How to Build a Human: Adventures in How We Are Made and Who We Are. Philip Ball

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       FIRST INTERLUDE

       THE HUMAN SUPERORGANISM

       HOW CELLS BECAME COMMUNITIES

      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.

      * * *

      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.