According to Milton(7) in a 1904 publication, the physiologists Elliot and Barclay-Smith had declared that the human gut was closer to that of a herbivore than an omnivore. They were either not believed or were forgotten, or perhaps people thought their conclusions imprecise? In the following century far more comparative biology has been performed, and despite (or because of) thousands of dissections, research still seems to be inconclusive.
There are axiomatic issues facing comparative biologists that make it impossible for their venture to yield usefully precise results. The further afield one goes in species gap between man and the subject animal for comparison, the less the chances that one will discover a species that is similar enough to man to be of use. The chimpanzee seems to have become a focus for many anthropologists, but examination of its teeth, faeces and a taste of the food it eats, should be enough to convince anyone that we don't share the same diet as chimps. Chimpanzees are simply not human ancestors, and have followed their own very different evolutionary path for millions of years. In any case, chimps have their own food cultures and environmental challenges, and they can learn new behaviours by observing humans, all of which could confound comparisons.
Perhaps because of the chasm between the great apes, other have chosen to focus on mans more recent ancestors by examining fossil finds, or by looking at contemporary hunter-gatherers. Unfortunately as Lewin commented(25) there is no way of knowing whether fossil finds are our ancestors, or simply our cousins(p. 59). As each new fossil find is incorporated into the evolutionary tree, the tree becomes more complex, and the simple linear view of our alleged ancestry that is presented, has to be adjusted. Lewontin says that "most fossils of different ages cannot be connected in a linear sequence, but represent a small sample from a lot of parallel lines."
Furthermore, since both enculturation and habitat discordance are confounding influences even in the chimpanzee, what chance have we of finding a naturalistic diet amongst ancient hominids? In addition, even if we could, how would we convince people that their diets were actually healthy with no medical records, or soft tissue to analyse? A similar issue applies to studies of contemporary hunter-gatherers - are they actually healthy? Certainly, there are lessons to be learnt, but a wealth of empirical nutritional research is sometimes ignored in the hope of finding a dietary ideal in hunter-gatherers or apes.
Before embarking on a mission to create a diet classification, it is essential to have rigid mutually exclusive quantified categories defined, and some kind of systematic method. The absence of such a precise system inevitably led to the confused naming free-for-all that has occurred. The term omnivore seems to apply to a vast array of different diets that range from small amounts of insect matter to regular and complete meals of carrion. Due to this imprecision, scientists would do well to refrain from using the term, as Chivers (and others) have pointed out.
Where does this leave the human? We can show a substantial range of adaptations to plant based diets, and in particular to juicy fruits. We can also show some not very close anatomical similarities to smaller primates that do make up a significant proportion of their diet from animal matter. However, there seems to be no compelling evidence of human adaptations to consume animal foods, and plenty of evidence of a lack of adaptation. This seems to make a bad case for humans being grouped in along with pigs, dogs or bears and other irrefutable “omnivores”. The behavioural evidence is perhaps the least scientific evidence, and yet in many ways is seems quite compelling. Taken as a whole, the evidence seems to suggest a foli-frugivous diet as natural for humans, with a digestive system that can probably tolerate small amounts of animal matter. It is therefore imprecise to call humans omnivores, but not necessarily entirely mistaken.
At the beginning of the article a criteria was suggested for testing whether humans are omnivores based on the health impacts of consuming significant amounts of animal products. Amongst the worldspopulations, it is in general the Western populations that consume the most animal products. The epidemiology of these populations has clearly established that diets high in animal products are directly associated with the epidemics of degenerative diseases such as cardiovascular disease and cancer.
The problem is that epidemiology is based on statistical associations which do not reveal causative factors. So far, comparing meat eaters with vegetarians has failed to produce conclusive evidence(32). Even so, evidence does seem to suggest that reducing meat intake is associated with healthier outcomes. Studies that compare meat eaters and vegetarians are confounded by the fact that vegetarians have lifestyles which are healthier in many ways versus the average meat eating person - in any case vegetarians may still eat significant proportions of animal products. A better study would be on a population where the level of consumption of animal foods varies, while other lifestyle factors remain comparable. However, this would still not overcome the problems inherent in making statistical associations.
Even if it were possible to overcome the problem of using statistics, it might be suggested that contemporary animal rearing methods produce unhealthy animal foods. Indeed there is now a popular movement towards more "natural" rearing of animals, in the belief that such animals make healthy food. There seems to be little substance to this claim. Traditional populations that consume large amounts of animal products, from free roaming animals, have been found to suffer with extensive cardiovascular disease(33). If it is further proposed that their intakes are excessive, then this leads to the question of what level of animal food intake is believed to be free of adverse effects, and then onto the question of why humans should be adversely affected by eating more animal foods if they are supposedly adapted to such a diet.
Given all of the above information, the evidence for humans being omnivores is not compelling. Humans have no clear-cut adaptations to consume animal foods, and regular consumption of animal foods is associated with unhealthy outcomes. In contrast the evidence presented is completely consistent with the claim that humans are specialised frugivores.