Humans: Carnivore, Herbivore, or Omnivore?

[unspecified42]

Since one of our fellow BPers has made the statement that humans are herbivores, and I refuted that with references to a great deal of evidence to the contrary, rather than hijacking the Bullfighting thread, I figured I would go ahead and give it its own arena.

You would be referring to me, but you are misinterpreting what I said. I didn't say that humans were herbivores- I said we were originally herbivores. Clearly we have adapted to ability to digest meat, but I seriously doubt that our original ancestors were meat eaters.

So:
Evidence for omnivory in humans:
Dentition: Cutting surfaces and grinding surfaces evenly arranged. Molars are weak, not up to the task of grinding large amounts of plant matter. Canines are small, but canines are used for grasping prey, not for cutting meat, so are not necessary for an animal that does not kill using its teeth. Number of teeth reduced in modern humans. Jaw size also reduced, indicating a diet that is not high in fibrous or tough materials. Jaw PRESSURE in modern humans is quite impressive, however. We can crack a nut (or a bone) if we have to. In a fossil record, modern humans would be easily identified as omnivores.

Actually there is really nothing about our dentition that suggests it evolved for the purposes of eating meat. You suggest that it does because we could be capable of crushing bone, how that just isn't really true. Our jaw strength is not really that impressive at all, and certainly paltry in comparison to predators who use their teeth for crushing bones. We would have a tough time with nuts, really. A diet that requires lots of bone crunching would leave us with broken teeth at a very young age.

Here is something that I’m sure I will repeat several times before I’m done:

Humans are clearly not ruminants. You are correct that we are not grazers and don’t have the teeth of a cow, nor do we have multiple stomachs for the digestion of tough plant matter. However, we are well designed to eat soft vegetation and we lack pretty much every adaptation of carnivorous and omnivorous animals (compare our jaws and dentition to that of other omnivores such as bears and pigs).

We have no teeth that aid in ripping meat from bone as omnivores and carnivores do. Our canines are so small as to be rendered useless. The shape of our jaw is all wrong for eating meat as well. Our jaws are designed for chewing and grinding- the configuration of our mandibular joints are much more complex and less stable than that of a carnivore. Instead of a simple joint which allows the jaws to scissor together (couple with large, sharp teeth designed to cut meat), humans have a complex jaw which brings dull, flat teeth together in a single plane so that all teeth meet at once for grinding and chewing. Our jaws also move side to side to aid in this grinding motion, just like other herbivores.

The shape of our oral cavity also suggests that meat is not a major dietary item. If you walked up to a deer you would have a very difficult time getting any meat from it. Our oral cavities are very small and not pronounced as they would be in animals designed to eat me (even compare us to the omnivorous pig). You could get your mouth around a leg or something, but you would find it very difficult to take a nice big bite out of the abdomen, unlike, say, a wolf or hyena or alligator.

Jaw size is reduced, but this is related to the fact that it allows us more room for a larger brain. This likely did happen after we started eating meat (and had the fat intake necessary to grow such a large brain). The fact that we have to get our wisdom teeth removed is evidence that we once had jaws that were more capable of handling fibrous matter. Not all evolutionary changes are advantageous (for example, humans have a much more difficult time giving birth than most other animals because our upright position requires a narrower pelvis, which makes birthing more difficult).

Intestinal length: Herbivores use one of two strategies to enable them to digest plant matter--either an extremely long and elaborate digestive system (such as in a cow), or a shorter system that food is passed through more than one time (such as in a rabbit). Carnivores have a short digestive tract, because meat is high in nutrients and extremely simple to digest. Omnivores have a digestive tract that is intermediate between the two. In fact, the human digestive tract is closely comparable in length and structure to that of a pig--a devout omnivore.

You are comparing our digestive tract to that of a ruminant, which again humans clearly are not. You would like to compare our digestive tract to that of a pig, but it is erroneous to just pick one other animal to compare us to.
Take a bear for example. It is an omnivore, with the majority of its diet composed of soft vegetation. It does also eat meat, though, and because of that it has classic anatomical features of carnivores including a short gut length (about 5x its body length, as opposed to humans’ 12x) and jaws designed for eating meat (as well as the teeth, claws, and sheer brute strength to hunt)

Tools: Hominids and great apes are devout tool users. Tool use itself is an interesting adaptation that allows a species to take advantage of a variety of food resources. Some species rely on tool use almost exclusively in order to procure food, such as woodpecker finches. Great apes regularly use tools to procure their food--to crack nuts, for example, to fish for termites, and to kill small animal prey. Our closest living relatives, bonobos and chimpanzees, both kill and eat monkeys fairly regularly to supplement their diets. Female orangutans have also been observed catching and eating small animal prey, and fish. The oldest stone tools we have found created by homonids were clearly designed for pounding, cutting, and scraping. Cutting and scraping are needed for preparing meat, and not for much else. Throughout the progression of homonid tool development, we find increasingly sophisticated tools for preparing meat and hides, and for hunting (such as spearheads).

Saying that great apes a ‘devout tool users’ is a bit misleading. Great apes occasionally use tools, and they rarely use them to kill other animals. Mostly they’ll use sticks to retrieve termites and rocks to break open nuts. This is nowhere near the complexity of a bow and arrow or hunting spear. Also, humans are not descendents of great apes. We share common ancestry, but we had an evolution divergence many many years ago. We are separate species with separate diets. It is clear that great apes have evolved adaptations that make it easier for them to eat meat (their canines are much larger than ours, for example).

Cutting, pounding, and scraping are also things used to break down tough plant matter before consumption. Civilizations evolved meat-eating ways. Preparing meat and hides has nothing to do with our diets, but the fact that we had to develop complex tools used to kill animals suggests that it is not an original adaptation.

Behavior: Humans hunt. Even children, left to their own devices, will stalk small animals. Catching butterflies isn't just about the 'pretty'. We have all the instinctive equipment necessary to wait patiently for long periods, to track, and to persist. We enjoy the process of hunting, like all predators.

Herbivorous animals such as rabbits, horses, apes, etc also frolick and play with one another, particularly as juveniles. They chase and pounce as well. It is a way for them to establish dominance within a group and learn how to interact with members of their group, not necessarily a way to practice hunting.

Physical fitness: Although you wouldn't know it by looking at the average cubicle-dweller, humans are incredible athletes. We are not particularly fast, but we don't have to be fast--we have tremendous endurance, and we can run for miles. Many prey species cannot. As a result, one of the oldest methods of hunting is known as persistance hunting. It involves selecting a large, fit male of a horned animal species, and following it until it drops dead of heat prostration and exhaustion. (The heavy horns tax the animal's strength faster). This is not a rare occurrence, or luck--it is a regular hunting style still practiced by the San people in Africa today. The San are our genetic ancestors, according to gene mapping...they are the oldest surviving lineage of humans on the planet. Humans can climb, dig, swim, jump, and run for miles without stopping. We are superb predators and are capable of grasping any opportunity presented to us.

Many carnivores and omnivores alike have high endurance for a variety of reasons.

I disagree wholeheartedly that humans are ‘superb predators.’ Aside from our intelligence, we are not well designed for tracking. The fact that we walk on two feet greatly reduces our stealth. As you've pointed out, we aren't all that fast, either. We have very poor senses of hearing, smell, and vision when compared to the rest of the animal world. We have nothing aside from tools to help us kill an animal, and we have very poor anatomical defenses for our own bodies. We wouldn't do well in a fight with a prey animal.

We can climb, sure, but we aren’t that good at it really. You could probably make it up an apple tree to pick some fruit, but if you are going to be battling some animal up there so that you can overpower it and eat it you’re likely to be quite disappointed.

Culture: The oldest existing human cultures on Earth are omnivorous. In fact, there are no primitive herbivorous cultures. There are primitive carnivorous cultures, however (the Inuit, for example).

Evolutionarily speaking, even Inuits are modern cultures. It is true that there are some cultures that could not survive without meat. However, these cultures did not evolve in the areas where they now live, suggesting that the meat eating was an adaptation and not an original intent.

Nutrition: Humans require vitamin B-12 in their diet, which is a vitamin that herbivores manufacture in their own gut. B-12 is found primarily in meat. B-12 is a very important vitamin for human health. Non-animal sources of B-12 are few and far between, and were largely or entirely unavailable to primitive peoples (such as yeast). No successful herbivore is unable to manufacture vitamin B-12 for itself.

The human demand for B-12 is very low and readily available through sources that early humans almost certainly ate; insects and eggs (argue that eating insects make us omnivorous if you want- I won't disagree. But I think it's pretty clear we're talking about eating higher members of the animal kingdom). A well balanced vegetarian (though not vegan) diet provides adequate amounts of B-12 (and all other necessary dietary vitamins and minerals)

Other various adaptation (such as digestive fluids and saliva):
As has been pointed out, meat is easy to digest. Vegetable food sources are difficult to digest, so most herbivores will concentrate their efforts on making the vegetation they find palatable actually digestible. It's much harder to digest a leaf than it is to digest a mouse, so more changes are needed to enable the digestion of plant matter.

This is the flaw of omnivory:
You can eat many things, but you will not be perfect at eating any of them. Humans are poorly adapted to eating both meat AND plant matter, but they are supremely adaptable because they can derive proper nutrition from eating both.

You are putting the cart before the horse a bit here. It is more likely that animals evolved senses of smell and taste to make what is digestible also palatable and not the other way around. We probably were able to digest berries before we found them delicious. The fact that we have a highly developed sense of taste is further evidence that we are well-evolved plant eaters, as carnivores lack well developed taste buds.

Humans are not poorly adapted to eating plant matter. We are not designed to eat things like corn and grass, but we are very well designed to digest succulents such as fruits and vegetables. We are so finely tuned for this purpose that we have a well developed sense of taste that makes us find the sweet sugars in these plants highly palatable when raw. The anthropological evidence from early cultures shows the use of cook fires used to prepare meat to make it palatable, suggesting that humans do not naturally find raw meat appealing. This was lucky for them, really, because their bodies are very poorly adapted to consume meat, particularly raw. We are highly susceptible to disease and parasites that are often spread through undercooked meat. Carnivores and omnivores are far less susceptible to this problem. Consider your omnivorous pig who will happily eat a carcass it happens across in the woods. Eating spoiled meat causes severe illness in a poorly-adapted human.
Digestive fluids and saliva are good things to point out, but neither in a human suggest that we are designed to eat meat, actually. Meat eaters do not have saliva that has digestive enzymes to help with predigestion of plant matter, whereas herbivores (and humans) do. Also, our stomach acids are considerably weaker than a carnivore’s, as are herbivores.

Recent research is also beginning to unlock the secret of our ‘useless’ appendix. Many herbivores have much larger appendixes which are useful in digesting plant matter. Our appendixes are shrunken and we survive just fine without them, but the evidence suggests that they are vestigial organs which once had a purpose when we were herbivores but not are losing their usefulness and evolving away.



All evidence I have seen points to an evolutionary pattern of herbivore to omnivore. Early humans were well adapted to eat plant material. As our brains grew and we evolved socially we developed tools that we were able to use to kill and consume animals. We discovered fire that made eating meat not only palatable, but also finally safe. The ability to eat meat allowed us to migrate to other areas of the world and survive in harsher climates and to flourish and continue our evolution. Our brains grew bigger, we had to put less effort into foraging, and we were able to put away fat stores for lean times so we didn’t have to hibernate or decrease our activity when food was scarce. We had the tools available to us so we had no need to evolved physical adaptations like other omnivores. We relied less on plant material so our bodies began to do away with unnecessary anatomy (reducing the appendix and shortening the gut somewhat) just like an eyeless cave fish did away with its vision.
While we clearly are able to eat meat in small quantities without ill effects (and even with some benefit, provided it’s the right meat), I see no evidence that suggests that early humans were built for hunting or consuming meat but rather quite a bit of evidence that humans evolved the ability later in their evolutionary history.