How to Build a Human: Adventures in How We Are Made and Who We Are. Philip Ball
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separation so that each daughter cell receives the full complement. Meiosis is even more complicated, because the existing chromosomes have to be divided precisely in two and shipped off to their respective destinations.
Actually it’s worse than that. Meiosis happens in two stages, and the overall result is that a single, diploid germ cell replicates its chromosomes once and divides twice to end up with four haploid gametes. As in mitosis, the process by which the chromosomes are divided uses a spindle-like structure made from fibrous protein. The chromosomes become attached to the fibres and are drawn towards opposing poles of the spindle located in the two lobes of the dividing cell.
Crucially, the chromosomes undergo some shuffling in this process. The pre-meiosis germ cell, recall, has one of each of the 23 types of chromosome from the mother, and a second copy of each from the father. Which of the poles of the spindle each chromosome is drawn towards is random, and so the diploid cells made by division of the germ cell have a random combination of maternal and paternal genes.5 The haploid gametes that eventually emerge from the process then have a thoroughly scrambled single set of chromosomes: with 23 pairs of chromosomes in all, there are 223, or about 8 million, possibilities. These are combined with a similar range of options in the other gamete when egg and sperm unite, so you can see that having sex is a good way to produce genetic diversity.
Formation of the so-called primordial germ cells happens early in the development of a human embryo, around two weeks after fertilization. This is even before the gonads have started to form, which is to say, before the embryo has yet “woken up” to which sex it is. It’s as if the embryo is putting these cells aside while deferring the matter of whether they will be eggs or sperm. The gonads themselves will guide this process, sending out chemical signals that tell the primordial germ cells which sort of gamete to become. They’re ready to do that around week six of gestation, by which time the germ cells have migrated across the developing embryo to their destination. For yes, that development involves not merely cell division but also cell movement, a physical sorting in space to arrange the parts in the proper disposition.
Germ cells were first postulated by the German zoologist August Weismann in his 1892 book The Germ-Plasm: A Theory of Heredity. As that title suggests, this was a hypothesis as much about evolution as about embryology. The “plasm” here reflects the widespread notion, before Boveri and Sutton’s chromosomal theory of inheritance, that heredity was somehow transmitted via the “protoplasm” substance inside cells. As we saw earlier, Charles Darwin speculated that the particles responsible for inheritance, which he called gemmules, were collected from the body’s cells and transmitted via sperm and egg. Weismann was a staunch advocate of Darwinism, but he was convinced that there was a fundamental distinction between the somatic cells that made up the body’s tissues and the special cells called germ cells that gave rise to gametes. Any changes to the “plasm” of somatic cells could therefore play no part in heredity. To demonstrate that changes to the body of an organism are not inherited, Weismann cut off the tails of hundreds of mice and followed their offspring for five generations, each time removing the tails. Not once were any offspring born without tails.6 Any notion that “acquired characteristics” could be inherited, as in the pre-Darwinian theory of evolution proposed in the early nineteenth century by Jean-Baptiste Lamarck, could no longer be sustained.
In Weismann’s view, then, somatic cells are irrelevant to evolution. They are destined to die with the organism. But germ cells beget more germ cells – there is an unbroken line of germ cells (the germ line) down through the generations. It’s often said that the germ cells are thus immortal, although that’s an odd formulation – by that definition, we are all immortal simply by virtue of being able (if indeed we are) to produce offspring.
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In the story of how to make a human “the natural way”, the fertilized egg is often portrayed as the end – at least, until the happy day that the baby emerges. All our traditional stories of people-making rely on that quantum leap from fertilization to birth. The dire moral warnings about pregnancy that loomed over adolescence (and in some cultures still do) make this the equation: bring together sperm and egg and you’ll get a baby! It’s a warning (sometimes needed, for sure) to experimenting teenagers, but becomes more like a promise in the narrative of IVF: to make that longed-for baby, all you need to do is unite the gametes. And if it doesn’t turn out that way, something has gone wrong. There is a single and inevitable road from fertilized egg to infant, and anything else is an aberration.
This is misleading. To put it starkly, most acts of non-protected penetrative sexual intercourse do not produce a baby – and when I say most, I mean 99.9 per cent. Even most fertilized eggs do not become babies – about 2 to 3 in 10 confirmed pregnancies abort spontaneously in miscarriage, but even those figures mask the 75 per cent or so of fertilized eggs that never get to the point of registering as a pregnancy at all, either because they don’t develop into a multi-celled embryo or because the embryo fails to implant in the uterus. That’s a puzzling thing about humans: we are unusually poor, within the animal kingdom, at reproducing. You have to wonder whether all the attention we give to sex is because we are so spectacularly bad at getting results from it.
Even to say “bad” is perhaps to collude with the moral imperative of the fertilized-egg-to-infant story; let’s just say that we are an anomaly, for reasons imperfectly understood. This calls into question the idea that sex really is “for” reproduction, as some religious moralists insist. If we were inclined to see procreation as a divine gift and imperative, one would at least need to grant that God expects us to have a heck of a lot of rehearsal.
The baby grows, of course, from a fetus: even children’s books tell us that. But in the common view the fetus is simply a baby – a person – that has not yet fully developed. Its proportions might be a little odd, its limbs blunter, but it is recognizably human. The classic images made in the mid-1960s by Swedish photographer Lennart Nilsson and presented in the book A Child Is Born (1965), have defined the view of our in utero existence ever since. They show the fetus floating freely in space, often lacking even an umbilical cord, like the iconic image from Stanley Kubrick’s 2001: A Space Odyssey three years later. Perhaps this “child” even sucks its thumb. But these images were actually made by artful arrangement of aborted fetuses – they were not in fact living organisms at all, much less in utero. They were curated to tell a reassuring story. (At least, so it might seem until you realize that it’s a story in which the mother has been edited out.)
By the time a fetus looks even vaguely human (which is what, loosely speaking, distinguishes it from an embryo), most of the important stuff has happened. Most of the dangerous hurdles have been cleared. And most importantly, the developing organism is already anthropomorphic, relieving us from any need to grapple with the strangeness of an entity evidently made of cells, which we might want to call human but would struggle to justify that intuition.
Yet it is the early embryo that reveals the true versatility, the genius, of our cells – and the unfamiliarity of the moment when those cells are not merely what we are made of, but what we are.
It might surprise you to discover – it surprised me – that when a woman first has a fertilized egg (a zygote) inside her body she is not technically pregnant. This is not some perverse biomedical fine print; it simply makes no sense to see things otherwise. A pregnancy test would show nothing, nor will it for the first four days or so after fertilization. The zygote divides by mitosis into two, then four, then eight cells and so on, and at this point these cells can form all the tissues needed in the embryo: they are called stem cells and are said to be totipotent.
In other words, every one of these cells could potentially become a separate embryo. In the early days of embryology, that was by no means clear. The German zoologist Wilhelm Roux thought, for example, that cells are headed towards different fates from the first division of the zygote. In 1888, he reported experiments on frog embryos at the two and four-cell stage, in which he destroyed one of the cells by lancing it with a hot needle. A single remaining cell from a two-cell embryo would then, he said, grow into a half-embryo, suggesting that it had even at that stage become assigned as the progenitor of that part of the body plan alone.
But