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Abstract. 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.One of the most striking and enigmatic aspects of paleontology has been the sudden appearance of advanced and diversified metazoan organisms in the early Cambrian. This subject has been the object of considerable research and speculation and numerous hypotheses have been proposed to explain the phenomenon. Most of these hypotheses assume that advanced Precambrian Metazoa existed but have not been preserved in the fossil record. This lack of preservation has been attributed to the unavailability of CaCG3, acid oceans, metamorphism, "Lipalian" interval, or the absence of Precambrian coastal sediments. Simpson' has evaluated most of these suggestions, and more recently Cloud2 has summarized and discussed the problem at length. Among Cloud's more important conclusions are the following: (1) "There are as yet no records of unequivocal lMletazoa in rocks of undoubted Precambrian age." (2)