For heavy-breathing animals like us, Earth was unlivable for most of its history. Only when oxygen filled the air and dissolved in the ocean more than half a billion years ago was the door opened to the evolution of animals much more active than a sponge. Now researchers may have found a surprising trigger for the rise of animals: globe-girdling ice ages that threatened to wipe out all life on Earth.
Biogeochemist Noah Planavsky of Woods Hole Oceanographic Institution in Massachusetts and his colleagues report online today in Nature finding a possible link between the "snowball Earth" glaciations—episodes in which ice encased or nearly encased the planet—and the rise of life-energizing oxygen more than 500 million years ago. They measured the phosphorus content of iron-rich minerals formed in ancient oceans as a gauge of the phosphorus content of seawater. During the past 3 billion years, phosphorus abundance varied little, they found, with one exception: a surge to levels several times higher than usual lasting from about 750 million years ago to about 635 million years ago.
A phosphorus surge at that time would fit nicely into an emerging scenario for animal evolution. Researchers have noted that two snowball-Earth episodes roughly coincided with the appearance of the earliest animals in the fossil record. That suggested that the glaciations triggered the evolutionary leap, but how?
In the scheme offered by Planavsky and his colleagues, the snowball ice sheets would, as their modern counterparts do, grind up continental rock that would release phosphorus when the glaciers retreated. That phosphorus would wash into the ocean, where it would fertilize algal blooms that could drive a surge in the production of organic matter and oxygen. And the added organic matter that settled into the mud on the ocean bottom would leave additional oxygen behind, eventually boosting atmospheric and oceanic oxygen.
An apparent glitch in that story is phosphorus's short lifetime in the ocean. According to one view, ice on a snowball Earth retreated only once, at the end of a 10-million-year-long glaciation. That would produce an ephemeral phosphorus pulse incapable of influencing evolution. But co-author and biogeochemist Timothy Lyons of the University of California, Riverside, says that even a single snowball Earth event could have produced elevated phosphorus concentrations, as ice sheets repeatedly advance and retreat during a snowball episode.
A phosphorus link between glaciations and oxygen, and therefore evolution, "is a fascinating possibility," says biogeochemist Donald Canfield of the University of Southern Denmark in Odense. "But they don't yet have the continuous [geologic] record that would prove it. In principle, it's a testable hypothesis; that's the work's value."