Debate: On the place of mankind in evolution, ethics and nutrition

A shepherd with his flock in the Netherlands. Peter Nicolai

Having reached an advanced level of communication through thinking, humans have been trying for a long time to differentiate themselves from the rest of the biological world. Until the end of the 19th century it was more or less thought that humanity was on top of the evolutionary tree.

Archaic phylogenetic tree. ENS Lyon

This anthropocentric vision describing humans as the ultimate step of evolution goes together with moral and religious developments described in several religions that propose that humanity was chosen by God or indicating that God has created us in his own image or else that God can occasionally present Himself under a human appearance. The last of these is not unique to monotheist religions as in polytheist Greek or Latin religions, where gods can easily acquire a human appearance, in particular to seduce attractive young women. Beyond the anecdote, these dated phylogenetic trees and those much more ancestral beliefs reflect the propensity of humankind to consider itself as superior to other biological organisms. Or is it to reassure ourselves?

Superior, or insecure?

Why on earth do we feel that need to prove that we are superior to other species? – doesn’t it hide a profound insecurity? Are we really so superior to other biological species, and if not, why did we invent such a fable?

An interesting level of answer to this question concerns animal nutrition, including that of humans. The whole animal chain depends on molecules containing carbon, nitrogen and sulphur, which are present in macromolecules built by other biological organisms capable of feeding on inorganic forms as carbon dioxide (CO2), nitrate (NO3-) and sulphate (SO42-). The most important macromolecules from a catalytic point of view in the cell are the proteins made by combining a repertoire of 20 amino acids. However, animals can synthesize only 11 of them. This is only one example among others, and we could also mention the needs for vitamins that are strikingly different between animal and plant species. It is clear that the whole animal chain is totally dependent on photosynthetic organisms for its survival and that without plants, we would die out very quickly.

A significant proportion of animal species (carnivores, omnivores) can or must feed on other animals. However, ruminants that are preyed upon by carnivores survive only by consuming plants. It is likely that during millennia the mixed nature of the human diet posed no ethical problems whatsoever. However, moral limits have rapidly emerged – for example, cannibalism is a strong taboo in modern societies. Likewise, the use of animal food for humans has certainly posed a number of ethical/moral problems, in particular because it’s much easier for a human to identify with other animal species, in particular mammals – broadly speaking, we have all four limbs, two eyes, one nose, etc. That’s not the case with plant species, which on the surface look extremely different and appear more like strangers to us.

One solution to this dilemma is to postulate that human beings are superior to the rest of the creation. Actually, one will note that the primitive phylogenetic tree shown above places safely humankind on top of the hierarchy. To further solve this moral riddle, it has been proposed that not only is man on top of the creation and at the image of God but also he is the only being with the capacity of communicating and reasoning. As a consequence, morally we have the right to prey upon “inferior” species.

Genetics upsets the apple cart

Of course, this convenient piece of reasoning has been shattered in the 20th century through progress made in genetics, genomics and behavioural sciences. Genetics and genomics have allowed us to refine the phylogenetic trees, their modern aspect being conform to the scheme below. In this modern version, Homo sapiens belongs to Opisthokontes and it is very clear that this branch is in no way more special than the other ones. On top of that, if we zoom on that branch, we’re not on the top of it but rather share a position at the end of a twig with all the other species. (Incidentally the human genome is more than 95% identical to the one of the chimpanzee, our closest relative.)

Thus if one want to defend the consumption of meat on moral grounds, one has to argue that our behaviour is justified because we’re the only species capable of higher reasoning. This certitude has also evaporated following the work of behavioural scientists who have shown that many other animal species possess elaborate communications skills and intelligence. An excellent recent article in The Conversation details a thorough analysis of this topic and the reader is strongly referred to it. PROVIDE A LINK

ENS Lyon, CC BY

It is more and more obvious that all these arguments weigh heavily on how human societies deal with nutrition issues, and there is a notable trend in Western societies moving toward a more plant-based diet.

Still, the moral acceptability of the consumption of plant products is once again based on the concept that plants would be inferior organisms compared to animals, but is that really biologically true? In reality the answer is extremely cruel for the animal world overall and mankind in particular. On a genomic standpoint, a key figure is the number of genes programmed by a given genome. We know now that the human genome codes for roughly 23,000 genes, yet the first plant sequenced, Arabidopsis thaliana, a weed of no agronomical interest and with a very stunt stature, has 27,000 genes. Amazingly, a tree like poplar contains nearly 40,000 genes, nearly twice the content of the human genome. This is a first humiliation, but it does not stop there.

The genes of eukaryotic terrestrial plants are incredibly close in their sequence and organisation to those of their animal homologues, and the proteins derived from these genes are also extremely similar. Plants are also autotrophs for carbon nitrogen and sulphur and they exhibit a growth plasticity that cannot be matched by animal systems, explaining in part the increase in gene number. These are only two examples illustrating the superiority of plants to animals when it comes to metabolic and functional aspects.

Speaking of plants

Some might say that, even if this is all true, plants have no sensitivity and do not communicate. This also is wrong: Plants do in fact communicate with one another as well as with their environment. For example, when a plant is attacked by a pathogen (fungi, viruses, herbivores) it releases volatile compounds (gases such as terpenes or ethylene) that signal to the plant next door that an attack is underway and allows them to turn on their defence mechanisms. Another example is that plants emit compounds that enable them to recruit pollinators (bees, etc.) needed for their reproduction. We are certainly only in the infancy of deciphering the communication of plants with other biological species.

These biological considerations lead to the following concept: although plants appear extremely different from us, in reality they are much closer than what we think and they are in no way inferior beings. It follows that when you feed exclusively on plants, you kill life as much as you do by using animal food. This is actually recognized in variants of Hindi philosophy that recommend their followers to eat only parts of the plants that allow it to not die in the process, such as fruits and grains. Fortunately, plants have an excellent the capacity to regenerate themselves…

From a biological point of view, it is neither more ethical nor more moral to feed on plants than on animals – in both cases we have to destroy lives to construct our own. Sadly, in terms of nutrition the law that often prevails is that the strongest wins, but that is also the way we survive and for that we have to pay a moral price. One superiority of mankind is that we are possibly the only ones able to grasp that, but it’s also our major weakness.