How does multicellularity evolve? Scientists who study a family of green algae that includes unicellular Chlamydomonas and multicellular Volvox are beginning to find answers to this question.

(A) Young Volvox adult, with about 2,000 small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix (ECM), just under the somatic cell layer.