About 350 million years ago, evolution took one small step for fish, and a giant leap for every terrestrial animal since. It may all have been made possible by plants.

Prehistoric oxygen levels extrapolated from ancient mineral sediments suggest aquatic life went into overdrive after plants boosted atmospheric oxygen levels. Perhaps oceans became so fiercely competitive that some fish sought safe haven outside them.

Some scientists have proposed as much, but the new research, published Sept. 28 in the Proceedings of the National Academy of Sciences, provides the first solid evidence.

"Before this paper, there was essentially no experimental evidence for how oxygen accumulated through animal history. It was only predicted by theory," said Tais Dahl, an evolutionary biologist at the University of Southern Denmark's Nordic Center for Earth Evolution.

Dahl and study co-author Donald Canfield analyzed prehistoric seafloor samples gathered from around the world and dating to between 1.7 billion to 400 million years past. They were especially interested in molybdenum, a mineral widespread in Earth's soil and carried off by erosion. At sea, the particles circulate for about one million years before coming to sedimentary rest.

As they circulate, the particles' atomic configurations are subtly changed by concentrations of atmospheric and aquatic oxygen, making their stratified deposits a record of Earth's oxygen composition. According to Dahl, it's a far more detailed record than can be read in carbon, the traditional source of oxygen extrapolation.

"As you walk back in time, the uncertainty of those models becomes larger and larger," he said. "If you're off by a little bit at a given time, you end up being completely off." Indeterminate carbon records have given rise to two competing interpretations of Earth's prehistoric oxygen levels, and thus the evolution of its life.

Each accepts that planetary oxygen levels first spiked about 550 million years ago, coincident with the first mobile, symmetrical life forms – a benchmark in animal complexity, set until then by sponges. But after that, the interpretations diverge.

The first, traditional view holds that planetary oxygen levels continued to rise steadily, reaching near-contemporary levels well before Earth's life diversified again, some 400 million years ago. In this narrative, it was only a matter of time – another 50 million years, give or take – before a few lagoon-dwelling creatures ventured onto land. Terrestrial life was a clockwork eventuality. Plants provided more oxygen, but weren't essential.

According to the other interpretation, oxygen levels stayed steady from 550 million to 400 million years ago, when the forerunners of modern plants evolved and flourished. Only then did oxygen jump, allowing fish – until then a small, relatively insignificant part of the animal kingdom – to take large, highly predatory forms.

This is the interpretation supported by Dahl and Canfield's molybdenum patterns. Plants, which release oxygen both while they live and as they decompose, are the key.

"The low oxygen level early in animal history limited evolution for fish. After this second oxygenation event, we begin to see large, predatory fish up to 30 feet long," said Dahl, who speculated that resulting predatory pressures could have driven the first amphibians onto land. "In principle, you could connect this all."

Note 10/12/2010: The original article did not accurately convey the speculative nature of causal links between ecological competition in Earth's early oceans and evolution of land animals. My apologies for this mistake.

Images: 1) Dunkleosteus, a 30-foot-long fish with some of history's most powerful jaws, lived just before the first land animals./University of Texas, Arlington. 2) Tiktaalik, considered to be a bridge between aquatic and terrestrial vertebrates./Zina Deretsky, National Science Foundation. 3) A leaf from a gingko tree, remarkably little-changed in 350 million years./Flickr, Geishaboy500.

See Also:

Citation: "Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish." By Tais W. Dahl, Emma U. Hammarlund, Ariel D. Anbare, David P. G. Bond, Benjamin C. Gill, Gwyneth W. Gordon, Andrew H. Knoll, Arne T. Nielsen, Niels H. Schovsbo, and Donald E. Canfield. Proceedings of the National Academy of Sciences, Vol. 107 No. 39, September 28, 2010.

Brandon Keim'sTwitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on an ecological tipping point project.