How did multicellularity evolve?

Multicellularity arose independently at least 25 times across the tree of life. The genetic and developmental changes that turn loose aggregations of cells into integrated organisms — with differentiated cell types, programmed death and germ-soma distinction — are partly understood from comparative genomics but not predictable.

Experimental evolution in yeast has produced multicellular lineages in months, hinting at the speed of the transition.