How to Build a Human: Adventures in How We Are Made and Who We Are. Philip Ball
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divorced are partly heritable, meaning that the differences between individuals can be partly traced to differences in their genes.
Far from alarming us, this shouldn’t surprise us. We have always been content to believe that, for example, some people seem blessed with talents that can’t obviously be explained by their environment and upbringing alone. By the same token, some seem hardwired to find particular tasks challenging, such as reading or spatial coordination.
Yet perhaps because we have a strong sense of personal agency, autonomy and free will, many people are disturbed by the idea that there are molecules in our cells that are pulling our strings. They needn’t worry. It is precisely because genetic propensities are filtered, interpreted and modified by the process of growing a human cell by cell that they don’t fully determine how our bodies turn out, let alone how our brains get wired … let alone how we actually behave.
Genes supply the raw material for developing our basic cognitive capabilities – to put it crudely, they are a key part of what allows most human embryos to grow into bodies that can see, hear, taste, that have minds and inclinations. But how they exert their effects is very, very complicated. In particular, very few genes affect one trait alone. Most genes have influences on many traits. Some traits, both behavioural and medical (such as susceptibility to heart disease), seem to be influenced – in ways that are imperceptible gene by gene, but detectable when their effects are added up – by most of the genome. That’s why the popular notion of a “gene for” some behavioural trait is misguided. In fact, it means that there may be no meaningful “causal” narrative that can take us from particular genes to behaviours.
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This is precisely why we need to resist seductively simple metaphors in genetics: blueprints, selfish genes, “genes for”. Of course, science always needs to reduce complex ideas and processes to simpler narratives if it is going to communicate to a broader audience. But I’ve yet to see a metaphor in genomics that does not risk distorting or misrepresenting the truth, so far as we currently know it. Fortunately, I do not think this matters for talking about the roles of genes in making a human. We will deal with those roles as they arise, without resorting to any overarching story about what genes “do”.
I haven’t even told you yet the worst of it, though. It’s not simply difficult to articulate clearly what, in the scheme of growing humans the natural way, genes do. For we don’t exactly know how to define a gene at all.
This isn’t a failure of biology, but a strength. It’s tempting to imagine that science can’t be fully coherent if it can’t define its key terms. But the most fundamental concepts are in fact almost invariably a little hazy. Physicists can’t say too precisely or completely what time, space, mass and energy are. Biologists can’t say what a gene or a species is. For that matter, chemists aren’t fully agreed on what an element or a chemical bond is. In all cases, these terms arose because it seemed as though they had a very specific meaning, but when we looked more closely we found fuzzy edges. Yet the reason we coined the terms in the first place was because they were good for thinking with.
That remains true. A gene is a useful idea, perhaps in much the same way as words like “family” and “love” and “democracy” are useful: they are vessels for ideas that enable us to have useful conversations. They are usually precise enough.
Here, then, is a definition good enough to let us talk about genes in the role of growing a human from cells. Think of a gene as a piece of DNA from which a cell is able to make a particular molecule, or group of molecules, that it needs in order to function. By passing on copies of genes, cells can pass on that information so that the progeny doesn’t have to rediscover it from scratch.
If you raised your eyebrows at “so that”, good for you. In such phrases, biology is given a false purpose, an illusory sense that it pursues goals. It is nigh on impossible to talk about biology – about growth, development, evolution – without some mention of aims. Try to remember that this is always mere metaphor. The way that the laws of the physical world have played out on our planet is such that entities called cells have appeared that have a propensity to pass on genes to copies of themselves. This is remarkable and marvellous. No one really understands why it happens – why reproduction, inheritance and evolution is possible – and that’s why we find it necessary to tell stories about it. All we can say is that there is absolutely nothing that forces us to invoke any supernatural explanation for it. The gaps that remain would make an extremely peculiar shape into which such an account might be squeezed.
Here’s another thing worth knowing about genes: a gene on its own is useless. It can’t replicate,10 it can’t even do the job that evolution “appears” to have given it. Frankly, there is no real point in calling a gene on its own a “gene” at all: the name connotes an ability to (re)produce, but a lone “gene” is sterile, just a molecule that happens to resemble a part of the DNA in a chromosome. It’s common to say that a gene is a piece of DNA with a particular sequence, but the truth is that such a physical entity only becomes a gene in the context of a living system: a cell, at minimum. Genes are central ingredients of life, but by the time you reach the level of the gene there is nothing left that is meaningfully alive.
No, life starts with the cell. And that’s why a gene only has meaning by virtue of its situation in a cell. Does this, then, mean that the cell is more fundamental to biology than the gene? You might as well ask if words are more fundamental to literature than stories. It is “stories” that supply the context through which words acquire meaning, making them more than random sounds or marks on paper.
And by “context” here I don’t just mean that a gene has to be in a cell in order to represent any biologically meaningful information. I mean also that, for example, the history of the cell, and of the entire organism, might matter to the function a gene has. A gene that is “active” at one point in the organism’s growth might represent a quite different message – have a different implication – than at a later or earlier point. Yet the molecular machine (the protein) encoded by the gene may be identical in the two cases. The gene doesn’t change, but the “instruction” it represents does.
You might compare it to the exclamation “Stop it!” Is that an instruction? Well, of course you don’t know from that enunciation alone what it is you are supposed to stop, but perhaps you might regard it as a generic instruction to desist from the activity you’re engaged in. But what if you hear someone shout “Stop it!” as you see a football rolling towards a cliff edge? Is that an instruction to desist in anything, or on the contrary an injunction to action? You need to know the context.
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The gene-centred narrative of life is just one example of our urge to somehow capture the essence of this complicated, astounding process – to be able to say “life starts here!” Science’s reductionist impulse gets a bad press, but breaking complicated things down into simpler ones is a tremendously powerful way of making sense of them. I think that what many people who complain about reductionism are reacting against is not so much this process of analysis – of taking apart – in itself, but the tendency then to assert “this is what really matters”. Science has sometimes been a little slow to recognize the problem with such assertions. When one group of physicists started insisting it was going to find a Theory of Everything – a set of fundamental laws from which the entire physical universe emerged – others pointed out that it would be nothing of the kind, because it would be useless in itself for predicting or explaining most of what we see in the world.
It’s not just that we should resist the temptation to see reductive analysis as a quest to identify what is more important/fundamental/real in the world. Sometimes the phenomenon you’re interested in only exists at a particular level in the hierarchy of scale, and is invisible above and below it. Go to quarks and you have lost chemistry. With genes and life it is not quite that extreme – but at the level of genes you are left with only a rather narrow view of some of the entities and processes that underpin this notion we call “life”. Life remains a meaningful idea from the macro level of the entire biosphere of our planet right down to the micro level of the cell. Within those bounds it encompasses a whole slew of factors: flows of energy