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own. This would make it impossible for all humans to interbreed, as is the case at present, and confine people to seeking partners within genetically similar groups. Such an outcome would be another step in fragmenting the human population into different species.

The weakening of drift and its mutation-reducing effect might be offset, to some extent, by human intervention, in the form of genetic engineering. Biologists may soon learn how to modify eggs, sperm or the early embryo so as to insert corrective genes that remedy future health defects. New genes inserted into the human genome on a widescale basis to replace existing genes might have the same mutation-shedding effect as genetic drift. Suppose the genetic modification eventually takes the form of adding many new genes, packaged in the form of an extra chromosome that could be introduced into a couple's eggs and sperm prior to an in vitro fertilization procedure, which a few decades hence has supplanted the quaint and hazardous method of conceiving at random.

This extra chromosome would include a suite of genes for correcting all genetic diseases diagnosed in the prospective parents. It would carry genes to fortify the immune system, to fend off cancer and to combat the cruel degenerative diseases of age. The in vitro fertilization procedure and the individually tailored genetic engineering would be expensive, but critics who claim that only the rich will benefit might be confounded should governments find the procedure to be so cheap, compared with the lifetime of health care costs it averts, that they offer it free to all citizens.

The early versions of the extra chromosome, to continue the scenario a little further, are allowed only to carry genes that correct threats to health. But when the first generation of humans to carry twenty-four pairs of chromosomes turns out to be entirely normal and robustly healthy, various enhancements of desirable qualities are allowed. The extra chromosomes carry genes that promote longevity, improve the symmetry and beauty of the body, and enhance intelligence, though all within carefully prescribed limits. After various adjustments, the technology is brought to a high level of perfection. The only downside is that the people with twenty-four chromosome pairs cannot interbreed with those carrying the old fashioned number unless the latter agree to genetic modification, which many resist. Once again speciation, the division of the human population into two or more species, is the unintended outcome.

Two choices lie ahead. One is between directed human evolution and the natural kind, the other is whether to allow or promote speciation. The idea of directing human evolution, by modifying the germline, may seem adventurous, but evolution's method rests on the outcome of two chance-driven processes, mutation and drift. It could be contended that despite the madness of its method, evolution has not done too badly so far. But evolution works with glacial speed. With germline modification, on the other hand, just as in the breeding of domesticated animals, human intervention can reach a desired outcome much more quickly.

The most serious disadvantage of actively managing the human germline probably lies in the risks incurred by unintentionally suppressing evolution's vast capacity for novelty. By creating mutations at random, and testing each out to see if it works, evolution comes up with innovations that no one would think of. Those in charge of modifying the human germline, on the other hand, doubtless constrained by medical ethics to avoid all risk, would inevitably freight the genome with their conservative preferences.

Speciation, the other major issue in the human evolutionary future, is another powerful way of generating novelty and hence of improving the essentially unfavorable odds that the human species will last a long time. Our previous reaction to kindred species was to exterminate them, but we have mellowed a lot in the last 50,000 years. A bifurcation into land people and sea people—mammals have returned to the sea several times already —might not necessarily lead to conflict, nor would that of separately evolving populations on Mars and Earth. More problematic would be different human species occupying the same environment, especially if one were somehow deemed inferior or bound in helotry to the other.

There is no one human evolutionary future but many possible paths, some to be shaped by chance, some by choice. We have come so far. There is so much farther to go.

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