In 1677, Anton van Leeuwenhoek used the newly invented microscope to examine animal spermatozoa. His observations soon led to descriptions of human sperm, seen swimming in fresh seminal fluid. But, with their home-ground lenses, microscopists of that time imagined that they could see a homunculus inside the sperm head; and artists drew pictures of what they thought they saw. There is a fine woodcut by Hartsoeker, in the Wellcome collection in London, of a perfectly formed tiny man with over-sized head, curled up in a foetal position, ready to hatch from the sperm head.
In the 18th century, an obscure rabbi, Elijah Pinhas ben Meir, used these findings as ethical justification against the practice of masturbation: "So the sperm contain little men, fashioned in man's image. Now we can see how the words of the old sages who wrote the Talmud 1,500 years ago were true. Clearly, destruction of the seed is in effect killing the homunculus and is akin to murder." Had sperm really contained little men, the destruction of semen would indeed have been like murder. But the rabbi's polemic was based on flawed observations, on bad science. Ethical values must be based on the truth, on the best factual assessment of the natural world that we have. And that is still one of science's greatest challenges in the coming century.
There is hardly an area of science where such challenges are greater than in human reproduction. Only a fool believes he can predict what will happen, even in his own field, with any accuracy. And at the turn of the millennium, when every newspaper is importuning reluctant "experts" to write of their view of the future, it seems foolhardy to offer my own predictions. But the subject of human reproduction is a microcosm for many of the ethical problems raised by science. And society's attitude to such problems in future and their various solutions will have a profound influence on scientific events. It is already largely forgotten that in vitro fertilisation (IVF), the "test-tube babies" advance pioneered by Drs Edwards and Steptoe, was almost banned before it really began. Had Parliament accepted the arguments of some right-wingers in 1985, many families, now happily complete, would not exist today.
Humans are among the most naturally infertile animals, but the reasons for that have only just started to surface. One of the most unexpected facts is that human existence is even more fragile than we thought. Only about one in every five human embryos seems capable of implantation and development into a full-grown baby. The majority perish - but why is mysterious. A significant proportion, at least one-quarter of all human embryos, is demonstrably malformed. Such embryos have abnormal numbers of chromosomes, which usually, perhaps fortunately, makes them incapable of independent life. Nearly all such embryos die. A few - around 1 per cent - survive. They emerge from the womb with severe abnormalities, such as Down's syndrome.
It seems that Nature herself is capable of efficient antenatal diagnosis, aborting those embryos she considers unfit. Nature practises eugenics much more frequently and effectively than those doctors called upon by their patients to undertake pregnancy termination, who are vilified by "right to life" groups in consequence.
Using IVF techniques, it is already possible to examine embryos at the stage of development when they are often destroyed by nature. Such pre- implantation diagnosis enables doctors to treat families at risk of having a child with a fatal gene disorder. Initially, DNA tests were used to establish the sex of an embryo in those families who had lost children from some sex-linked disorders, such as muscular dystrophy. Screening for cystic fibrosis and one or two other fatal genetic disorders is now also just possible. Within a few years of completion of the human genome project, we should be able to screen for a great number of the 6,000 or so single-gene defects that regularly cripple, then kill babies and children. …