I sometimes think about this at the scale of our bodies, mine and yours. We are more fluid and less predictable than we like to think about. Our tissues are full of species living cryptically in our folds. Not only do we share a common ancestor with the bacteria and fungi that inhabit us—as all life does—but our evolutionary journey cannot be disentangled from the needs and pressures of our microbial counterparts. Our bodies were not terra nullis, a blank slate taken over by microbes after natural selection sculpted us into our final form. There is no human species without accounting for a
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