See Introduction/aim of research for the background to this work
When considering human diets, food requirements and choices it is essential to look at the evolutionary history of our species. This requires looking back at least the couple of hundred thousand years of homo sapiens time on earth and even the previous hundreds of millions of years since our ancestral divergence from other primates. With the relatively recent changes to diet that agriculture and industrialisation have brought, it is especially valuable to consider what has been eaten for most of our evolutionary history.
In evolutionary terms, species have a choice between concentrating on eating from a relatively wide or a relatively narrow range of foods. There is then a choice as to whether to concentrate on higher or lower quality foods sources. Due to the danger of poisoning, eating many different foods is challenging for a species as it needs to develop strategies for determining what is safe to eat. Mostly, this is accomplished with some kind of learning process and the more foods to learn about, the bigger the brain needs to be to store and process this information. The benefit to the many-foods strategy is that the species is more secure when there are failures in particular food sources. However, the support of a bigger, more complex brain is a heavy burden on an organism, for example a human brain accounts for around 20% of the body’s energy use (Manning 2004). The choice between high and low quality diets offers a trade-off. High quality foods are usually more difficult to obtain and will generally be subject to more competition as a food source. Being easier to digest though, the species can afford a simpler, smaller and more economical digestive system. This allows the animal greater mobility, beneficial when searching for this type of food. Lower quality diets tend to be easier to find (like grass and mature leaves) but require more complex and specialised digestion apparatus (like the cow’s rumen in which cellulose is broken down by bacteria before being available to the cow). The ancestors of humans and today’s primates chose the complex high-quality diet strategy but our direct descendants took this strategy to extraordinary levels.
Since the time hominins devolved from our common ancestor with modern primates around 500 million years ago there has been a massive increase of brain size, intelligence and novel ways to increase food availability and security. This was achieved by, for example, the making and using of tools, development of complex communication abilities, learning to control and use fire and eventually the ability to plant and grow crops and domesticate animals. Meat in the diet of the earliest hominins provided a beneficial high-quality, energy and nutrient-dense supplement to plant foods, which in the wild are rarely nutrient-dense. In environments containing a plethora of poisonous plants threatening the eater with unpleasant to even lethal consequences, animal meat must have been a relatively safe choice when freshly killed.
Despite evolving from mainly vegetarian ancestors and having a digestive system which is more like that of a herbivore, hominins gained some of the benefits available to carnivores by regularly adding meat to their diet (Milton 1999). This may have given them the necessary advantage to further enlarge their brains and increase their intelligence. However, as capturing animals for food is a relatively difficult activity, especially for an animal without the evolutionary capabilities of a specialised carnivore, it is likely that hominins became better hunters after their brains and intelligence grew, allowing more complex use of tools and cooperation in hunting. Therefore, initially animal food that allowed the development of the species may have been the most easily obtained such as invertebrates and shellfish. These also happen to be easily eaten without cooking which was a skill developed much later. There has been speculation that the high levels of long chain poly-unsaturated fatty acids (LCPUFAs) found in fish and shellfish could have been instrumental in the evolution of the hominin larger brain. This is because the brain is mostly made up of the same LCPUFAs, specifically arachidonic acid and docosahexaenoic acid. While these can be synthesised from other lipids such as linoleic acid and α-linolenic acid which are available in other foods, the transformation is less efficient. Therefore a diet abundant in these ideal brain nutrients may have been the factor that allowed the brain to enlarge while the rest of the body remained roughly the same. The East African Rift Valley with its abundance of fresh-water lakes would have given access to this kind of diet of fish and shellfish (Broadhurst et al. 1998).
Kaplan et al (2000) identify four key characteristics that distinguish humans from other species: long life-span; long juvenile dependency period; social support from older group members and male support through food-provisioning. They theorise that these characteristics and our advanced intelligence are “co-evolved responses to a dietary shift toward high-quality, nutrient-dense, and difficult-to-acquire food resources” (Kaplan et al. 2000). Their research goes on to demonstrate the extraordinary level of skill and learning that is required for traditional human hunting strategies. By analysing modern hunter-gatherers, they found that men’s hunting skills did not peak until they reached their 30s, well into adulthood. Due to the considerable nutritional pay-off hunting provides, in most of the studies it was found that adult male hunters were the greatest providers of food energy in their communities. Females concentrated on gathering, plant foods especially, an activity that is easier to achieve while child-rearing.
It is difficult to prove exactly when hominins began to control and use fire but evidence points to its use by around 500,000 years ago, possibly as early as 1.7 million years ago (James 1989). Cooking of foods which developed some time after that was another key factor that allowed our ancestors to advance as they did. Cooking offered unique advantages to the diet and appears to have been an essential part of our evolution. Cooking of food equates to a type of pre-digestion, reducing the energy required to digest food in the body and this may have allowed the human intestinal tract to shorten over time. Cooking made some previously inedible foods edible, considerably reduced some of the dangers from food-based pathogens and was revolutionary for our gastronomic progress, multiplying the sensual aspects of food through different cooking methods. The cooking of meat has an especially transformative action in terms of taste, safety and ease of eating/digesting. It is noticeable that very little meat is eaten without cooking or similar processing in either modern or traditional diets.
Professor Sonia Ragir (2000), writing in the Journal of Evolutionary Anthropology puts the use of fire for food preparation as one of the four key factors in the transformation of our evolutionary ancestors into modern humans along with: the search for and eating of rhizomes and tubers (which are not generally eaten by other primates and can provide significant amounts of nutrition when processed correctly); the use of tools for hunting, trapping and butchering sometimes large animals; and the group-sharing of meat that had been hunted.
As our hominin ancestors spread out around the world, their unique flexibility and ability to survive on different food sources allowed them to thrive in otherwise hostile environments. There is extensive archaeological evidence of their lives from diverse parts of the world. As they travelled further from the equator, and encountered more extreme environments, the importance of meat in the diet became greater due to the seasonal and less extensive nature of wild plant foods.
Studying exactly what our ancestors ate between the emergence of homo sapiens and the historical recording of dietary norms presents a challenge due to the lack of preservation of most food evidence over that time-scale. Some data can be gained from archaeological studies and carbon analysis of human remains. Study of remaining hunter-gatherer communities can also give valuable insights, although very few of these groups have been unaffected by the thousands of years of human agricultural development.
Various studies such as Richards et al (2000) have analysed hominin bones in Europe for the stable isotope (δ13C and δ15N) ratios which can indicate the proportion of animal to plant food in their diet (Richards et al. 2000). These studies suggest that European Neanderthal and later hunter-gather diets were almost completely carnivorous. Further research such as done by Richards (2003) find a sudden shift in British diet at the beginning of the Neolithic period, around 5000 years ago. Again analysis of the stable isotopes in bone remains show an almost complete abandonment of seafood in the diet for land-based foods. This shift would have occurred around the time that agricultural and animal husbandry techniques arrived in the area.
When Cordain et al (2000) analysed the diets of 229 modern hunter-gatherer societies listed in the Ethnographic Atlas they found that “most (73%) of the worldwide hunter-gatherers derived > 50% (56–65%) of their subsistence from animal foods (hunted and fished), whereas only 13.5% of worldwide hunter-gatherers derived > 50% (56–65%) of their subsistence from gathered plant foods” The importance of animal foods in this analysis generally increased with greater distance from equatorial regions. These studies show how important animal-derived foods have been in our evolutionary diet especially in the higher latitudes.
With this high dependence on meat and long history of consumption in the evolutionary diet, it is interesting to note that there been virtually no evolutionary adaptation of our bodies to this diet (Milton 2000). Our digestive system remains almost the same as our completely herbivorous ancestors and we see none of the digestive adaptations that carnivorous animals have. While our digestive system is not best suited to processing lots of meat, it copes well enough that there has not been evolutionary pressure for adaptation. Similar change of diet without significant adaptation can be seen in reverse in some animals such as the giant panda which has a carnivorous evolutionary history but has evolved to eat a vegetarian diet.
High meat consumption can be difficult to sustain for humans as protein is not a practical primary source of energy for the human body due to the high energetic cost of metabolising it. Therefore, a diet high in lean animal meat can be inadequate to provide full nutrition. To get around this, animals with higher fat content need to be eaten or plant foods can provide more of the dietary energy. Larger herbivores typically have greater stores of fat and overall food energy making them especially valuable as a food source. A mammoth, for instance could have provided the food energy of a hundred large deer (Smil 2003) and supplied a gigantic supply of food energy compared to that expended in obtaining it. This preference for large herbivores such as the mammoth may be implicated in the extinction of these species, as seen for example in the North American mega-faunal extinction at the end of the Pleistocene era (Smil 2003).
Due to the protein metabolisation problem, plant foods have always been a valued part of the diet when available. Plant carbohydrates are also probably the most efficient source for convertion to the glucose that our large brains require as energy. In areas where prey was predominantly lean and large herbivores rare or absent, plant food had to provide significant proportions of the total dietary energy. Large starchy roots have played an important part in many cultures due to the concentration of food energy they can provide. Often these roots need processing such as grinding, leaching or cooking to remove inedible substances. Our reliance on diet for other essential nutrients such as vitamin C has meant that varied diets encompassing available edible fruits, nuts, seeds, tubers, leaves etc. helped ensure good nutritional health. Diets with a mix of different foods and food groups have helped prevent boredom and nutritional deficiency which is perhaps part of the reason our bodies reward us with pleasure when we eat diverse, quality foods.
When agricultural practices were developed to the east of the Mediterranean, the ability to have a greater control over the supply of edible plants must have been especially valuable in an area lacking the botanical abundance of tropical areas. The other field of agriculture, livestock farming with its domestication of animals provided an easier and more reliable way of supplying meat in the diet and added the possibility of a whole new food source through the collection of animal milk.
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