process of breeding food plants.

So even without postulating major and sudden changes in the genome, those which suddenly introduced a “game changer” mutation so far as the brain was concerned, it’s possible to see that humans could have become progressively smarter simply if the natural variation in smartness among a population was selectively reinforced over generations, the same way we breed dogs. Of course, this doesn’t mean that you can breed dogs to a human level of intelligence, there may well be certain required mutations, and these have to happen first. You can’t select for what doesn’t exist. But at a point where we don’t fully understand the nature of these key mutations, we can’t really talk about how essential they were. Perhaps they could happen in other species too. Perhaps there are a dozen different ways to get the same result, and if mutation “A” doesn’t happen, mutation “X” can provide the similar benefits. I’d rather not speculate about this until we have more information to speculate with.

So I think such articles (as the one referenced above, which talks about the relation of diet to human evolution) speculate about the remainder of the problem, the mutually reinforcing effect of the selection of traits which are part of the natural variability of a population, and the behavioral consequences of selecting such traits. You set a species on a certain path, on which a greater reliance on the brain cuts out some options while expanding others, and the options that are promoted require even greater brain power to work well. It’s interesting to speculate what put us on this path, why we seem to be the only species on it. What set of circumstances came together at the right time for this to happen. The drying climate and spread of grasslands, the change from an arboreal to a savannah type lifestyle, the appearance of bipedality at this critical juncture when these big new ecological niches suddenly opened up, the development of more and more hand flexibility with a greater range of movement in the opposable thumb (compared to other primates), the social interactions, etc. There were so many changes happening at roughly the same time to the same species, somewhere this set us off on a path to bigger brains.

The article that set me off on this line of thought, of course, talks about something narrower. It talks about the relationship of cooking to gender roles, the development of the male-female bond, which is marriage today. This seems less an evolutionary question than an anthropological one. The evolutionary part is the importance of cooked food in the development of our brains, which I have speculated about. The anthropology part is relating this importance to something else, namely male-female pairing. I am not qualified to speculate about the anecdotal evidence offered about some primitive societies where food is more important than marital fidelity. Nor do I have any evidence that women tended the home fires, though it seems likely if men were the hunters and spent less time at the home camp. This seems to be supported by fossil evidence, such as hunting related injuries, as well as by anthropological evidence. You can speculate that the importance of cooked food was critical enough to shape our behavior patterns in other ways, such as pair-bonding between males and females. But just because a theory seems to make sense doesn’t mean it’s true, so I guess we’ll need to see some more physical evidence before placing much value on it.