Are we it? Have we already seen the best of humanity? Was Plato or Shakespeare or Einstein or Buddha or Lao Tzu or the prophet Mohammed as clever as any human being is ever likely to be? Modern athletes with their minutely cultured hearts and limbs don't run the 100 metres significantly faster than Jesse Owens did in 1936. So is this as fast as people can ever be? In short: has our evolution stopped; and if so, why, and if not, what lies in store? Or might genetic engineering allow us to breed our own superspecies, if not in God's image, at least according to the demands of market forces?
To begin at the beginning. Darwin's great contribution in The Origin of Species was to propose not simply evolution, but a plausible mechanism: it happens, he said, "by means of natural selection". The individuals best able to cope at any one time are those most likely to survive and leave offspring. So as the generations pass, each lineage of creatures becomes more and more closely adapted ("fitted") to its particular surroundings. Natural selection requires an appropriate mechanism of inheritance - one that ensures "like begets like" (that cats have kittens, and horses give birth to foals), but also provides variation, so that not all kittens and foals are identical. Darwin's near contemporary, Gregor Mendel, working in what was then Moravia and is now the Czech Republic, provided just what was needed: he showed that inheritance works by transmitting units of information, now known as genes. Genes encapsulate the characters of the parents, but they are recombined in the offspring through the machinations of sex and are also prone to random change, of the kind known as mutation. So they provide all the variation that is required.
Darwin did not know of Mendel's work (he had Mendel's account on his desk, but failed to cut the pages), but 20th-century biologists put the two together and, by the 1940s, generated "neo-Darwinism". Creatures that reproduce through sex continually swap and recombine their genes, so all the genes in all the individuals in a sexually breeding population form one great "gene pool". Natural selection operates on the pool as a whole (these neo-Darwinists said). It knocks out individuals who contain less helpful genes, but favours those whose genes are especially advantageous. Thus the "bad" genes tend to be lost as time goes by, while the ones that promote survival and reproduction spread through the pool. Over time, the composition of the gene pool changes and so the creatures change as well. The neo-Darwinian model has been modified somewhat, but that general picture obtains.
There is no destiny in evolution, Darwinian or neo- Darwinian. Natural selection is opportunist and answers to the here and now; it has no mind for the distant future. The fossils tell us that our ancestors grew taller over the past five million years, from about a metre to nearly two, while our brains have puffed up from an apish 400ml or so to 1,400ml - easily the biggest in proportion to body weight of any animal. Perhaps this has made us more like God. But there is nothing in natural selection to suggest that our ancestors did more than adapt to whatever their surroundings threw at them, or to imply that we will grow more godlike as the future unfolds.
Neither will we go the way of The Eagle's Mekon, arch-enemy of Dan Dare: a green homunculus with a head as big as a dustbin and legs like cribbage pegs. Before Darwin, the Frenchman Jean-Baptiste Lamarck proposed a different mechanism of evolution, through "inheritance of acquired characteristics". He observed rightly enough that bodies adapt to whatever is demanded of them, so that blacksmiths, say, acquire bigger muscles. But he was wrong to propose that a blacksmith passes on his hard-earned biceps to his children. If the children want to be tough, they have to do their own smithying. By the same token, thinking won't make our brains grow bigger, in any heritable way, and physical indolence will not shrink our descendants' legs. So our evolution is not shaped by destiny, nor by our own endeavours, nor by our self-indulgence. Neo-Darwinian mutation and selection (plus large slices of chance) are all there is.
But in us, the neo-Darwinian mechanism seems logjammed. Some genetic variants are being lost, as small tribal groups continue to die out; and others are constantly gained by mutations, some of which persist. There are fluctuations: genes that confer resistance to Aids are gaining ground in Africa, for instance, while Kenyans are currently breeding faster than Italians, so any genetic variants that are peculiar to either group must be increasing or falling. But the permanent losses of genes through extinction of minorities are small compared to the whole pool, and while the particular genes of Kenyans may wax in one century, they may wane in another. Most importantly, there is no consistent pressure to push our gene pool in any particular direction. Nobel prize- winners and professional basketball players are lauded, but do not typically leave more offspring than the ordinary Joe. Infant mortality is still high in some societies but, in genetic terms, it strikes randomly because the poor are not genetically distinct.
Genetic logjams certainly happen, as is clear from the fossils. Some lineages of clams remained virtually unchanged for tens or even hundreds of millions of years. Today's leopards and impala are more agile and brainy than their ancestors of 50 million years ago, but they have not changed much in the past three million years. People anatomically indistinguishable from us were living in Africa at least 100,000 years ago.
Yet the deadlock could be broken. Through global war or some other ecological disaster, human beings could again become isolated into island groups, and natural selection could then go to work on each of them separately to produce a range of neo-humans, each adapted to its own island. We should not assume that any of the islands would especially favour brains, which require a great deal of nourishment. Agile climbers of fruit trees might fare best, and so we might again become more simian.
Or human beings might take their own genetic future in hand - which, in principle, has long been within our gift. The same techniques that made wheat from wild grasses and Aberdeen Angus from aurochs could transform humanity, too, in any direction we might care to prescribe - albeit over longer periods, given that we have such an extended generation interval, and many of the characters we might be most interested in undoubtedly have a complex genetic basis. Eugenics, the deliberate transformation of the human gene pool, was popular 100 years ago through most of Europe, and de rigueur in the US. Only the Catholics spoke out consistently against it, and the socialists H G Wells, George Bernard Shaw and Sidney and Beatrice Webb were among its most incongruous advocates. Up-to-date Edwardian matrons spoke approvingly of "eugenic" marriages.
The eugenicists were interested not so much in breeding super-people as in preventing the "decline" of the species through the perceived reproductive prodigality of the "feeble minded" (who were taken to include a great many foreigners and a large section of the working class). Hitler revealed the political perils, however, as he wiped out the people who did not meet his own particular criteria and matched blonds with blondes like prize porkers. So eugenics has gone out of fashion and now is virtually taboo. But in various ways, the new biotechnologies seem to open new possibilities and have concentrated minds afresh: cloning, genomics, genetic engineering.
Genetic engineering is the biggie: the transfer of particular stretches of DNA from one individual to another. The first phase of the Human Genome Project was completed last year, and it is already beginning to show which pieces of DNA correspond to which particular genes and which, therefore, are worth transferring. Cloning qua cloning is not directly pertinent, but it does provide techniques that will generally be necessary if genetic engineering is ever to progress beyond its simplest stages. Genetic engineering is already commonplace in bacteria, increasingly in food crops (GMOs), and in laboratory mice. It has at least been essayed in farm livestock and, in principle, is certainly applicable to humans. So where might it lead?
Most simply, doctors already try to repair the affected tissues in people with particular diseases: for example, to correct the damaged genes in the lungs of patients with cystic fibrosis (CF). Genetic changes made to the lungs (if and when this becomes possible) would not be reflected in the eggs and sperm, and so would not be passed on to future generations. Some argue that genetically transformed lung cells could escape, to be breathed in by the rest of us. But apart from this hypothetical hazard, no third parties are involved. The ethical problems therefore seem minimal.
More radical would be to repair the CF gene in a very young embryo, so that the whole person who subsequently develops would be genetically changed. His or her sperms or eggs would develop from cells that were already transformed, so the genetic alteration would be passed down the generations. Biologically and ethically, this is far more heavy-duty than ad hoc tissue repair.
Whether CF cells are repaired ad hoc, or in a young embryo and so passed on, such procedures are clearly in the realms of therapy. CF is a disease that causes suffering: to correct the gene is to attempt a cure. Western medicine is rooted in the belief that therapy, to correct unmistakable illness, is good.
But some already speak not simply of repairing what is obviously damaged, but of improving (according to their own or their clients' judgement) on what already works well enough. By analogy with traditional medicine, this would move us from physic to tonic - a distinction clearly spelled out by controllers of sports, who allow insulin to correct diabetes (some of the greatest athletes have had diabetes) but forbid steroids to pump up muscles that are already perfectly functional. At the end of the line lies the "designer baby", built to a specification in the way that Ferrari builds motor cars. In Remaking Eden, Professor Lee Silver of Princeton University in effect advocates such a course, proposing that "GenRich" (genetically enhanced) individuals, primed to gain honours at Princeton and/or to outreach Michael Jordan at the basketball hoop, will be tomorrow's elite. There are plenty of people with cash to spare for such indulgences, says Silver, and plenty of molecular biologists anxious to oblige; and, he says, where the market presses, reality should and indeed must follow.
Yet for all the hype and hand-wringing, the evolutionary impact of these new technologies will surely be virtually zero. The genetic repair of damaged embryos would affect the future, at least in a few families, but it is very difficult to see why anybody should ever want to do such a thing. A person may carry the CF gene (say) yet half of his or her sperm or eggs will be free of it. Even if a carrier marries another carrier, one in four of their embryos will be totally free of the damaged gene. It would be far easier in principle to induce superovulation, fertilise the eggs in vitro to produce a batch of embryos (as is already standard practice for IVF births), and then select the ones that do not contain the mutant gene at all. Only these healthy embryos would then be implanted into the mother. Techniques of the kind that have been developed largely in the context of genetic engineering are employed for diagnosis, but no actual genetic transformation takes place.
Critics, though, have perceived indirect evolutionary consequences if we contrive to rescue babies with damaged genes who would otherwise have died. Those damaged genes, they argue, would once have been purged from the human lineage, but now they survive, and surely this will weaken the pool as a whole. This argument is similar to that of the old eugenicists who feared the genes of the "feeble minded", and is at least equally misguided.
Most of the genes that cause "single-gene disorders", including CF, have no adverse effects unless they are inherited from both parents. The unfortunate individuals with a double dose are called "homozygotes". The "heterozygotes" - those who inherit the "bad" gene from only one parent - carry that gene and may pass it on to their offspring, but they are not diseased themselves.
Most "bad" genes are rare, but a few are common. The genes that cause sickle-cell anaemia occur frequently in people of African descent, while an astonishing one in 20 Caucasians carries the CF mutant. But assuming random mating (as biologists say), each CF carrier has only a one-in-20 chance of mating with another CF carrier; so only one in 400 Caucasian marriages will bring two carriers together. Only one in four of their offspring will inherit the bad gene from both parents, and so be homozygous for CF; so only one in 1,600 children in a Caucasian population will actually manifest the disease. It would be possible to sterilise those children (as if they did not have problems enough already) or to let them die, as they would do if neglected. But it makes no genetic sense to eliminate one in 1,600 children while leaving the carriers, who are so much more common, intact. Indeed, before modern medicine came along, nature had been assiduously eliminating the unfortunate homozygotes for many thousands of years (ever since the CF mutation first occurred) and yet it is still with us.
Some eugenic zealots could track down all the carriers, and eliminate them; although, if such zealots were Caucasian, they might well find that they themselves were carriers. It's easy to see intuitively, too, that the rarer the gene - and most are far rarer than CF - the more dramatically the heterozygous carriers outnumber the homozygous sufferers. Besides, at least 5,000 different syndromes have been described that are caused by mutations in single genes, and there must in reality be many more, because all our genes are prone to mutation. Thus it is estimated that every one of us is liable to carry an average of five damaged genes that would cause disease if we had children by some similar carrier. To eliminate all "bad genes", we would need to wipe out the entire human species. In short, genetic zealotry is born of nonsense. Humane, sensible medicine implies no genetic risk for our species as a whole.
The designer baby, however, the child conceived like a custom car, is metaphorical pornography that, we may note in passing, is perpetrated not by the much-maligned "press", but by the scientists themselves, many of whom have their eyes on megabucks and argue the market mantra that what people are prepared to pay for is by definition good. Fortunately, it is also ludicrous. The listing of genes through the Human Genome Project does not "open the book of life" as some idle geneticists (not the Cambridge scientists who actually did the work) have claimed.
If we think of genes as words, then what we have is an incomplete lexicon. An individual's apportionment of genes - the genome - should be construed as an arcane work of literature with its own syntax, puns, allusions, redundancies, colloquialisms and overall "meaning" of which we have almost no inkling, and may never understand exhaustively. On present knowledge, or even with what we are likely to know in the next two centuries, it would be as presumptuous to try to improve on the genes of a healthy human baby as it would be to edit sacred verse in medieval Chinese if all we had to go on was a bad dictionary.
So all in all, human beings seem likely to remain as they are, genetically speaking, barring some ecological disaster; and there doesn't seem to be much that meddling human beings can do about it. This, surely, is a mercy. We may have been shaped blindly by evolution. We may have been guided on our way by God. Whichever it was, or both, the job has been done a million times better than we are ever likely to do. Natural selection is far more subtle than human invention. "What a piece of work is a man!" said Hamlet. "How beauteous mankind is!" said Miranda. Both of them were absolutely right.
Colin Tudge's latest books, In Mendel's Footnotes and The Variety of Life: a survey and a celebration of all the creatures that have ever lived, are now available in paperback