Abstract
The Berkner and Marshall hypothesis suggests that the atmosphere was virtually anoxic until the late Precambrian, and that the abrupt and coincidental appearance in the fossil record of the major animal phyla reflects the rise of PO~2~ to a level that supported animal life. In a more recent version the rise of PO~2~ is supposed to permit previously softβbodied animals, which had arisen during the anoxic period, to form fossilizable hard parts. The animal kingdom is conceived as fundamentally and primitively anaerobic and only secondarily aerobic. These ideas were incorporated into the Thiobiotic Theory, which supposes that a meiofauna endemic to the anoxic sulfide layer in marine sediments is a relict community of the preβnormoxic Precambrian. This fauna consists of a number of simple metazoan phyla, including freeβliving platyhelminths (but not cnidarians) which this theory regards as representatives of the ancestral Eumetazoa.
The development of geological and ecological thinking about these hypotheses is summarized and the pertinent physiological evidence for and against them is briefly reviewed. I conclude that the available information does not support either the Berkner and Marshall Hypothesis or the Thiobiotic Theory, and is inconclusive on the identity of the ancestral animals. Moreover, several aspects of respiratory and metabolic biology are far more consistent with a fundamentally aerobic than anaerobic way of animal life.