he Cambrian "explosion" of multicellular animal life is one of the most significant evolutionary events in Earth's history. But what was it that jolted Earth system enough to prompt the evolution of animals? While we take the presence of oxygen in our atmosphere for granted, it was not always this way.
The Neoproterozoic era that preceded the Cambrian explosion of life was witness to a dramatic rise in oxygen levels. It has been widely assumed that the rise in atmospheric oxygen was the essential precursor to the evolution of animals. But the work of Graham Shields-Zhou and Lawrence Och of University College London shows that the rise of oxygen was chaotic and nonlinear. Tectonically, the Neoproterozoic Earth was in the throes of the breakup of a supercontinent, Rodinia, and climatically, it had plunged into a snowball state, with ice-covered oceans extending from pole to pole.
In their March GSA Today article, Shields-Zhou and Och summarize geochemical and biological data that suggests that oxygen-depleted waters characterized the scattered seas that lay trapped beneath this global ice sheet. It may well have been the ability to survive in this harsh and variable climate that constituted the vital first step in the evolution of animals.
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