some bits of our cells have a different ancestry from the rest of the genome. But it’s not just that. Because of the way they exchange genetic material, prokaryotes have wantonly promiscuous genes, which can jump between ‘species’ with relative impunity. Indeed, the very concept of a species starts to lose its meaning in these life forms. As we go deeper and deeper in time, we might therefore expect to start tracing more and more genes back through radically different routes.
