The Protomolecule

The protomolecule is a replicating agent that an alien civilisation launched at Earth 2.5 billion years ago to hijack its early biosphere; it was captured instead by Saturn's moon Phoebe. Protogen discovers it and releases it on Eros, and it assimilates the station before Miller steers Eros into Venus. There it builds a vast structure that launches beyond the orbit of Uranus and becomes the Ring, a gateway to a network of gates. In Caliban's War it is used to build soldiers, and Winston Duarte later uses it on himself.