There are several reasons why we can’t live on plants alone. Here I’m going to focus on a prime and compelling reason – the size of our guts. The chimpanzee is our closest relative in the mammalian world and chimps eat principally plants. As can be seen in the above photo, a chimps guts are naturally more extended than ours. The reason why lies in both the quantity of plants the chimp has to eat everyday and in the fact that plants are hard for us mammals to digest, given that a principal component of plants is cellulose. We do need cellulose in our diet – it gives us the fibre our guts need. And, in our long evolution,there was a time when we lived principally on plants, as explained in the interesting book Catching Fire. How cooking made us human by Richard Wrangham.
In the second chapter he says our last ancestor that was principally a plant eater was the australopithecine and in support of this hypothesis he says
fossils show that australopithecines had broad hips and a rib cage that was flared outward toward the waist. Both features indicate the presence of capacious guts, held by the rib cage and supported by the pelvis.
So it seems that australopithecines had the digestive volume necessary to cope with a plant based diet, as do all of the others in the ape family. We have evolved considerably since those days, and now not only do we have a smaller stomach than the other great apes, but also a smaller colon. In our colon, bacteria ferment the plant fibre, producing fatty acids that we go on to use for energy. This fact amuses me in these fat phobic days; because of the way we digest our fibre, even lettuce ‘contains’ fat. That aside, because our colon is relatively small, and our digestive system relatively fast, we cannot retain enough fibre in there to give us sufficient energy to fuel us through our day.
The great apes have different ways of maximising their nutrition from the plants they eat. All involve long chewing times. But also, for instance, gorillas eat their own poo – the second time through enables them to obtain maximum nourishment from their diet. Now, to be fair, the above book is about how discovering cooking made us the humans we are, and cooking makes both meat and vegetables more digestible, and so even vegetarians cope better if they eat cooked vegetables as opposed to only raw ones. But we are still left with the problems of the quantity of vegetables we have to eat if we eat purely vegetables – and nuts – even if they are cooked.
Pure protein has the same number of calories as pure carbohydrate. And the purest protein we can eat is white fish like cod, since that contains very little fat. Oily fish and meat all contain fat in various amounts, and this means that per mouthful of food we get more calories from meat and fish than we do from vegetables. The whole low calorie diet idea is based on the fact that fat contains more calories per ounce or gram than either pure protein or pure carbohydrate does. So to fill our small tummies and give us enough energy, if we were to live on pre-farming plants alone – that means no grains or potatoes – we will have to eat a huge amount of food every day – and spend an awful lot of our waking hours just eating.
If we choose to avoid animal protein, we do have to eat plenty of grains or other starches to fill us up within the small capacity of our guts. We have developed foods such as grains, maize and potatoes out of ancient wild plants over the past 10,000 years and these ‘modern’ foods present us humans with digestive difficulties because the plants that we ate during our long evolution to becoming homo sapiens were more like those that the great apes eat now. And modern foods like grains and potatoes are bred from original plants that we either could not eat because they were poisonous or they presented us with real digestive difficulties.
For example, all grains are developed from grasses. And animals that have evolved to eat grasses all have a large gut. For example, horses, cows, goats, sheep or hippopotamuses. This gives them the digestive capability of breaking down plants very rich in cellulose.
I realise that vegetarians are passionate about their cause. But until we evolve a different digestive system, we are unfortunately stuck with the guts we have, and those guts need animal or fish based protein, plenty of vegetables and/or fat for us to thrive. Our guts, under the increasing amounts of overlying blubber, are simply too small and too fast moving to cope with huge amounts of vegetables for things to be otherwise.