The thesis is developed that a low oxygen level Precambrian atmosphere presented early-evolving metazoan organisms with physiological connective tissue priorities resulting from the important molecular oxygen requirements in the biosynthesis of collagen hydroxyproline. Shells, cuticles, and carapaces which are not mandatory metazoan prerequisites but which directly or indirectly demand substantial connective tissue collagen are oxygen expensive, low priority features. A marked increase in atmospheric oxygen level near the beginning of the Paleozoic would eliminate oxygen-collagen priorities simultaneously and on a world-wide basis in all metazoan stocks providing evolutionary pressure for enlarged musculatures and associated "hard parts." This could explain the sudden presence in the fossil record of the early Cambrian of advanced and diversified metazoans, the earlier forms of which were essentially unpreservable.