Breathing room for early animals

If life has been present on our planet for much of its history—more than three and a half billion years—why did it take so long for animals to appear? This question emerged long ago from the incipient fabric of the fossil record, which embodies the sudden appearance of metazoans in sedimentary strata deposited near the end of Precambrian time—and scientists have wrestled with this issue now for well over a hundred years (1). For the past 50 or so years, the most popular and pertinacious hypotheses have concerned atmospheric dioxygen (2⇓⇓–5). Because of their aerobic physiology, the idea was that O2 levels were perhaps too low to have supported animals until sometime near the end of Precambrian time, and that rising O2 levels thereafter provided a quantum evolutionary leap of sorts for aerobic biology culminating in the “late” evolution of animals (3). A recent study of middle Proterozoic sedimentary rocks in China, however, favors a different view—suggesting that O2 levels capable of supporting animal physiology were present more than 500 million years before the appearance of animals (6). Animals evolved in an aerobic world; that much is clear. They need large O2 fluxes to support the metabolic demands of their multicellular body plans (7), and O2 is used copiously during the biosynthesis of their tissues (8). Exactly how much dioxygen did early animals require? Phylogenetic relationships near the base of the Metazoa remain unsettled, but based on the degree to which Porifera (sponges) provide a useful measure of the aerobic requirements of early animals, experimental observations illustrate that the O2 concentrations needed appear to be exceptionally low, perhaps 0.5–4% present atmospheric levels (PAL) (Fig. 1) (9). A similar view was garnered from environmental observations of oxygen minimum zones (OMZs)—subsurface …