Every animal in the Natural World is bound to live according to its own nature. This is dictated by 550 million years of evolution since the emergence of complex organisms including the very first ancestor of all animals on the planet today. To live according to its own nature means to live in its natural habitat, to eat what it has evolved to eat, to grow, mature, reproduce, age and die according to the way in which every detail of every aspect of its life has been defined and refined by the environment and living conditions of its ancestors over hundreds, thousands, millions of years.

As conditions change, natural selection ensures adaptation. On this question, there are only two possibilities: adapt and evolve or perish and disappear from existence. And this is not a matter of choice or of willingness: it is a biological adaptation that takes place on its own without the conscious intervention of the organism subject to the process.

In the Natural World, through the entire history of life on Earth, it has been thus. It has been thus for the very first living microorganisms, for the very first photosynthetic cyanobacteria, for the first eukaryotes and the first multicellular organisms, for all algae, fungi and plants, and for all animals. It has also been true for humans, for the human animal, the human chordate, the human mammal, the human primate. True, until very recently.

This is what I understood on that day when I watched, shocked and amazed, that female sheep swallow up her own placental tissues dripping with blood almost immediately after she had given birth to that helpless lamb that was lying unmoving in the shade of a large oak tree.

The earliest dates for which we have evidence that people settled in relatively large settlements and sustained themselves by cultivating cereal grains and tending to herds of domesticated animals is about 10 to 12 thousand years ago. Before that, all human populations throughout Africa, Asia and Europe were seasonal nomads that followed the animals on which they relied for food, clothing and tools.

An exception to this might be the south Pacific (Melanesia and Polynesia), lands that were settled some 50-60 thousand years ago, but that, unfortunately for those settlers, had little food resources: no large seeded grasses, no easily domesticable animals, and very few fruit-bearing trees or edible wild vegetables. They found a starchy tuber that could be eaten after some processing to remove the toxins it contains. So, this is what they survived on, and this is primarily what they still survive on today.

Needless to say that even with the intake of minimal amounts of protein from some animal sources, and today they have domesticated pigs, these people–all the different clans and tribes that evolved in that part of the world–have always been extremely restricted in their evolution by the need to devote so much time to their most essential requirement for surviving longer than a few weeks or months. Moreover, can anyone be surprised by the fact that this is the part of the world where cannibalism has always been practiced and still is practiced to this day?

If you were starved of protein, not just for a few days, a few months or even a few years, but for your entire lifetime, generation after generation, of course you would eat your dead rivals and enemies. There’s no question or doubt about it. It would be a waste not to. Not only that, but you would also certainly go out of your way to find and make rivals and enemies in order to maximise your meat base. No question or doubt about that either. Naturally, this is what we see there: hundreds of small tribes sharing the scarce natural resources in this inhospitable land by intense rivalry and continuous warring. You would do the same. I know I would. You can be sure of that.

The fundamental difference between humans and all other animals is that they are bound–forced by natural selection–to eat only what they have evolved eating. Humans are not only able to disregard this biological framework to which we really are in fact bound as all other animals are, but to actually eat and live in ways that are completely contrary to what is prescribed to them, to us, by our evolutionary history and by natural selection. This is true whether the constraints are imposed upon us by our environment, climate, geography and available resources, or defined by the beliefs that shape our worldview.

The former is the dominant in most of the world, but the latter is definitely pervasive in industrialised countries where, for practical purposes, there are no constraints on the availability of foods at any time during the year or any moment during the course of one’s whole life, really. And this is what we are addressing here: not the scarcity of food and the restrictions on dietary regimens in those struggling to get enough food for themselves and their dependents, but the effects on our health of restrictions we place on our own diet based on beliefs.

This fundamental difference is well illustrated by the fact that large carnivore like jaguars, panthers, tigers and lions eat meat exclusively. They never think about it, they don’t consider what they feel like having for supper, they don’t sometimes go grazing a little grass or other plants here and there: they always only eat meat, and have been for millions of years. Consequently, these large felines have only sharp teeth without any flat ones for grinding fibres, they have a shorter and simpler digestive tract that measures about 7 metres (compared to about 10 metres for humans), their proportionally larger stomachs secrete such strong concentrations of hydrochloric acid that the pH inside it after meals drops to values around 1 (compared to  around 2-3 for humans), the lowest (most acidic) on the logarithmic pH scale, and their livers have much greater capacity (about 10 times the one we have) to concentrate uric acid out of the bloodstream and excrete it in the urine.

Cows, bisons, buffalos; sheep, goats, lamas, alpacas; horses, ponies, donkeys, mules and so many other animals eat a diet that consists of basically only grass and grass seeds, have completely different adaptations: they have large thickly enamelled flat teeth for grinding over and over again, and for hours on end throughout the day, those tough cellulose structures of the plant that lock in the nutrition they need to extract, they have extra long digestive systems, some of them with several stomach-like sacs along the way, that actually allows the chewed up grasses to travel back and forth a number of times to maximise the extraction of nutrients, and they have a purely alkaline digestive system, secreting no hydrochloric acid at all, simply because this is what is most suitable and necessary for the optimal digestion and absorption of the sugars, minerals and vitamins present in the grass they live on.

These are just a few examples of evolutionary adaptations to a diet of only meat seen in obligate carnivores like large felines or in herbivore grazers, but they are most appropriate because they pertain to the digestive system on which is built every other system and on which our health and survival depends most directly.

Like feline carnivores, herbivores do not think about what they will eat for their next meal, what they feel like having for breakfast or for lunch. They always eat the same things, grasses and other little leafy plants, and in the late summer, fall and winter, the seeds of the grasses and other plants that have dried and gone to seed. How much of each depends on where they live and how the climate is. It never depends on their thoughts and feeling about what they should eat. And if we were to offer the lion or the tiger something other than fresh meat, a nice big bowl of freshly cut grass or grass seeds like oat kernels, for example, they wouldn’t touch it because for them, it is not food. If we were to offer a cow or a horse a big juicy steak from a gazelle or antilope they would in exactly the same way not even look at it or sniff it because for them, this is not food.

All animals eat only the foods that they have evolved to eat in order to live healthy for the right amount of time to allow them to reproduce and raise their offspring to the point where the offspring can themselves do the same for the next generation. For millions of years this process takes place and refines every detail of the unique characteristics of their bodies, of their physiologies and their biochemistries, of their physical aptitudes and their psychological makeup. Animals do not comprehend this: they know it in their natures, they know it in their instincts, they know it in their very bones.

We, humans, have the ability to comprehend this, at least when it is taught or explained to us, but because we think, we analyse, we believe, we rationalise, we justify and we convince ourselves and others of basically anything we want using more or less clever logic, more or less sound analyses and rationalisations, and, in the end, more or less convincing arguments and justifications. And we excel at this. We excel at it remarkably.

What comes of it? We end up eating and drinking whatever we believe we can or whatever we believe we should, whatever the reason or lack of reason. We eat bread and jam every morning because this is what we’ve always done, because this is what our parents always did, because this is what everyone around us has always done, and because it tastes so good. We eat at McDonald’s, Burger King, Taco Bell or Pizza Hut at lunch because it’s fast, convenient, and also because it tastes so damn good. We feed ourselves and our kids pasta with jarred tomato sauce for supper because it’s the easiest meal we can make, everyone loves it, and it leaves us with a feeling of being full and satisfied. We eat only plant foods. We eat only animal foods. We eat only raw foods. We eat only brown rice. We eat only salad. We eat no fat. We eat mostly fat. We eat no carbs. We eat mostly carbs. We eat in this way or in that fashion. We eat in all sorts of ways for all sorts of reasons and we somehow never ask ourselves what has this body evolved to eat: what we should eat.

In this respect, the situation between humans and all other animals is, at this stage, radically different. So different it couldn’t be more different: animals instinctively eat only what they have evolved eating and therefore evolved to eat; we eat only what we feel like eating or what we think or believe we should. We have lost our food instincts and overrun them with beliefs. We do not care to ask ourselves what our evolutionary history, that of our species as well as that of our personal ancestry, tells us about what we have evolved eating, and we trust the word of food “scientists” that tell us preposterous things such as eating egg yolks and animal fats causes heart disease, or that eating large sweet fruits and whole grains is good for us, or that we should drink milk to have strong bones, or that a big brain like ours needs lots of sugar. All preposterous. All mistaken. All unfounded. But we believe. And we listen. For decades on end before the weight of evidence begins to turn the light around. All the while getting fatter and sicker eating inappropriately for our constitution.

There are at least two ways by which we can approach the problem of trying to figure out what our long past ancestors would have eaten and preferred eating through the millennia given the constraints imposed upon them by the environment and climate: we can consider the archaeological evidence we have gathered, and combine that with as much as we have learned in the realm of evolutionary biology and physiology, trying to trace back the evolution of the different systems of the body, in particular the digestive system, coupled with the evolution of our brain; the other approach is to look at the energetics of survival and work our way through a series of deductions based on what we know and what we can learn from this process itself.

One of the important differences between our closest cousin, the modern chimpanzee, and ourselves is that a chimp eats mostly raw, fibrous plant foods (2/3 stems and leaves and 1/3 small fibrous fruit), and spends many hours each day chewing through these in order to feed itself. As a result, very strong jaws and thickly enamelled teeth together with a long digestive tract through which all these fibrous and nutrient-poor foods must pass as slowly as possible to extract as much as possible out of them. Naturally, this requires a specific kind, and well-developed intestinal flora. As is also natural to expect, and as is in fact the case, the intestinal flora of microorganisms upon which animals depend for proper digestion, and ultimately for survival, develops and adapts to the foods eaten that make their way through the intestines, on the long term, of course, but also on the short term.

What we see in the fossil record is that, following the Miocene that lasted for about 18 million years from 23 to 5 million years ago and that was dubbed the golden age of the apes because they flourished all over the world, there were, in different parts of the world, between 13 and 9 million years ago, several genera of hominoids (something between apes and hominids), and that the earliest members of our group lived at the end of the Miocene and beginning of the Pliocene between 7 and 4.5 million years ago. Molecular studies on DNA also suggest from a completely independent analysis (rate of DNA mutations) that our line must have branched off from the common ancestor we share with chimpanzees around 6-7 million years ago. So it is pretty clear that this is the time around which this separation of lineages must have occurred.

There are two lines of structural changes used to evaluate and follow the evolution we are trying to trace from that oldest ancestor to the modern forms in our genus Homo: The first looks at changes that, in the structure of the skeleton, especially in the hips, legs and feet, but also in the shoulders, arms and hands, betray evidence for an upright walking posture and manual dexterity as opposed to structures consistent with knuckle walking and tree climbing; the second looks at changes in the upper spine, skull, jaws and teeth that also indicate upright posture (skull) and less ape-like features including smaller canines, smaller top and brow ridges, and a flatter and taller face and forehead. Both lines of evolutionary changes lead to the following scenario as the most likely.

Currently, the best contenders for the title of our last common ancestor with the chimp are Sahelanthropus tchadensis (dated at 6-7 million years), Orrorin tugenensis (dated at 6 million years), and Ardipithecus (kaddaba at 5.8-5.2 and ramidus at 4.5-4.3 millions years). All of these fossil species, no matter how little evidence there actually is in some cases, show strong evidence for evolutionary adaptations to upright walking based on the shape of the hip bone or femur or feet or skull. Teeth and skulls also show smaller canines and larger and thicker molars both of which indicate that they ate tougher more fibrous foods like leaves, stems and roots.

As is very clearly illustrated in the figure below, from the oldest australopithecines (africanus), the trend towards larger, flatter and even more thickly enamelled teeth, wider and stronger jaw bones, and thicker skulls with powerful top ridges and sideways flaring cheekbones all constructed to sustain the pressure generated while chewing, continues to later species and peaks in Paranthropus Boisei, believed to be the last of the australopithecines, and probably the most robust of the toughest fibre-chewers ever. But while the trend towards narrower hips, longer femurs, thicker heel bones and higher foot arches, all needed to increase mechanical efficiency in upright locomotion, continues to be evident in the later species, we see a reversal in the trend towards better fibre-chewers, in the shrinking of teeth and jaws, the disappearance of the top ridge and flaring cheekbones, and the decrease in brow ridge in the fossils of Homo habilis and in the very well preserved 1.6 million year old Turkana or Nariokotome Boy, the best specimen we have of our ancestral species Homo ergaster.

This is most naturally and sensibly interpreted as the adaptation from a chimp-like diet based primarily on fruit and other plant foods with the rare feasting on animal flesh from  group hunts of thought to be important mostly in establishing a clear social order in their hierarchical structure, in the oldest australopiths; to a change in diet towards tougher and more fibrous and naturally less desirable leaves and stems, fallback foods, as they are called, that were available to them after migration out of the depths of the forest and into the dry savannah; and to eventually the shift towards more fibre-less animal foods, rich in calories from fats and protein, only a very small amount of which was necessary for survival in comparison to the amount of fibrous and nutrient-poor plant foods.

The implications are clear and also obvious: 1) More fibrous nutritionally-poor plant foods led to adaptations for chewing them but also for processing them internally and must have been associated with a longer much more herbivore-like digestive tract and system. 2) More nutritionally-rich fibre-less animal foods led to the loss of the need for large teeth, powerful jaws and thick skulls, and also must have led to a shrinking of the digestive tract and evolution of digestive adaptations needed to process animal protein and fat, which would include the need for hydrochloric acid in the the stomach to breakdown protein, and bile from the liver to emulsify fats, as well as a new bacterial flora which would have also been entirely different depending on the diet. And 3) the more animal foods were eaten, the more the brain grew in volume, both in absolute terms and relative to body size.

These are the most important conclusions from this exploration of our earliest evolutionary history as a species, which also very closely tie-in with our reflections about what we choose to eat and the reasons we invoke or construct in justifying these choices to ourselves and others, because it shows us as plainly and straight-forwardly as is possible to imagine, that in order to live healthy and thrive throughout our life over its natural lifespan, we are bound to eat what our ancestors have evolved eating in exactly the same way as all other animals are, and that this is dictated by our anatomy, physiology and biochemistry, independently of what we think and of what we believe.

In the next part, we will explore the question of energetics and food selection, what a hominid would naturally do–what you and what I would do–when faced with the need to seek out food for its own survival, and come back to my own story in more practical terms. And if you are interested in reading more about the topics we touched upon in this article, I recommend Ian Tattersall’s Masters of the Planet, Daniel Lieberman’s The Story of the Human Body, Jared Diamond’s The Third Chimpanzee, and Yuval Noah Harari’s Sapiens: A Brief History of Human Kind. Darwin’s On the Origin of Species is truly remarkable in scope, in detail, in depth and in foresight. Even if it doesn’t relate specifically to the details of the evolution of our genus Homo, it is the foundation of the broadest context in which we as intelligent and literate being understand evolution of all species everywhere since the emergence of life on this planet.

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