Diet

Paleoanthropologists are concerned with diet evolution because it is so strongly correlated to other variables (like brain size, body size, intelligence, activity levels, geographic range, tooth shape, skull shape, etc.) and, not insignificantly so, because food intake is the primary means of survival. Diet has also changed considerably throughout human evolution. As we have already discussed, tools and teeth lend clues to hominin diet. However, there are several other useful methods for reconstructing hominin diet (others are discussed in the section on Scavenging and Hunting in this chapter).

There are ecological and energetic rules involved in diet. Energy from the sun is incorporated into plant matter, which is then incorporated into the flesh of herbivores and is finally taken up by carnivores. At each level of the trophic pyramid (i.e., the ordering of groups of animals according to diet, with carnivores on top of the pyramid) there is a decrease in energy available per unit of biomass. As a general rule, primates with large body sizes tend to eat vegetation because of its abundance relative to nectar and insects, which are sufficient for nourishing small-bodied species.

We, literally, are what we eat. Trace elements in fossil teeth and bones tell of an individual's trophic level. For instance, the metal strontium (Sr), which is usually studied in relation to calcium as the ratio Sr/Ca, is taken up by plants through ground water but is only taken up by animals that eat the plants. Low levels of Sr indicate carnivory and high levels indicate herbivory. The same type oftrophic information can be gleaned from levels of barium, magnesium, and zinc as well. The hominins at Swartkrans have omnivorous (intermediate) Sr levels, which contradicts a common assumption that australopiths and Paranthropus were purely herbivorous.

Stable isotopes from teeth and bones also tell a diet story. Carbon isotopes are particularly informative and are based on the characteristics of the two different paths of carbon dioxide fixation in photosynthesis. The C3 photosynthetic pathway differs chemically from the C4 pathway, so C3 plants have detectably different ratios of carbon isotopes than C4 plants. C3 plants are mostly trees, shrubs, herbs, and temperate grasses. C4 plants include maize, some millets, sorghum, and warm grasses. Animals that eat these different plants show the different ratios. For example, giraffes are browsers that eat mostly C3 plants and zebras are grazers that eat mostly C4 plants. Therefore, fossils of individuals that have higher carbon isotope ratios (C4) are labeled grazers and those that have low ratios (C3) are browsers or they are eaters of grazers or eaters of browsers, respectively.

Paranthropus shows a mixed carbon signature like aardvarks that eat primarily termites (which fall under the C4 signature). So Paranthropus, like its chimpanzee relatives, could have incorporated termites into its average diet. Analysis of bone tools from South African cave sites supports this notion. The tools are remarkably similar to experimentally made termite fishing wands with polished, rounded tips and parallel microscopic striations from the soil. Carbon isotopes also indicate Paranthropus ate tropical grasses and sedges, woody fruits, shrubs, and herbs. Differing signatures that alternate in the teeth indicate their diet was seasonal and switched between diverse plants depending on the time of year.

Of course, direct evidence of diet can be found in the contents of an individual's stomach as well as in fossilized dung (coprolites) and vomit. Seeds in the hominin coprolites from Terra Amata (380 Kya) suggest that the camp site was seasonal since the particular seeds were only available during the summer. Unfortunately animals do not normally fossilize with preserved organs. If fossil animals did preserve stomachs, the contents of the stomachs would be direct evidence for diet, or at least for the last supper. The famous bog bodies of Great Britain and Denmark often had full meals preserved in their stomachs.

Food-related diseases can tell volumes about an individual's diet as well. Such is the case of KNM-ER 1808, an adult female H. erectus's partial skeleton from Koobi Fora, Kenya. Her long bones had a covering of woven bone, which is a sign of disease. Bone should be smooth, not bumpy as hers were and the pathology is consistent with modern cases of hypervitaminosis A, a disease caused in modern humans by ingesting an overdose of vitamin A. But, in the early Pleistocene, the disease was likely incurred from eating too many carnivore livers, as carnivore livers are high in vitamin A relative to anything else available on the East African savannah. If a H. erectus was able to overdose on an animal product around 1.5 Mya, it is clear that they were skilled at obtaining meat by then.

Tapeworms

The human tapeworm offers an intriguing angle on the evolution of meat-eating. Domestic animals have long been blamed for giving humans tapeworms (Taenia). So when genetic analyses were performed on tapeworms from a variety of animal hosts, scientists expected to find an origin for the human tapeworm around the time that agriculture first became prevalent around 10,000 years ago. On the contrary however, the results showed that human-specific tapeworms are not descendents of tapeworms living in modern domestic animal hosts. Human tapeworms evolved further back with the shift to meat-eating behavior when humans joined the carnivore guild.

The tapeworm life cycle involves two different hosts. First it lays eggs on the ground that are eaten by a grazer (like an antelope) which is the tapeworm's "intermediate host." The egg forms an embryo inside the muscle of the animal. When a carnivore, or "definitive host," eats the intermediate host it ingests the young tapeworm that grows into an adult tapeworm and leaves the carnivore with the feces. The tapeworm, then, has been transported back to the ground where it will lay eggs and continue the life cycle.

Tapeworm gene sequences are most similar between humans and hyenas and big cats in Africa, so hominins probably picked up tapeworms from eating the intermediate hosts of tapeworms specific to those carnivores of the prehistoric African savannah. Molecular clocks of the sequences point to an origin around 1.71-0.78 Mya, which is during H. erectus times. --

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