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 O_2 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 O_2 levels capable of supporting animal physiology were present more than 500 million years before the appearance of animals (6).