How to Grow a Human - Philip Ball

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under selective conditions, or ejecting it after sex. Some experiments on genetic outcomes of fertilization seem to imply the egg somehow selects sperm with a particular genotype, defying the conventional idea that once it gets to this stage the union of gametes is random.

      It’s by means of narratives about sperm and egg getting together that we instruct our children about the “facts of life” – the definitive phrase seemingly designed to fend off the awkward questions they might otherwise ask. Questions like: Why this way? For doesn’t it seem an awfully messy, contingent and chancy way for genes to propagate, requiring a costly investment in wardrobe, grooming products and expensive meals? If children knew that parthenogenesis (development of the ovum without fertilization) was a reproductive option in the animal kingdom, I suspect many would think it dreadfully unfair that it is not available to humans. (Not only children, actually.)

      So why these facts? Why go through the rigmarole of sex, if it doesn’t seem to be a biological sine qua non of reproduction?

      No one really knows the answer. The usual one is that sexual reproduction, by combining the genes of two individuals, permits a beneficial reshuffling that can help stave off genetic disease. Organisms like bacteria that reproduce by simple cell division, generating clones, will gradually accumulate gene mutations from one generation to the next. As most mutations are detrimental or at best have no discernible effect on fitness, this can’t be a good thing. But bacteria proliferate exponentially and rapidly, and it doesn’t much matter if many genetically disadvantaged lineages die off, so long as there are a few that acquire mutations that improve fitness. In a rapidly growing bacterial colony, the mutations give the population a chance to explore a significant amount of “gene space” and find good adaptations. It’s for much the same reason that bacteria have evolved mechanisms to transfer genes directly from one to the other in a non-hereditary process called horizontal gene transfer.

      Organisms that reproduce slowly and sparsely, like humans, don’t enjoy a bacterium’s capacity to “try out” many mutations. But sex is a way to spread them more rapidly, by allowing new combinations of gene variants to be created at a stroke from one generation to the next.

      Clonal reproduction is also risky as it tends to put all a population’s eggs in one basket (to rather abuse a metaphor). Along comes some virus that exploits a bacterium’s vulnerabilities, or a change in conditions such as drought or extreme cold, and the whole colony could be wiped out – unless a few individuals are fortunate enough to possess gene variants that can withstand the threat. That’s also why genetic diversity is good for a population. And again, sexual recombination of genomes provides some of that.

      So sex is a way for slowly reproducing organisms like us to eject “bad” genes and acquire “good” ones. It recommends that the organisms become dimorphic – that there be two distinct sexes, to ensure that an organism doesn’t end up combining its own sex cells, which would rather defeat the object. Or at any rate, there must be more than one sex: there’s no obvious reason why it should be limited to two, and indeed some fungi have thousands of different “sexes”.3

      From this basic requirement for a distinction between types of sex cell (gametes) stems all the rest of the exciting and confusing features of sexual dimorphism. It helps if the two sexes are distinguishable at a glance, to save the wasted effort (since generally it does cost time and effort, sometimes considerably so) of trying to mate with another individual with which that is not possible. (It also makes complete sense that those impulses and signals will not be equally strong, or present at all, in all individuals, making homosexuality a natural and common phenomenon in animals.)

      Once those differences exist, they are apt to get amplified, diversify, sometimes way out of proportion. If you’re going to have sex, it’s likely that your behavioural traits will evolve to let you spot the fittest partners and to advertise your own fitness. Each sex will evolve methods of assessing mates, and outward indicators of fitness will elicit attraction in the opposite sex. Some of these make physiological sense: a lot of muscle suggests dominant males with good survival skills, wide hips in females imply superior child-bearing capacity. Other sexual signals may end up being rather arbitrary – it’s not obvious why body hair on our male ancestors would of itself confer any survival advantage. (Perhaps this was an example of “useless” signalling of fitness, like the peacock’s tail?) Some displays are simply about standing out from the crowd, like exotic bird plumage. Other facets of sexual attractiveness may be subtle: it seems likely, for example, that symmetrical facial features indicate that one’s developmental processes, which commonly unfold independently in the mirror-image halves of the body, are robust against random variations, giving the individual better health prospects. For all the elaborateness of some mating rituals in other animals, they might – if they could – count themselves lucky that these sexual signals and responses don’t get refracted through culture as they do with humans to the point where it can all get overwhelmingly confusing.

      Now, this is certainly one way to talk about the evolution and origin of sex, but it invokes an uncomfortable amount of teleology. Sex doesn’t really exist in order to create genetic diversity. Nothing happens in evolution in order to produce a particular end result. It makes intuitive sense for us to speak like this, but the fact is that sex evolved because those early organisms that became able to fuse their cells and chromosomes somehow produced more robust populations than those that lacked this ability. Sexual reproduction might be a more or less inevitable consequence of evolution by natural selection, once it gives rise to a particular kind of complex organism, much as snowflakes are an inevitable consquence of the laws of physics and chemistry playing out in a particular environment. Evolutionary biologists say that sex is a successful evolutionary strategy, although this again imputes a sort of foresight to evolution that it doesn’t possess.

      While these arguments for the value of sex surely have a lot going for them, they can’t be the whole answer. Sex is not essential for all higher vertebrates. Parthenogenesis occurs in many different types of animal, including insects such as mites, bees and wasps, and some fish, reptiles and amphibians. In a few such cases, reproduction can happen either with sex or without. Sometimes that’s by design – for example, parthenogenesis is thought to be an option for mayflies as a defence against a lack of males. (The same useful trait arises in the women of the male-free society in Charlotte Perkins Gilman’s utopian feminist novel Herland (1915).) In other cases it occurs by accident, unfertilized eggs just happening rarely to develop into embryos. Komodo dragons are among the larger creatures that can reproduce this way.

      The reasons and mechanisms for parthenogenesis are varied and sometimes rather complicated. But one thing you can say for sure is that, in organisms for which it can take place, evolution has not seen fit to rule it out. To put it another way, there is nothing obviously necessary about sex, and assessing the benefits of sexual reproduction over other methods of propagating is a subtle and perhaps context-dependent business.4 As far as evolution is concerned, it is just a matter of whatever works.

      * * *

      There’s a complication with sex. Each of our body cells has two sets of chromosomes, and therefore dual copies of each gene, one inherited from each parent. But if one of these cells in a female simply fused with one from a male, the resulting cell would have four sets of chromosomes. That’s too many, and the cell couldn’t function properly. So organisms that reproduce sexually have evolved special kinds of cells that possess only one copy of each chromosome. These are the gametes, and they are found only in the gonads: the ovaries and testes.

      Gametes are made from a specialized type of cell called a germ cell. The germ cells have doubled chromosomes (they are said to be diploid) just like somatic cells, but in a special kind of cell division called meiosis they divide into gametes in which these chromosomes have been carefully segregated into two. Cells with just a single set of chromosomes are said to be haploid.

      Normal cell division (mitosis) involves replication of the chromosomes accompanied by their 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

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