When does a person begin? I will try to give the most precise answer to that question actually available to science. Modern biology teaches us that ancestors are united to their progeny by a continuous material link, for it is from the fertilization of the female cell (the ovum) by the male cell (the spermatozoa) that a new member of the species will emerge. Life has a very, very long history but each individual has a very neat beginning: the moment of its conception.
The material link is the molecular thread of DNA. In each reproductive cell, this ribbon, roughly one meter long, is cut into pieces (23 in our species). Each segment is carefully coiled and packaged (like a magnetic tape in a minicassette) so that under the microscope it appears like a little rod, a chromosome.
As soon as the 23 paternally derived chromosomes are united, through fertilization, to the 23 maternal ones, the full genetic information necessary and sufficient to express all the inborn qualities of the new individual is gathered. Exactly as the introduction of a minicassette inside a tape recorder will allow the restitution of the symphony, the new being begins to express himself as soon as he has been conceived.
Natural sciences and the sciences of law speak the same language. Of an individual enjoying a robust health - a biologist would say that he has a good constitution -of a society developing itself harmoniously to the benefit of all its members, a legislator would state that it has an equitable constitution.
Nature works the same way. The chromosomes are the tables of the law of life, and when they have been gathered in the new being (the voting process is the fertilization), they fully spell-out his personal constitution.
What is bewildering is the minuteness of the scripture. It is hard to believe, although beyond any possible doubt, that the whole genetic information necessary and sufficient to build our body and even our brain, the most powerful problem-solving device, even able to analyze the laws of the universe, could be epitomized so that its material substratum could fit neatly on the point of a needle!
Even more impressive, during the maturation of the reproductive cells, the genetic information is reshuffled in so many ways that each conceptus receives an entirely original combination that has never occurred before and will never again. Each conceptus is unique, and thus irreplaceable. Identical twins and true hermaphrodites (abnormal individuals having both male and female reproductive organs) are exceptions to the rule: one man, one genetic make-up; but interestingly enough, these exceptions have to take place at the time of conception. Later accidents could not lead to harmonious development.
All these facts were known long ago and everybody was agreeing that test-tube babies, if produced, would demonstrate the autonomy of the conceptus, over which the test tube has no title of property. Test-tube babies now do exist.
If the ovum of a cow is fertilized by a bull's sperm, the conceptus, floating freely in liquid, starts its cattle's career. Normally it would travel for a week, through the Fallopian tube, and reach the uterus. But thanks to technology it can travel much farther, even across the ocean!
The best shipping equipment for such a two-milligram cattle being is to introduce it inside the Fallopian tube of a female rabbit. (Air freight is much less than for a pregnant cow.) At destination, the minuscule animal is carefully removed and delicately settled inside the uterus of a recipient cow. Months after, the calf exhibits all the genetic endowment it received from its true parents (the donors of the ovum and of the sperm) and none of the qualities of its temporary container (the rabbit) nor of its uterine foster mother.
How many cells are needed to build an individual? Recent experiments spell out the answer. If very early conceptuses of mice are artificially disassembled (by a peculiar enzymatic treatment), their cells come apart. By mixing such suspensions of cells, coming from different embryos, one sees them reassembling again. If the tiny mass is then implanted in a recipient female, some little mice (very few indeed) manage to develop to term, completely normal. As theoretically expected and demonstrated, a chimeric mouse can derive from two or even three embryos, but no more. The maximum number of cells cooperating to the elaboration of an individual is three.
In full accordance with this empirical demonstration, the fertilized egg normally cleaves itself in two cells, one of them dividing again, thus forming the surprising odd number of three, encapsulated inside their protective bag, the zone pellucida.
To the best of our actual knowledge, the prerequisite for individuation (a stage containing three fundamental cells) is the next step following conception, minutes after it. All this explains why (British) Drs. (Robert) Edwards and (Patrick) Steptoe could witness in vitro (outside the living body, in a test tube) the fertilization of a ripe ovum from Mrs. (Lesley) Brown by a spermatozoa from Mr. (Gilbert) Brown. The tiny conceptus they were implanting days later in the womb of Mrs. Brown could not be a tumor or an animal. It was in fact the incredibly young Louise Brown, now three years old.
The viability of a conceptus is extraordinary. Experimentally, a mouse conceptus can be deep frozen (even to -269c) and, after careful thawing, implanted successfully. For further growth, only a recipient uterine mucosa can supply the embryonic placenta with appropriate nutriments. In his lifecapsule, the amniotic bag, the early being is just as viable as an astronaut on the moon in his space-suit: refueling with vital fluids is required from the mother-ship. This nurture is indispensable for survival, but it does not "make" the baby, no more than the most sophisticated space shuttle can produce an astronaut. Such a comparison becomes even more cogent when the fetus moves.
Thanks to a refined sonar-like imagery, Dr. Ian Donald, from England, a year ago succeeded in producing a movie featuring the youngest star in the world, an 11-week-old baby dancing in utero (in the uterus). The baby plays, so to speak, on a trampoline! He bends his knees, pushes on the wall, soars up and falls down again. Because his body has the same buoyancy as the amniotic fluid, he does not feel gravity and performs his dance in a very slow, graceful, and elegant way, impossible in any other place on the Earth. Only astronauts in their gravity-free state can achieve such gentleness of motion. (By the way, for the first walk in space, technologists had to decide where to attach the tubes carrying the fluids. They finally chose the belt buckle of the suit, reinventing the umbilical cord.)
When I had the honor of testifying previously before the Senate, I took the liberty of referring to the universal fairy-tale of the man smaller than the thumb. At two months of age, the human being is less than one thumb's length from the head to the rump. He would fit at ease in a nutshell, but everything is there: hands, feet, head, organs, brain, all are in place. His heart has been beating for a month already. Looking closely, you would see the palm creases and a fortune teller would read the good adventure of that tiny person. With a good magnifier the fingerprints could be detected. Every document is available for a national identity card.
With the extreme sophistication of our technology, we have invaded his privacy. Special hydrophones reveal the most primitive music: a deep, profound, reassuring hammering at some 60-70 per minute (the maternal heart) and a rapid, high-pitched cadence at some 150-170 (the heart of the fetus). These, mixed, mimic those of the counterbass and of the maracas, which are the basic rhythms of any pop music.
We now know what he feels, we have listened to what he hears, smelled what he tastes and we have really seen him dancing full of grace and youth. Science has turned the fairytale of Tom Thumb into a true story, the one each of us has lived in the womb of his mother.
And to let you measure how precise the detection can be: if at the beginning, just after conception, days before implantation, a single cell was removed from the little berry-looking individual, we could cultivate that cell and examine its chromosomes. If a student, looking at it under the microscope, could not recognize the number, the shape and the banding pattern of these chromosomes, if he was not able to tell safely whether it comes from a chimpanzee being or from a human being, he would fail in his examination.
To accept the fact that, after fertilization has taken place, a new human has come into being is no longer a matter of taste or of opinion. The human nature of the human being from conception to old age is not a metaphysical contention. It is plain experimental evidence.