Separate Evolutionary Origins Research Articles

Bismuth staining of Golgi complex is a characteristic arthropod feature lacking in Peripatus

THE traditional view for the origin of arthropods is that an exoskeleton has evolved once and the three major modern groups, Crustacea (for example, crabs), Chelicerata (spiders) and Uniramia (insects) had a common ancestor in the precambrian. This ancestor was itself related to primitive Onychophora (present day representative, Peripatus) and ultimately to predecessors of the annelids1–4. But, arthropods may have had two, three or several separate evolutionary origins if the external features thought to unite the groups have evolved in response to functionally similar requirements3,5. Studies on trilobite skeleton, for example, suggest two independent ancestral groups whose members were not themselves arthropods. These gave rise on the one hand to the Uniramia and on the other to the chelicerates which were linked to Crustacea through ancestral trilobites6. The discovery of a cell component characteristic of all cell types in insects7 prompted a systematic study of its distribution, to see if it could be of help in determining arthropod evolutionary relationships. We report here that a characteristic feature of arthropods, bismuth staining of the Golgi complex beads, is absent in Peripatus, thus supporting the established view of arthropod evolution.