The Essence of Humanity

Are we just another primate - The Third Chimpanzee according to writers like Jared Diamond(1) - or is there something special about us that distinguishes man from every other living creature, as philosophers - even 'Darwinians' like Dan Dennett(2) - believe? No one doubts that over the last 10,000 years or so, humans alone have been able to change their life styles out of all recognition: concrete cities and the internet, heart transplants and space flights. The question is, how? Is the difference between man and chimpanzee merely quantitative - a heightened ability to think and reason, for example - or do we also possess some distinct quality that makes us unique? I favour the former view, but do not believe that it is simply a matter of cerebral function (Homo sapiens, 'thinking' or 'wise' man).

Take the second point first. Chimpanzees and dolphins can be taught to work out quite complicated problems that undoubtedly require the powers of thought and reason (Fig 1). Indeed, there is considerable overlap between the mental agility of a bright gorilla and a backward child. I consider that it is not a single feature, but a fortuitous combination of several, that is the key to human behaviour. What might those attributes be?

The first is the mobile thumb, that permits humans to grasp objects with the 'precision grip', in which an article is held between thumb and the fingers, whereas other primates can only use the 'power grip', in which something is clasped between clenched fingers alone. The precision grip allowed early species of Homo to start manufacturing primitive tools and enables us tie our shoelaces. Without the mobile thumb there would be no Bayeux Tapestry and no Nigel Kennedy playing Vivaldi.

The second is a more elaborate vocal cord, that lets humans utter a variety of sounds denied to an ape. You will not hear the sounds of an Ella Fitzgerald or a Luciano Pavarotti among the chimpanzees of Gombe. Yet a human baby has a vocal cord similar to that of an ape, high up within the larynx. An interesting consequence is that babies - like chimpanzees - can swallow and breathe at the same time. You and I cannot.

The third is a larger number of cortical neurons (nerve cells in the top layer of the brain) in man than in ape - three times as many in a modern human as in a chimpanzee. It is these neurons that underly the thoughts and feelings of our mind.

None of these attributes alone accounts for the difference between man and chimp, but in combination they endow us with the essential features of humanity. I am reminded of a remark Peter Medawar made in regard to immune function: `One individual differs from all others not because he has unique endowments but because he has a unique combination of endowments'(3). We may also note that each attribute represents no more than a subtle difference between human and chimp: a slightly longer thumb and shorter fingers, a developmental change in the structure of the larynx at around one year of age, a gradually increasing number of cortical neurons (but a neuron in a chimpanzee is indistinguishable from that in a human).

If sapiens is not the most appropriate way to define a human being, what is? I propose that it is quaerens, 'searching' or 'questing' (and please note that a derivative of quest is conquest, a very human quality). Of course all animals search - for food and water, shelter and a mate - but in man the ability to search has become heightened as a result of the three attributes mentioned above: our search is triggered not just by need, but by curiosity as well. To search for the source of the Nile, to ponder the movements of the planets, has no direct bearing on our well-being. Yet it is such curiosity that underlies human behaviour. Man has explored the entire globe, and colonised most of it; the chimpanzee has remained within its environment. Man never stops tinkering with the way he lives - he seeks novelty in the houses he inhabits and the cars he drives, the food he eats and the clothes he wears; the chimpanzee has not changed its life style over millions of years.

You might say that searching is a particularly inappropriate characteristic with which to identify human demeanour, since it is shared not only with other animals, but with every type of plant and microbe. Quest is as fundamental a feature of living organisms as growth and division. Plants do not merely use the energy of light, they seek it out as actively as any sunbather on the Mediterranean (Fig 2). Bacteria don't just metabolise organic nutrients, they swim towards them (and away from toxic molecules). Indeed several of the genes involved in searching by plants and microbes have homology with genes of animals and man, where they play a role in nerve transmission and other processes. The common feature is a 270-long stretch of amino acids called the LOV (light, oxygen and voltage) or PAS domain (4) in the relevant proteins. It is this domain that 'senses' the stimulus and triggers the appropriate response, be it growth of plant stems or movement in bacteria or a nervous impulse in animals.

So is it not then paradoxical to focus on quest as the essence of humanity, when it is merely a somewhat enhanced ability to do so that distinguishes man from chimpanzee and all other creatures? I do not think so. On the contrary, the closeness between the genetic make-up of man and ape makes just such a subtle difference the most likely explanation for our very diverse behaviours. Let us examine this a little more closely.

The genomes of man and chimpanzee differ by some 2 - 5% (depending on exactly how one chooses to measure them (5)). This has led scientists to look for genes - there could be as many as a thousand - that are distinctly either 'chimp' or 'human'. I do not believe they will find them. All they will find are some mutations in genes that are common to ape and man. In short, I favour the second explanation for the genetic difference between chimpanzees and humans (Fig 3). What is my reason for this assertion? Not so much that no distinctly 'human' gene has yet been identified - we don't yet have the complete genome of the chimpanzee - but rather that those genes that play a role in the three key attributes I have mentioned, do not seem to be unique to humans. Let me give an example.

Some years ago it was discovered that between 2% and 5% of all children have a severe speech and language disorder. Their grammatical ability is badly impaired, they have difficulty in articulating words, and they are unable to control the muscles around their mouth properly. The condition is hereditary, and persists throughout life. A gene called FOXP2 has been identified as lying at the heart of the problem. A group of scientists from Leipzig and Oxford have shown that in chimpanzees and other apes, the FOXP2 gene is different from that in humans. But within all healthy humans studied, including individuals of African, Asian, European, South American, Australian aboriginal and Papua New Guinean descent, the gene is identical. Moreover it appears not to have mutated over the entire time span (some 200,000 years) that Homo sapiens has been around. In short, it is the first molecular marker for the development of speech and language to be identified. And speech and language are dependent on a combination of two of the three attributes I have identified as being typically human: a unique vocal cord and many cortical neurons. One might therefore suppose that the protein coded by the FOXP2 gene is significantly different in chimpanzees and humans. Indeed, genes like FOXP2 might account for much of the 5% disparity between the genomes of chimpanzees and humans. Far from it. The dissimilarity between the chimpanzee and human FOXP2 gene is just 0.03%, and the FOXP2

Figure 3

The genetic make-up of chimpanzees and humans.

protein of humans differs from its counterpart in chimpanzees by a mere 2 amino acids out of a total of 715. Many proteins that perform essentially identical functions in chimpanzees and humans are considerably more diverse, and we may conclude that much of the genetic difference between humans and chimpanzees represents no more than a build-up of 'silent' mutations (including polymorphisms) that have accumulated over the seven or so million years since the common ancestor of chimp and human inhabited the forests of Africa. The argument, of course, extends throughout evolution: there are no specifically `ape' as opposed to `monkey' genes, and so on. All creatures that ever lived are made up of genes that can be traced back to the first multicellular member of the animal kingdom some 500 million years ago. And even that organism inherited the genes responsible for its metabolism from an archaebacterium alive two billion years earlier. Evolution does not produce new genes characteristic of any one species: it merely modifies existing ones and adds a few variants.

What, you may now ask, are the consequences of human quest? Will we destroy ourselves through some catastrophic failure of our making - of our computers and our weaponry? The astronomer Martin Rees thinks so (7), but I disagree. In the long term, over the next billions of years, of course he is right. We - along with every other form of life - are doomed to extinction as flares from the dying sun boil every ounce of water off the face of the earth, though that should not trouble you, or your grandchildren, for many generations yet to come. In the short term, however, I am more optimistic. Man's pragmatism, that prevented nuclear disaster during the Cold War, surely protects him against calamities triggered by current weaponry or by rogue computers. And so far as the possibility of extinction by a massive asteroid or a lethal virus is concerned, man is unquestionably the species most likely to survive: as a result of his incessant quest for answers to the most formidable problems. He’s been successfully exploring the realms of science for two thousand years, and he’s not going to stop now.