The origin and evolution of segmentation (Reprinted from Trends in Biochemical Science, vol 12, Dec., 1999)

Arthropods, annelids and chordates all possess segments. It remains unclear , however, whether the segments of these animals evolved independently or i nstead were derived from a common ancestor. Considering this question invol ves examining not only the similarities and differences in the process of s egmentation between these phyla, but also how this process varies within ph yla, where the homology of segments is generally accepted. This article rev iews what is known about the segmentation process and considers various pro posals to explain its evolution.