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We are not precisely who we thought we were. We are composite creatures, and our ancestry seems to arise from a dark zone of the living world, a group of creatures about which science, until recent decades, was ignorant. Evolution is trickier, far more intricate, than we had realized. The tree of life is more tangled. Genes don't move just vertically. They can also pass laterally across species boundaries, across wider gaps, even between different kingdoms of life, and some have come sideways into our own lineage — the primate lineage — from unsuspected, nonprimate sources. It's the genetic equivalent of a blood transfusion or (different metaphor, preferred by some scientists) an infection that transforms identity. "Infective heredity." I'll say more about that in its place. | 
