Political revolutionaries turn the world upside down. Scientific ones more often turn it inside out. And that, almost literally, is happening to the idea of what, biologically speaking, a human being is. The traditional view is that a human body is a collection of 10 trillion cells which are themselves the products of 23,000 genes. If the revolutionaries are correct, these numbers radically underestimate the truth. For in the nooks and crannies of every human being, and especially in his or her guts, dwells the microbiome: 100 trillion bacteria of several hundred species bearing 3m non-human genes. The biological Robespierres believe these should count, too; that humans are not single organisms, but superorganisms made up of lots of smaller organisms working together.
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