GOD at the top, followed by the angels, then we humans: that was the pecking order at the top of the Great Chain of Being. Then came the apes and various classes of 鈥渓esser鈥� non-human animals, and plants. It鈥檚 a hierarchy that has been in trouble ever since Darwin鈥檚 day, and now you could be forgiven for thinking the great chain is about to dissolve foreverin a vast sea of As, Cs, Gs and Ts.
In case you missed the news, scientists in the US have carried out the most extensive comparison of human and chimp genes to date (see 鈥淟osing the ape within鈥�). What separates us, the findings indicate, is not some supernatural essence of humanness but an accumulation of worldly mutations in perhaps a few thousand genes.
There is a vast amount still to learn. Just how many genes have altered between the two species? Which mutations are most influential 鈥� ones that alter the biological functions of genes or those that change the way genes are expressed? But with the human genome in the bag and the chimp genome being deciphered, it is only a matter of time before the answers start to appear.
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And what then? The story of our species鈥� past will no longer rely on a string of scrappy human fossil remains. If today鈥檚 new breed of molecular anthropologists succeed in finding and dating the key mutations, we will be able to link all the great human nativities of the past 鈥� the emergence of Homo erectus, the birth of sapiens 鈥� to sets of specific genetic changes, each tied to key human traits. We will at last see our genomes for what they really are: modified recipes for making chimps.
But here鈥檚 the rub: the great chain will live on because in the end human feelings of superiority flow not from the amount of DNA we do or don鈥檛 share with other beasts but from our capacity for reason and language. Nor should we expect genomics to sweep away the belief that the boundaries between species are sacrosanct. In fact, over the next few years, people鈥檚 concerns about scientists riding roughshod over such boundaries are only likely to increase.
Consider their attempts to create sheep-human hybrids, or chimeras, by injecting human stem cells into sheep fetuses (see 鈥淕rowing human organs on the farm鈥�). The completely laudable aim of the research is to solve the shortage of human transplant organs that blights or kills so many patients. And the resulting sheep chimeras are hardly scary monsters. But this will not stop the 鈥測uck factor鈥� kicking in.
Is chimerism unnatural? Yes, say those in thrall to gut reactions; no say those who take their cue from the fact that species are fluid rather than fixed entities and that we do not find mules morally repugnant. But such arguments get us nowhere: assuming something is OK just because it happens in nature is just as false as assuming something is wrong because it doesn鈥檛 happen in nature. What ought to matter more is that there is as yet no guarantee that the human-like tissues from such chimeras would generate functional grafts for transplant or be safe from the threat of animal viruses. These problems, though, dog all xenotransplantation research. It would be unfair to use them to single out this one approach for a ban.
So how should we deal with chimerism research? At present the number of human cells inside these sheep is small, but to protect animal welfare in the long term there needs to be a full debate about where to set the limits. For example, we might want to ask scientists to minimise or even eliminate the number of human-like cells they incorporate into the brains or gonads of lab animals. The idea of a chimeric sheep developing human-like awareness may be as far-fetched as the idea of two such lab animals mating to produce human offspring. But why take the chance?
The pace of chimp genome research makes it even more vital to have effective, proportionate policies in place. Richard Dawkins, for instance, has suggested that scientists will one day not only have the information needed to reconstruct the genome of the last common ancestor of apes and humans: we might even have the know-how to implant the genome into a human egg to create this ancestor. Even if this is fanciful, scientists will certainly be tempted to test out in mice or monkeys their theories about the evolutionary significance of specific sets of human mutations. How far should they be allowed to go? Now is the time to decide.