These are just our closest cousins. The apes of Asia – orang-utans and gibbons – have entirely different sex lives again. So do the many and various species of monkey, presenting a bewildering variety of social and sexual stratagems, each one suited to its habitat and food. Forty years of field primatology have confirmed that we are a unique species, completely unlike any other. There is no exact parallel to the human scheme. But in the animal kingdom, there is nothing exceptional in being unique. Every species is unique.
ENTER GENETICS
The argument about human exceptionalism, swaying between Darwinian similarity and Cartesian difference, shows no sign of ending. Each generation is doomed to fight the same old battles. If you arrive in the world in a time when people have strayed a bit far into anthropomorphic similarity, then you can find a fresh argument for how different animals and people are. If the air is full of difference, then you can champion the similarities. Philosophy is like this: eternally unsettled and only occasionally disturbed by new facts.
Then came an unexpected threat to this pleasant debate. A threat of a resolution. A threat of defining once and for all, at root, what the difference is between a person and a chimpanzee; what you would have to do to a chimpanzee to make it into a person.
It happened about the same time that Jane Goodall was undermining the exceptionalism of human behaviour. Almost completely forgotten until rediscovered in the 1960s was an extraordinary experiment done by a Californian named George Nuttall in 1901 while at Cambridge University. He noticed that the more closely related two species were, the more their blood produced the same immune reaction in a rabbit. He injected blood from, say, a monkey into a rabbit repeatedly for some weeks, then a few days after the last injection extracted serum from the rabbit’s blood. That serum, mixed with the blood of a monkey, caused it to thicken as the immune reaction set in. Mixed with the blood of a different animal, it thickened more according to how closely related the species were. By this means Nuttall established that human beings were more closely related to apes than they were to monkeys. This ought to have been obvious from the lack of a tail and other features, but it was still controversial at the time.
In 1967 at Berkeley, Vincent Sarich and Allan Wilson revived Nuttall’s biochemical techniques in a more sophisticated form and used them to construct a ‘molecular clock’ that measured the actual length of time since two species had shared a common ancestor. They concluded that human beings had shared a common ancestor with the great apes not 16 million years ago, as was then conventional wisdom, but only about five million years ago. Anthropologists, whose fossils implied a more ancient split, reacted with contempt. Sarich and Wilson stuck to their guns. In 1975, Wilson asked his student Marie-Claire King to repeat the exercise for DNA in order to find the genetic differences between human beings and apes. She came back disappointed. It was impossible to find differences, she said, because human and chimpanzee DNA was so astonishingly similar: close to 99 per cent of the DNA in a human being was identical to that in a chimpanzee. Wilson was thrilled: the similarity was more exciting than the difference.
That figure has meandered a little since the 1970s. Most estimates place it at 98.5 per cent, although two recent detailed studies of actual stretches of genome came to a figure of 98.76 per cent.26 However, just as the 98.5 per cent was seeping into the public consciousness, Roy Britten wrote a dramatic paper in 2002 showing that it was out by a mile. He confirmed that if you count only substitutions – i.e., letters in the text that are different between human and chimpanzee genes—you do indeed get a figure of 98.6 per cent. But if you then add in the textual insertions or deletions, the figure drops to 95 per cent.27
Whatever. It was still a terrible shock to science to discover just how small was the genetic distance between the two species. ‘The molecular similarity between chimpanzees and humans is extraordinary because they differ far more than many other [closely related] species in anatomy and way of life,’ wrote King and Wilson.28 An even greater shock was in store in 1984, when Charles Sibley and Jon Ahlquist at Yale found that chimpanzee DNA was more like human DNA than it was like gorilla DNA.29 This was a moment of human dethronement similar to Copernicus placing the Earth within the solar system as just another planet. Sibley and Ahlquist placed the human species within the ape family as just another ape. From having our own distinct ape lineage stretching back 16 million years, we were now forced to admit that not only did we share a common ancestor not much more than five million years ago, but that we were the most recent branch of the family. Our common ancestor with the chimp lived after the common ancestor of both with the gorilla and long after the common ancestor of all three with the organg-utan. Incredible as it may seem, chimpanzees are more closely related to human beings than they are to gorillas (a conclusion that Britten’s reanalysis of the precise number does not alter). Nothing in the anatomy or fossil record of the African apes suggested such a possibility. Human beings are not the odd ones out.
Time has dulled these shocks. But there are more coming. Reading the DNA of a human being, alongside that of a chimpanzee, might once and for all define the difference between them. At the time of writing, the complete genome of the chimpanzee is not yet available. Even when it is, it may prove tricky to work out which differences are the ones that matter. The human genome contains about three billion ‘letters’ of code. Strictly speaking, these are chemical bases on a molecule of DNA, but since it is their order, not their individual properties, that determines what they produce, they can be treated as digital information. The difference between two individual human beings amounts, on average, to 0.1 per cent, so there are three million different letters between me and my neighbour. The difference between a human being and a chimpanzee is about 15 times as great, or 1.5 per cent. That equates to 45 million different letters. That is about ten times as many letters as there are in the whole Bible, or 75 books the length of this one. The book of digital differences between our two species, unannotated, would fill eleven feet of bookshelf. (The bookshelf of similarities, by contrast, would stretch to 250 yards.)
Look at it another way. Scientists now reckon that there are about 30,000 human genes. That is, scattered throughout the genome are 30,000 distinct stretches of digital information that are directly translated into protein machinery to run and build the body: a gene being a recipe for a protein. Chimpanzees almost certainly have roughly the same number of genes. Since 1.5 per cent of 30,000 is 450, then it seems to follow that we have 450 different, uniquely human genes. Not such a big number. The other 29,550 genes are identical in us and chimps. But this is actually most unlikely. It could instead be that every single human gene is different from every single chimp gene, but only 1.5 per cent of its text is different. The truth is bound to lie somewhere between the two. Many genes will be identical in closely related species; many will be slightly different. A very few will be utterly different.
The most visible difference is that all apes have one more pair of chromosomes than people do. The reason is simple enough to find: at some point in the past, two middle-sized ape chromosomes fused together in the ancestors of all human beings to form the large human chromosome known as chromosome 2. This is a surprising rearrangement, and it almost certainly means that chimp-human hybrids would be sterile if they could survive at all. It may have helped create what evolutionists delicately call ‘reproductive isolation’ between the species in the past.
But the rearrangement of the chromosomes does not necessarily imply a difference in genetic text at that spot. Although the chimpanzee genome is still largely