The Evolution of Human Diet

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Chimp, Australopith and
Human Teeth Compared.

The evolution of human diet followed a major zig (as in zig-zag) in a wholly unexpected direction, followed by the most significant biological innovation to ever occur among multi celled animals: The invention of cooking. I’m actually going to point you to two papers on this topic, and provide a brief summary of the ideas here.

Let’s start with the bold assumption that humans evolved from a chimpanzee-like animal. This is tantamount to saying that the last common ancestor of chimpanzees and humans was, essentially, pretty much like a chimpanzee. At another time, I’ll write a post on why this is a good assumption, but for now lets just go with it. Some large percentage of human evolution experts like this assumption, a bunch of others hate it (which is the usual pattern for most ideas in human evolution).

A mammal’s diet is reflected in physiological attributes that can be discerned from the fossil record. Body size, the nature of the teeth and associated muscles, possibly the shape of the mouth’s cavity, and even the overall size and shape of the gut may be closely connected with diet.

If we draw a direct line from a presumed chimpanzee-like ancestor to modern humans, we can make some inference about dietary changes by looking at these features. Chimp and human teeth are fairly different at first glance. Chimps have large canines, which could influence chewing, but let’s ignore that for now and assume that the loss of fighting canines is related to something else. The shape of the tooth row is different, with Chimps’ molars being roughly parallel, while human teeth form a more continuous arc. That probably has noting to do with diet and more to do with the canines and their use in a somewhat elongated snout (and some changes in human skulls). Chimp teeth are relatively thin on enamel and they are not especially high-cusped or large. This is not too different from human teeth. In other words, it is hard to look at the molars of chimps and humans and speculate about huge differences. The incisors of chimps are very large compared to humans, and that is likely to be a dietary difference. The large incisors are thought to be useful in processing fruit, both for biting into hard fruits and for keeping the wadges of tough fibrous fruit into the mouth during prolonged bouts of chewing.

Humans range in size from about chimp-size to much larger. This mean that humans have a higher demand for energy for both growth and maintenance of the larger size, which in turn would place more demands on the human diet (in terms of total energy). Chimp guts seem to be larger relative to body size suggesting that chimps eat a lower-quality food that requires more processing in the gut.

But why compare chimp and human indicators of diet when we can actually observe chimp and human diets? No good reason, actually, other than to prepare the groundwork for comparisons that cannot be made by direct observation (the diets of extinct hominids).

Living chimps like to eat fruit, and they eat what is known in the biz as “Terrestrial Herbaceous Vegetation” (hereafter: THV) as a fall back food when fruit is not available. Forest salad is better than starving. Chimps also eat some meat. But what does the typical human diet consist of?

This is a problem, because as we look at different human populations, we see a huge amount of variation. One way to make sense of the diversity of human diets is to start by only considering the diets of hunter-gatherers. But among hunter-gatherers, there is still a huge range of diets. Some human forager groups eat almost all meat, others eat very little meat, for example. OK, so we can simplify this even more by focusing on human hunter-gatherers who live in tropical and subtropical regions, since people have moved into temperate and colder regions only several thousand (or tens of thousand) of years ago. But we still encounter a large amount of variation. With respect to meat eating, we’ve only eliminated the most extreme cases such as the Inuit.

One thing that obviously sets human diets apart from all other mammals is that a very large percentage of the food that humans eat is cooked. This is important. Consider these two probable facts:

1)An important percentage of the food that all human groups consume is cooked and can only be eaten if cooked; and
2)Many environments in which human foragers live or have lived provide food that is insufficient for human sustenance unless it is cooked.

In other words, not only do we eat a lot of cooked food, but without that cooked food there are probably many regions of the world where our species would simply not survive.

Putting it yet another way: The ability to cook food transforms most environments on this planet into one habitable by humans. THIS is a link to a paper that suggests that the origin of cooking (which involved the controlled use of fire) is indicated in the fossil record by a strong biological signal. This signal consists of a huge increase in body size accompanied by an impressive reduction in tooth size. These two changes are at odds with each other: How do you manage a huge increase in energy requirements for growth and maintenance at the same time as a decrease in the basic apparatus for obtaining energy? There are other physical changes as well, and broad implications including changes in mating system, suggested in our paper. Enjoy!

Above, I talked about drawing a straight line from a chimp like ancestor to modern humans, but many of you knew that I was just leading you on. You know this because one of the major findings of the last 35 years of human origins research is that our lineage has undergone change not in a single direction (leading to “us”) but rather in various different directions at different times. The key characteristic of the rise and diversification of the australopiths and their close relatives was probably NOT becoming bipedal. Oh sure, that was important and everybody goes gaga over that interesting fact. But the most important adaptive innovation was probably megadonty and related adaptations. This involved the surfaces of the teeth becoming larger, the enamel thicker, and teeth that are normally not used in chewing being recruited to varying degrees to doing the job of the molars (along with the giant molars themselves, of course).

Concurrent with this was NO change in body size (at least none that follows any sensible pattern); a probable increase in sexual dimorphism indicating a mating system shift from something like a living chimp (multi-male multi-female) to something more like a gorilla (single reproductive male and a harem of females), as well as some other details like a larger mouth interior signaled by a high arching palate.

Again, I’m going to send you to a paper, which you can download HERE. In this paper, we argue that the primary novel adaptation of these australopiths was the use of plant underground storage organs (roots and such) as the principle fall back food. In other words, a chimpanzee like ancestor, forcing on fruit as it’s primary food and THV as it’s fall back food probably continues eating fruit (and some meat, we suppose) but changes it’s fall back food from leaves to roots.

There are many important implications of this, some speculation some not. For instance, there are more roots in drier environments. As you measure root availability from the rain forest, wooded savanna, open savanna, and arid areas, you get more and more as you move along a transect from wetter to drier habitats. A chimpanzee like animal would be confined to the rain forest not only because its primary food (fruit) is more abundant there, but perhaps more so because it’s all-important fall back food … the food it eats to avoid starving in a bad season or year … is confined to the rain forest. Chimps would not be able to eat most of the non-rain forest plant leaves. But if you swap roots for THV, it is now actually preferable to leave the rain forest to seek this fall back food.

The eating of USOs as a fall back food essentially served as a kind of conveyor belt moving these hominids from the rain forest into other, more open biomes. We suspect it shaped social structure and behavioral ecology of the australopiths as well. In some way, it could be said that these early hominids were more different from either chimps and humans than humans and chimps are to each other (… hey, I said SOME ways…. ).

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