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Science & Nature SpecialThe chimp genome

Language, learning and logic

14 June 2003

12:00 AM

14 June 2003

12:00 AM

Everyone knows that the Earth is not at the centre of the universe and that mankind has descended from the apes. But what about this: according to the latest estimates, we share 98.8 per cent of our DNA with the chimpanzees. What distinguishes us from our closest living relative is due to a 1.2 per cent genetic distance.

Now the race is on to decipher the chimp genome, a draft of which will be published later this year. By superimposing the human genome on the chimp’s, researchers hope finally to shed light upon the genetic basis of human nature. In fact, some hints of genes underlying uniquely human traits, such as our big brains as well as our ability to use language, have already been discovered.

We shared our last common ancestor with the chimpanzee about six million years ago. This is a recent split in evolutionary terms, and thus the main reason for the genetic proximity between Pan troglodytes and Homo sapiens. Nevertheless, a lot has happened along the evolutionary road leading to mankind. We became bipedal and grew opposable thumbs; we developed speech and, of course, grew our absurdly large brains that outweigh a chimp’s three times over. The result is mankind’s most distinguishing feature: our use of culture based upon the extensive use of symbols to transmit ideas. We communicate through material artefacts, abstract symbols such as words or numbers, or, lately, information fired down a glass-fibre cable. Culture is an accumulative process that means everyone is standing on the shoulders of giants.

So why didn’t chimps go down that road? The simple answer is that evolution doesn’t care about whether you are smart. All that counts is to survive in your environment, and this is what chimps do. After all, they are able to perform quite complex tasks: they use stones to crack open nuts; they chew sticks into brushes and poke them into anthills to ‘fish’ for ants; some have even been seen using leaves as a plate to catch their own faeces which might contain undigested food items. Perhaps even more impressively, chimps live in complex societies where bonding and cheating are reminiscent of Machiavellian politics; some females have been observed to provoke an argument among males so that they can feed undisturbed. All of this makes chimpanzees perfectly adapted to their natural and social environment.

But we would have to go back a long way in the human lineage to see this sort of lifestyle. A time-traveller visiting the East African savannah two million years ago would see our ancestors already fully bipedal, putting out such masses of stone tools that archaeologists refer to this period as the Oldowan industry (after the site where the tools have been unearthed). Meanwhile, the ancestors of chimps were probably doing what chimps still do today: chewing sticks, hammering on nuts and cheating on each other.

But the big bang in human evolution was yet to come. Only with the rise of modern humans about 200,000 years ago do we see the emergence of artefacts other than tools, such as red ochre used as body paint. In other words, we began to embark on the evolutionary road to culture. Now, the intriguing thing about the human’s ability to embrace culture is the fact that it is universal. It would thus defy any biological logic if the prerequisites, such as cognitive abilities and language, did not have a genetic basis.

Since all of our ancestors are gone, we cannot analyse their brains. The only thing we can do is look inside the skull of a chimpanzee. But what do we find? Of course, the brain is smaller, but size doesn’t necessarily matter (Neanderthals had bigger brains than we do, and still they never went beyond producing stone tools). Moreover, even the organisation of the chimp brain is by and large the same. The overall anatomy is scaled down by just one third. In fact, as Wolfgang Enard at the Max Planck Institute in Leipzig has shown, chimpanzees and humans even seem to use the same genes in their brains. So the fact that it is we who study chimpanzees and not vice versa must be due to something else.

A gene only holds the information for how to build a protein. As such, a gene is indeed just like an instruction – or, perhaps better, a recipe. For simplicity, let us assume that the recipe for how to make a brain contains two genes, A and B. Now here is the interesting bit: genes A and B can build both a human and a chimp brain (and probably also a Neanderthal’s). It all depends on how to weigh the ingredients. If gene A manufactures more protein than gene B, we end up with a chimp brain. On the other hand, if we shift protein output to gene B, we end up with a human brain. Just as one can make different dishes with the same ingredients, nature can build different brains by altering protein production (or gene expression) of identical set of genes.

This is exactly what Enard had found: chimpanzee and human brains are made of the same genes, but during the evolution of the human brain the expression of those genes has been altered dramatically. A few mutations in genes that regulate the expression of a whole set of other genes might have brought about this difference. As a result, the human brain has tripled in size, while the proportion of different brain areas to the total brain volume has stayed the same. This is why 1.2 per cent genetic distance can make a big change indeed.

Now let us take another example – the evolution of language. It is surely central to the evolution of culture. The fact that language must have a genetic basis had already been recognised by the American linguist Noam Chomsky back in the 1960s.

For Chomsky, it simply seemed unrealistic to assume that a child of four years of age can string together hundreds of words using complex rules of grammar without there being a genetic factor at work. His so-called ‘poverty of stimulus argument’ is nicely illustrated by the rise of Creole languages among the descendants of slaves. Initially, different peoples speaking different tongues were forced to communicate by sign language. Yet the children of the slaves already began to invent their own language, using a primitive grammar. Over the generations, without any teaching or dictionaries, the pidgin dialects arose, and finally the Creole languages. They are languages in their own right, based on English or French but with their own complex grammar.

In the 1990s, researchers investigated a language disorder called KE that ran in families. Although they showed almost average intelligence, these individuals had subtle problems with language. For example, they could not distinguish similar words, such as pad from bad, or they had problems with grammar, such as the English ending -ed to signal the past. Even more bizarrely, when shown a picture of small elephants, KE patients struggled to complete the sentence, ‘Look at these small elephants. This one over here must be the …smallest!’

In a late triumph for Chomsky, a team of researchers at Oxford University showed that this language disorder is due to a mutation in a single gene called FOXP2. But perhaps even more impressive was a molecular analysis of the gene, again undertaken by Wolfgang Enard and colleagues. When they compared the FOXP2 sequence between humans and chimpanzees, they found that the human form differed from the chimpanzee’s by a single mutation that was dated to about 200,000 years ago. This is the time when language is thought to have arisen.

FOXP2 might be the most dramatic example of a genetic basis for a uniquely human feature, but it is by no means the only one so far. There is the olfactory gene, for example, active in chimpanzees and inactive in humans. The loss of this gene might have been rather random; our ancestors simply did not have to rely on smell any more, and as a consequence this mutation has gradually spread.

So far, researchers have usually stumbled across the genes that underlie human nature. The completion of the chimp geno
me will, without doubt, change that. It will also force us to accept that all that sets us apart from our closest living relative are some chance mutations that have, luckily or not, given rise to mankind.

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