It is an expanded version of a short story of the same title, originally published in the June 1983 issue of Analog and the winner of both the 1983 Nebula and 1984 Hugo awards for Best Novelette.

Inside Ulam's body, the noocytes multiply and evolve rapidly, altering their own genetic material and quickly becoming self-aware.

Through infection, conversion, and assimilation of humans and other organisms, the cells eventually aggregate most of the biosphere of North America into a region 7,000 km (4,300 mi) wide.

This civilization, which incorporates both the evolved noocytes and recently assimilated conventional humans, is eventually forced to abandon the normal plane of existence in favor of one in which thought does not require a physical substrate.

The only problem is that it's nigglingly close to the conclusion reached by an author extrapolating from a different start-point: Arthur C. Clarke in Childhood's End.