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See Inside February 2011

When Earth Was a Snowball: Global Glaciers May Have Sparked Evolutionary Burst

New evidence links melting glaciers with the evolution of life



Courtesy of Noel Gourmelen

It took a mere 85 million years—the geologic blink of an eye—for animals to evolve and radiate out over much of the world’s land and oceans. Although fossil records and molecular biology have provided much information on the spread of animal life, scientists have not been able to figure out exactly what sparked this massive diversification. New research shows that nutrient-rich runoff from massive melting glaciers may have provided the extra energy needed to fuel this dramatic evolution.

In the 1990s several scientists found evidence that much of Earth’s surface was covered with glaciers 635 million to 750 million years ago. They called their hypothesis “Snowball Earth.” Since then, many other studies have confirmed that it once may have been possible to ski from pole to pole. As the glaciers advanced, they scraped off the top layer of rock and soil on land and then released minerals and nutrients into the ocean as they retreated.

The moment of glacial runoff coincided with the rapid evolution of animal life. Biogeochemists Timothy W. Lyons and Noah J. Planavsky of the University of California, Riverside, knew as much, but what they could not understand was whether the runoff contained enough nutrients to spur animal evolution and whether the appearance of animals in the fossil record at this time was merely a coincidence. If they could measure phosphorus, a key nutrient in biological systems known to support the growth of microbes and algae, Lyons and Planavsky could surmise the total nutrient concentration. The problem was finding a way to measure the phosphate concentrations of oceans nearly one billion years old.

Lyons and his colleagues realized they could use iron-rich deposits from ancient, low-oxygen oceans high in dissolved iron to estimate how much phosphorus was in the water. “These iron-rich deposits scavenge phosphates in a very predictable and well-understood way,” says Lyons, who published his and Planavsky’s research recently in Nature. (Scientific American is part of Nature Publishing Group.) This discovery enabled the researchers to calculate marine phosphate concentration based on the phosphates in the iron-rich deposits. As the team expected, phosphate levels spiked in seven different samples around the world as the glaciers melted.

“A big pulse of phosphate would have supported a lot of life in the ocean,” Lyons says. This phosphate buffet would have encouraged the abundance of oxygen-producing algae and other organisms and increased oxygen levels spurring the explosion of animal evolution.

“This study links Earth’s geochemical systems with the evolution of life,” says Gabriel Filippelli of Indiana and Purdue Universities, who was not involved with the study. It also shows how one big chill might have changed life on Earth forever.

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