The natural world is full of great partnerships. Bacteria give animals the guts to digest all manner of otherwise inedible foods. Algae allow corals to harness the power of the sun and construct mighty reefs.  Ants cooperate to become mighty superorganisms. But the greatest partnership of all is far more ancient. It’s so old that we can only infer that it took place by looking for signals of history, embedded into the genomes of modern species. The details of how and when it happened are still the source of fierce debate but this was undoubtedly the most important merger in the history of life on Earth: a partnership between two simple cells that would underlie the rise