source of valuable nutrients. The earliest primates, being small, most probably had a predominately insectivorous diet. Small mammals lose body heat more quickly than larger creatures, so they need a mainly carnivorous diet in order to maintain the higher metabolic rate required to compensate for this heat loss. Plant foods generally take longer to digest. Thus a mainly plant-based diet was only possible for primates who evolved to a size that limited their heat loss and thus reduced their metabolic rate.
Initially, while continuing to obtain most of the protein they needed from insects, some primate species increasingly derived many of their energy requirements from plant resources such as nectar, gum, and fruit. Seeds and nuts provided an alternative source of proteins and lipids; eventually insects would play a less important role in the primates' diet as they exploited the plant foods available in the forest.
One peculiar aspect of the primate diet that was most probably acquired during this early period of evolution is the need to regularly include a source of vitamin C in what is eaten. Vitamin C is not a necessary component of the diet for most creatures, including some other mammals. It is probably safe to assume that the primates lost the ability to synthesize vitamin C because their diet was one that always included plenty of plants and fruit, which ensured the inclusion of vitamin C in most of what was being eaten. Color vision, a characteristic shared with the birds, probably also evolved during this period to enable the primates to locate and discriminate between poisonous and edible fruit (Crowe 2000, 18).
We know from today's primates that diet is closely linked with body size (Kay and Covert 1984), as explained previously. Small animals, because of their immediate energy demands, cannot tolerate the delay between eating the leaves and deriving energy from them. Hence smaller primates whose diet does include large quantities of leaves also have to eat fruit to obtain energy, as leaves contain fewer sugars that can be easily assimilated. Some primatologists have concluded that any species that came to rely mainly upon leaves for its survival must at some time have gone through an intermediate frugivore stage during the course of its evolution.
The largest of the living primates, the gorilla, has a largely folivorous diet. But leaves are a low-grade food. Depending on the species of plant, bacterial fermentation has to occur in either the stomach or in the intestines of folivorous primates in order to process the leaves before any nutritive value can be extracted. Therefore, the evolutionary increase in body weight seen in foli-vores was a necessary adaptation to accommodate modifications in the digestive tract. This adaptation is effectively a cul-de-sac as any radical changes in habitat resources can lead to the extinction of a species. Even when favored fruit is seasonally available the gorilla must continue to consume leaves, simply in order to maintain the gut micro-organisms it normally requires for digesting the plants that form the bulk of its diet during the rest of the year (Tutin 1992). This may well have been a factor that, much later in hominoid evolution, contributed to the eventual demise of the robust australopithecine (the upright ape preceding and probably ancestral to humans) during the early Pleistocene period (Crowe 2000, 18), as they were believed to be dependent upon similar resources (Foley 1989).
In contrast, frugivory (fruit eating) gives primates a flexibility of diet that allows them to avoid the specialization of either eating predominantly meat or predominantly leaves that is normally characteristic of most other mammals. It also enabled different primate species to adapt in varying degrees to their habitat, and to supplement their basic diet of fruit with insects, small mammals, or leaves and other similar vegetable matter.
One notable disadvantage of eating nothing but plant foods is that no single plant can provide all the amino acids required by the body as the building blocks to produce animal protein. A wide variety of plants must always be included in the diet to ensure that all the essential amino acids are present. This is reflected in the behavior of chimpanzees, whose dietary needs often lead them to engage in opportunistic hunting activities (Lawick and Goodall 1971, 182) to supplement their diet by eating meat. This is because the meat of all animals, unlike plant material, contains all the amino acids any other creature needs to synthesis their own body tissue. The diet of chimpanzees as a result is even more diverse than that of humans and this severely limits the habitats in which they can survive. In fact it is one of the main reasons why they are so endangered as a species. Our own ancestors, the slender australopithecines, on the other hand, probably owed their survival, after the demise of their robust cousins, to the fact that they were scavengers and possibly opportunistic hunters of small animals while inhabiting the fringes of the African savanna (Foley 1989).
The underlying factor here is that an exclusively, or predominantly, vegetarian diet can place a huge burden on animals whose habitat and particular digestive system limit the edible plant foods available to them. Climate and seasonality can both compound the problem. Once the early primates migrated to more temperate climes, suitable plants for primates' diets were both in short supply and often widely dispersed—this was especially so at certain times of the year.
Plants contain a wide range of structural tissues, such as cellulose, and contain chemical compounds that ward off predators. As a result, many plant tissues are inedible—and sometimes even poisonous—in their raw state. Nevertheless plants are an essential part of our diet and an important source of energy as well as of nutrients, vitamins, and essential trace elements. The effective exploitation of these diverse resources is therefore essential to our survival.
What made human survival possible in many regions was acquiring the use of fire. The control of fire was possibly first achieved by Homo erectus, the direct descendants of the australopithecines, maybe as long ago as two million years ago, while they were still confined to the African continent. Fire may not have originally been used to cook food but employed to keep dangerous animals at bay and to keep warm. The effects of fire on animal flesh and plants must have been observed in the aftermath of the fires that often swept across the savanna after lightening strikes during the routine thunderstorms.
When considering the exploitation of food plants alone, the control of fire was absolutely pivotal to our success as a species. Many otherwise inedible plants are made more palatable and more nutritious, and rendered free of toxins, by cooking. This means that, once our ancestors began to employ fire to cook their food, many plants that would previously have been inedible could be included in the diet; this vastly increased the potential resource base (Hillman 1999, personal communication). Cooking also helps to preserve most foods.
There is another important side effect of cooking; the process of cooking roots—and some other parts of plants—has the effect of bursting the cells, thereby releasing the nutrients stored by the plant to aid its growth when spring arrives. Therefore the advent of cooking not only made more plant resources available as food, but the nutritional value of those plants was also increased. Mastication of cooked material was also easier than that of the raw resource and this benefited the youngest and eldest alike and particularly those without a full head of teeth. Well-cooked vegetable matter can also be used as a weaning food. All these things must have aided the survival rate among those hominid populations possessing fire. Wrangham et al. (1999) has also suggested that access to additional nutrients, from root foods especially, could have helped fuel the evolutionary development of a larger hominid brain (Wandsnider 1997; McKie 2000, 110). Archaeological evidence for use of fire by Homo erectus is still controversial, but appears well established from at least 700,000 years ago.
Whilst relying heavily on scavenging, Homo erectus were also foragers, as were all humans until the advent of farming. Plants are particularly important as a source of carbohydrate and to a lesser extent fats in primate diets. This is especially so when the animals being taken in hunting are suffering from nutritional stress and themselves have depleted fat reserves (Speth 1990). On the African savanna this would have been a familiar scenario nearly every year during the dry season among the herbivores being hunted or scavenged, when arid conditions adversely affected the vegetation.
The importance of fats and carbohydrates in the human diet can best be explained by relating what happens when none are available. Proteins cannot be properly absorbed by the human body without the regular consumption of either fats or carbohydrates, which are needed to aid the metabolization of protein. When both of these nutrients are missing from the diet individuals may begin to show signs of protein toxemia. There have been extreme instances of people who have had nothing but protein-rich foods to eat over a period of several months becoming so disoriented that they seem to be suffering from a form of dementia; and yet once fats or carbohydrates are reintro-duced into their diet they make a rapid and complete recovery within a matter of days if not hours. In the longer term death can result (Speth 1990). It is this kind of problem that our hominid ancestors would have encountered first as scavengers and then later as hunters on the African savanna. It is a problem that some hunter-gatherers still experience today.
The range of nutrients that can be obtained from the foods available is obviously crucial to survival. At times people have to go to extraordinary lengths when processing their food to make up for the deficits in particular food groups. Sometimes, though, there seems to be little or no ergonomic advantage, with far more energy being used in the processing procedure than the amount of energy eventually gained from the food. But the importance of these processing procedures can sometimes be better explained by the need to maintain a balanced diet. This is because, as has been illustrated with reference to fats and carbohydrates, if one particular food group is under-represented or, worse, missing entirely, the results can be catastrophic.
One of the main dietary constraints in certain parts of the South American rain forests, as in other similar habitats, is a scarcity of food plants that can provide sufficient carbohydrates for human foragers. Katherine Milton has suggested that a number of indigenous species of roots from families such as Araceae and Marantaceae (see Roots and Tubers) could have been important sources of carbohydrate in the past, before the introduction of cultigens such as manioc, as well as the nuts of the babacu palm, which are available throughout the year, as are the seeds of the banana brava plant, Phenako-spermum guyannense. Various species of wild figs, which are highly nutritious, might have also made an important contribution to the diet in this respect (Milton 1992). Nuts like cashews and Brazil nuts would also have been available as a valuable addition to the diet in some areas. Depending on the region, fruit trees native to the American tropics, such as Pourouma cecropiifolia (uvilla), the pawpaw Carica papaya, and the avocado pear Persea americana, which has a particularly high content of both protein and fat, would have provided further useful food resources. In addition, some vines, such as the ceriman, Monstera deliciosa, also produced fruit.
Many wild fruits in this kind of habitat, on the other hand, are very small and require a great deal of time and effort to gather enough to meet dietary needs. The amount of energy gained from the food collected may be little more than the energy used to acquire the food. If foraging has been unduly prolonged or done too far afield, the problem is compounded. Situations such as this may occur periodically if trees previously relied upon for fruit are found to be barren, when, for example, weather has been unseasonable, as when rain is experienced during the dry season.
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