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Meat in the human diet: an anthropological perspective.

Article Excerpt
KEY POINTS

* Human ancestral diets changed substantially approximately four to five million years ago with major climatic changes creating open grassland environments.

* We developed a larger brain balanced by a smaller, simpler gastrointestinal tract requiring higher-quality foods based around meat protein and fat.

* Anthropological evidence from cranio-dental features and fossil stable isotope analysis indicates a growing reliance on meat consumption during human evolution.

* Study of hunter-gatherer societies in recent times shows an extreme reliance on hunted and fished animal foods for survival.

* Optimal foraging theory shows that wild plant foods in general give an inadequate energy return for survival, whereas the top-ranking food items for energy return are large hunted animals.

* Numerous evolutionary adaptations in humans indicate high reliance on meat consumption, including poor taurine production, lack of ability to chain elongate plant fatty acids and the co-evolution of parasites related to dietary meat.

INTRODUCTION

Anthropologists have long recognised that the diets of palaeolithic and recent hunter-gatherers (HGs) represent a reference standard for modern human nutrition and a model for defence against certain Western-lifestyle diseases. Boyd Eaton of Emory University (Atlanta) put this succinctly: 'We are the heirs of inherited characteristics accrued over millions of years, the vast majority of our biochemistry and physiology are tuned to life conditions that existed prior to the advent of agriculture. Genetically our bodies are virtually the same as they were at the end of the palaeolithic period. The appearance of agriculture some 10,000 years ago and the Industrial Revolution some 200 years ago introduced new dietary pressures for which no adaptation has been possible in such a short time span. Thus an inevitable discordance exists between our dietary intake and that which our genes are suited to'. This discordance hypothesis postulated by Eaton could explain many of the chronic 'diseases of civilisation'. (1) This review presents an anthropological perspective on what HG populations may have actually eaten.

Contrary to views that humans evolved largely as a herbivorous animal in a 'garden of Eden' type of environment, historical evidence indicates a very different reality, at least in the last four to five million years of evolutionary adaptation. It was in this time frame that the ancestral hominid line emerged from the receding forests to become bipedal, open grassland dwellers. This was likely accompanied by dietary changes and subsequent physiological and metabolic adaptations. The evolutionary pressure for some primates to undergo this habitat and subsequent diet change involving open grassland, foraging/scavenging, related directly to massive changes in global climatic conditions, primarily drier conditions followed by worldwide expansion of the biomass of temperate climate (C4) grasses at the expense of wetland forests, (2) accompanied by a worldwide faunal change, (3) including the spread of large grazing animals. Thus, the foods available to human ancestors in an open grassland environment were very different from those of the jungle/forest habitats that were home for many millions of years.

ANCESTRAL DIETS: ANTHROPOLOGICAL EVIDENCE

The lines of investigation used by anthropologists to deduce the evolutionary diet of our evolving hominid ancestors are numerous: (i) changes in cranio-dental features; (ii) fossil isotopic chemical tracer methods; (iii) comparative gut morphology of modern humans and other mammals; (iv) the energetic requirements of developing a large ratio of brain to body size; (v) optimal foraging theory; (vi) dietary patterns of surviving HG societies; and (vii) specific diet-related adaptations. Findings from each of these fields reveal a changing dietary pattern away from low-quality/highly fibrous, energy-poor plant stables to a growing dependence on more energy-rich animal foods, culminating in palaeolithic Homo sapiens being top-level carnivores. (4)

Changes in cranio-dental features

Early hominid fossil remains already show clear cranio-dental changes which indicate a move away from a specialised structure suited to coarse foliage mastication to a more generalised structure indicative of dependence on fruits and hard nuts but also incorporating changes that indicate meat consumption. Such changes included a decrease in molar teeth size, jaws/skull became more gracile, front teeth became well buttressed and shearing crests appearing on teeth, all indicative of less emphasis on grinding and more on biting and tearing of animal flesh....

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