How to Build a Human: Adventures in How We Are Made and Who We Are. Philip Ball
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child’s cells may enter the body of the mother.
In fact, some degree of exchange, known as microchimerism, is normal. Many women who are pregnant with sons, for example, acquire some cells with Y chromosomes. What surprised researchers when this exchange came to light in the 1990s is that these fetal cells may persist and remain active, albeit at a low level, in the mother for many years after the child is born. But while microchimerism affects just a small proportion of the body’s cells, a process like embryo fusion to create a tetragametic chimera makes a person who is genetically heterodox through and through: some organs and body parts come from one embryo, some from another.
If I were a mixture of the flesh of “two people”, what would that make me? It is tempting to say that such an individual is indeed a mixture of two people in one. But that seems a profoundly odd and unhelpful way to look at the matter, for in what meaningful sense were those twin embryos “people” before they fused? These clusters of cells have only given rise to a single individual. This is just one of the ways in which quirks of developmental and reproductive biology undermine a simplistic determination to invest an embryo with unique personhood. A “person” is a higher-order concept, not to be reduced to genes or cells.
Still, our habits of thought and even our laws are challenged by these discoveries. DNA analysis of a tissue sample from a tetragametic woman may fail to confirm that her biological children are “hers”, if the sample does not happen to share chromosomes with her gametes. Such cases have come to light through genetic testing to confirm maternity in applications for social welfare benefits in the United States, leading to harrowing accusations of false claims of parentage. Some of these cases have highlighted how strongly we invest notions of personhood and identity in the character of our flesh and genes. In his book She Has Her Mother’s Laugh, science writer Carl Zimmer describes two such cases, saying that the discovery of their chimerism left these women with “haunting questions not only about their families but about themselves”. One woman wondered if she was only partly the mother of her children, despite having given birth to them, and partly their aunt. “I felt that part of me hadn’t passed on to them,” said the other woman. As Zimmer explains:
We use words like sister and aunt as if they describe rigid laws of biology. But despite our genetic essentialism, these laws are really only rules of thumb. Under the right conditions, they can be readily broken.
Yet I wonder. Not all cultures do use these words this way. It is common in Chinese society, for example, to call a close female friend of the family “aunt” even without any blood relation, and in the West “sisterhood” and “brotherhood” are widely used to express sympathetic bonds irrespective of sibling connections. Many cultures have a flexibility of familial relations that does not inevitably reduce them to blood and birth.
No, the problem here is not that biology destroys our traditional categories and concepts of human life, but that we too often now fall into the trap of imagining that biology can and should arbitrate on socially mediated questions of self and identity, family and kinship, sex and gender. Biology has a habit of declining that role, handing back (so I like to see it) the responsibility and saying, “you, not I, are the ones who care about these issues, so you must decide them for yourself.”
To insist xthat the embryo is “us” from its first instants is to some degree a displaced religious impulse. It announces a moment of creation, as profound and abrupt as the fiat lux of the Old Testament. Because, let’s face it, the world did begin when you did, and it will end when you do: that’s a universal, experiential human truth. Symmetry alone then seems to demand a beginning that is as abrupt and all-encompassing as the end – a moment, in those monotheistic traditions, when the soul enters the body to match the one when it leaves.
But this concept of the embryo denies the true wonder of our origin, and is another expression of the flight from flesh that has been going on for centuries. The assertion of the soul as an immaterial thing, pre-existing and eternal, is a pre-scientific attempt to deal with the incommensurability between the life we lead and the life in our cells.
For the latter is truly something to be astonished at. It is contiguous with the moment life first appeared on Earth. Life is passed like a baton between living things and is not created afresh with their own beginning. In arguments about abortion and embryo research, we talk about “when life begins”, but that’s not what we mean. Life only began once, around four billion years ago, and no one knows how. It continued in an unbroken thread from primal slime and algae to the oddly shaped metazoans of the Cambrian, through to the shrew-like ancestors of all mammals, and on to our apelike forebears walking upright and wielding stone tools, and finally – for this brief, glorious moment – here you are. Life is just passing through you, so enjoy it while you can.
The ambiguity, anxiety and angst that arise when we contemplate the life of the one-cell zygote and try to reconcile it with the human form in order to formulate laws and moral codes are consequences of our being assemblies of cells living in community. So it’s worth considering how that came about.
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If there was to be a competition for the least appealing organism in the world, slime moulds would be a strong contender. Bacteria get a bad press as mere “germs” to be expunged, but they also have a certain cachet too now that we know their presence in our gut is so beneficial and that they have such superpowers: metabolizing radioactive waste and oil spills, surviving in hot springs and so forth. Slime moulds, meanwhile, appear to be nothing more than their name suggests: a slightly disgusting smear of living matter whose purpose seems incidental to anything useful or inspiring in nature.
These organisms are members of a group called Mycetozoa. They are a type of amoebae, single-celled entities so “primitive” that for years microbiologists argued about whether they were closer to animals, fungi or plants. Modern genetic studies suggest that in evolutionary terms they are most closely related to the former two kingdoms, but they sit right at the boundaries – which is to say, the Mycetozoa became a distinct group around the same time in evolutionary history that animals, fungi and plants went their separate ways.
This is what makes slime moulds in fact deeply interesting. They offer a glimpse of what might have gone on when life began to get truly complex: when single-celled organisms evolved into multi-celled ones. In other words, when cell communities started to become superorganisms like us.
Amoebae played a significant role in the history of how we came to understand living matter. The word was coined originally to denote any microscopic organism that doesn’t have a fixed shape. Bacteria do: typically they are cigar-shaped, like round-ended tubes. But amoebae are shape-shifting blobs that move by extending a part of their bodies into pseudopods (“false feet”). The term “amoeboid” has entered everyday speech to denote that kind of amorphous, oozy mass.
The amoeba Proteus: a cell of no fixed shape.
But amoebae aren’t really a well-defined class of organism at all. There are types of amoebae that are truly animals, or fungi, or plants, as well as protozoans, which are single-celled organisms more “complicated” than bacteria. (I’ll say shortly what I mean by that.) Some amoebae are parasites; some are slime moulds. Even some of our own cells display amoeboid behaviour, such as white blood cells that “eat” bacteria and other pathogens by engulfing and absorbing them.
Amoebae were first reported in the eighteenth century in studies of seawater under the microscope. In 1841, the French biologist Félix Dujardin