Evolution of the human diet

Eduzendium article

The evolution of the human diet is an important research topic within physical anthropology and nutritional anthropology. It involves evidence drawn from human biology, nutritional science, the paleoanthropological analysis of hominin fossil remains, and comparative studies in primatology. Key issues that have been investigated to date include the functional relationship of dentition and craniofacial anatomy to diet, behavioral adaptations to diet (such as the use of tools and fire), the metabolic consequences of increased encephalization, and the relative evolutionary importance of meat-eating. Ancient hominin diets are inferred through a wide range of techniques, such as biomechanics, dental microwear analysis, stable isotope analysis, and paleoenvironmental reconstruction.

Overview

The human diet differs from that of other living primates in several important ways. First, humans are highly omnivorous, exploiting a wide range of plant, animal, and fungal foods (although they do not tend to consume plants high in cellulose, unlike some primates). Second, the human diet is comparatively high-quality, or dense in energy and nutrients. Finally, there is not just one human diet, but instead a very wide range of diets situated within a highly diverse set of environments. Although all humans share the same broad dietary requirements for calories, macronutrients, and micronutrients, various populations have discovered or invented significantly different strategies for meeting those requirements.

This creates a conceptual problem for the evolution of the human diet: if modern humans, all of whom are morphologically and physiologically very similar, consume such a wide range of foods, then how is it possible to specify the particular diet of earlier humans?

Paleoenvironmental reconstruction

The reconstruction of ancient hominin environments provides a valuable foundation for inferring hominin diets, since diets are necessarily shaped and constrained by environmental factors. For example, if an environment is characterized by dry, open grasslands, then it is unlikely that the diet would have contained a large proportion of fruit.

Primatological and ethnographic comparisons

An analysis of extant primate diets can provide clues to the evolution of the human diet, since humans are themselves primates. Additionally, the diet of modern humans living in small-scale foraging societies may be similar to that of the earliest humans.

Morphological evidence

The physical qualities of the masticatory apparatus - teeth, jaws, and related cranial features - can be used to infer the mechanical properties of the diet. However, most morphological evidence is indicative of what an organism could eat, rather than what it necessarily did eat. Also, in evolutionary terms an organism's morphology is strictly adapted to its ancestors' environment, rather than its own.

Another aspect of morphology directly related to diet is the gastrointestinal tract (GIT). Digestive systems designed to efficiently digest meat, for example, look quite different from ones designed to process large volumes of fibrous plant matter. Intriguingly, modern human GITs appear quite carnivore-like in comparison to extant great apes, a fact which may be related to issues of bioenergetics and brain metabolism. Unfortunately, it is difficult to trace the morphological evolution of the human GIT, because soft tissues do not readily fossilize. However, it may be possible to infer some properties of hominin GITs from other aspects of thoracic morphology (such as the size and shape of the ribcage).

Dental functional morphology and microwear

The size, shape, and configuration of teeth are strongly indicative of diet. Teeth designed to fracture hard, brittle foods, for example, look quite different from those designed for shearing tough, elastic foods. Furthermore, food items often cause microscopic pits and scratches on tooth enamel, and the structure of these features is determined not only by the physical properties of the food, but also by the mechanics of mastication and external processing.

One way of quantifying dental functional morphology is to compare the shearing quotients (SQs) of primate molars associated with different diets. Shearing quotients are a measure of mesiodistal crest length compared with occlusal surface length. A high SQ indicates a more "pointed" or "jagged" dental topography, while a lower SQ indicates a "flatter" tooth. Comparative primate studies show that folivores and insectivores exhibit higher SQs than do frugivores, and that within frugivores, soft-fruit feeders have higher SQs than do hard-object feeders.

A problem with this technique is that it requires relatively unworn teeth - once crests or other "landmarks" have worn down, it is no longer possible to accurately calculate the SQ. Since most fossil teeth are indeed worn, the technique is therefore of limited utility for analyzing the diets of extinct species.

This problem can be surmounted through dental topographic analysis, a recent technique that involves scanning a tooth in three dimensions, and analyzing the resultant data through geographic information systems (GIS) software. The advantage of this technique is that it analyzes the entire occlusal surface of a tooth, rather than specific "landmarks." Thus, it can be used with equal effect on both worn and unworn teeth. Dental topographic analysis provides information on the total surface slope, aspect, area, angularity, and other topographic attributes of the tooth surface. These attributes can then be used to characterize functional aspects of occlusal morphology.

Chemical evidence

A powerful method of assessing diet is the direct chemical analysis of bones, teeth, and other remains. While the morphology and habitat of an extinct species may provide strong indications of what it probably or usually ate, chemical analyses provide direct evidence of what it actually ate. This is because the chemical properties of an ingested food item are reproduced with high fidelity in the tissues of the animal that consumed it.

For example, it is known that mammal metabolisms discriminate against strontium in the diet, in favor of calcium. Since plants contain both elements, an herbivorous mammal would be expected to express a lower strontium-calcium (Sr/Ca) ratio in its tissues than the plant. If the herbivorous mammal is then itself consumed by a carnivore, the carnivore would be expected to express an even lower Sr/Ca ratio, since its metabolism would further discriminate against the strontium. Thus, Sr/Ca ratios can provide indications of trophic level: high values signify a diet high in plant tissues, while low values signify a diet high in animal tissues.

Archaeological evidence

Although the archaeological record of early human evolution is quite sparse (especially before 1.8 mya), artififacts can provide valuable dietary evidence. Specifically, the form and wear patterns of early stone tools can suggest how hominins may have gathered and processed various plant and animal foods. Additionally, examinations of the cut marks on preserved animal bones may indicate hominin hunting or scavenging behaviors.