A Suit of Armor

Sometime, somewhere in the Precambrian era, a major milestone in the evolution of life on Earth took place. The soft cuticle in the segmented ancestor of animals we now call arthropods was hardened by the deposition of additional protein and an inert polysaccharide called chitin. The cuticular exoskeleton offered some protection against predators and other environmental hazards, and it conferred on its possessors a formidable array of other selective advantages. For example, a hardened cuticle provided a more secure site for muscle attachment, allowed adjacent segments and joints to function as levers, and vastly improved the potential for rapid locomotion, including flight. Of course, a suit of armor could not be uniformly hard; the animal would be as immobile as the rusted Tin Woodsman in the Wizard of Oz. Stiff sections of cuticle were separated from each other by thin, flexible sections, which formed sutures and joints. The cuticular exoskeleton had enormous evolutionary potential. Jointed extensions on each segment became appendages.

As the hardened cuticle evolved, or perhaps concurrently with it, many other changes took place in the bodies and life cycles of protoarthropods. Growth required a sequence of cuticular molts controlled by hormones. Coelomic compartments reduced their hydrostatic skeletal function, perhaps causing a regression of the coelom and its replacement with an open system of sinuses (hemocoel). Motile cilia were lost. These changes and others are called “arthropodization.” Some zoologists argue that all changes in arthropodization followed from the development of a cuticular exoskeleton. If several different ancestors had independently evolved a cuticular exoskeleton, then they independently would have evolved the identical suite of characters we associate with arthropodization. If this were the case, the huge phylum we call Arthropoda would be in reality polyphyletic. We agree with other zoologists who feel that the weight of evidence still supports singlephylum status.