Nor is it possible to define us genetically, as bodies made up of cells that share an identical genome—many of our symbiotic microbial partners are inherited from our mothers alongside our “own” DNA, and at points in our evolutionary history, microbial associates have permanently insinuated themselves into the cells of their hosts: Our mitochondria have their own genome as do plants’ chloroplasts, and at least eight percent of the human genome originated in viruses (we can even swap cells with other humans when we grow into “chimeras,” formed when mothers and fetuses exchange cells or genetic
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