Earth's 4.5-billion-year history is filled with several turning points when temperatures changed dramatically, asteroids bombarded the planet and life forms came and disappeared. But one of the biggest moments in Earth's lifetime is the Cambrian explosion of life, roughly 540 million years ago, when complex, multi-cellular life burst out all over the planet.

While scientists can pinpoint this pivotal period as leading to life as we know it today, it is not completely understood what caused the Cambrian explosion of life.

Now geologists Martin Kennedy of UC Riverside and L. Paul Knauth of Arizona State University believe they have found the trigger for the Cambrian explosion: a massive greening of the planet by non-vascular plants (plants with no roots, stems or leaves) that began roughly 700 million years ago.

This period, the researchers argue, set the table for the ensuing explosion of life through the development of early soil that sequestered carbon, led to the build up of oxygen and allowed higher life forms to evolve.

"Our evidence suggests this very important step took place just at the end of the Precambrian-that is just before animals appear in the fossil record," said Kennedy, a professor of geology in the Department of Earth Sciences and the director of the Graduate Program in Global Climate and Environmental Change. "It's likely not a coincidence because one of the important effects on the biosphere that the terrestrial realm has today is atmospheric oxygen control. Without such control, the atmosphere would not have had sufficient oxygen to allow animals to breath."

"Today, the terrestrial biota-trees and plants-are critical for controlling the atmospheric composition, but it hasn't always been that way," Kennedy said. "For most of Earth's history the terrestrial surface was largely barren."

According to him and Knauth, Earth became extensively occupied by photosynthesizing organisms during the end of the Precambrian. This greening was a key element in transforming the Precambrian world - which featured low oxygen levels and simple, bacteria dominant life forms - into the kind of world seen today with abundant oxygen and higher forms of plant and animal life.

Kennedy explained that the main contribution of a terrestrial biota is to produce soils, which, through weathering reactions, play an important role in controlling oxygen and carbon dioxide (and thus temperature) of the planet.

"It is interesting, too, that the profound temperature swings of the Earth leading up to the end of the Precambrian abruptly ceased," he said. "This is in keeping with our hypothesis of terrestrialization."

In order to understand what happened on Earth during the Precambrian, scientists have studied the isotopic composition of limestone that formed during that period. Limestone, a sedimentary rock composed largely of calcium carbonate, has three oxygen atoms for every carbon atom.

Knauth and Kennedy gathered all of the published measurements of carbon and oxygen isotopes in Precambrian limestone, and carefully plotted carbon isotopic data against oxygen isotopic data, a process that took three years. This exercise helped them formulate a very different type of scenario for what led to complex life on Earth and they were able to come up with a simple, alternative view of the thousands of carbon isotope measurements that previously had been taken by scientists as evidence of geochemical catastrophes in the ocean.

"With the 13,000 isotopic data points we present in our paper we could record the magnitude of the biological impact on the planet as well as the timing," Kennedy said. "We applied a new interpretation to all of this data that shifts the focus from oceanic controls-currently popular to explain the end Precambrian changes in the biosphere-to the terrestrial realm and its influence."

Rather than a world subject to periods of life-altering catastrophes, Kennedy and Knauth began to see a world that first greened up with primitive plants. This greening, they conclude, made soils which sequestered carbon and allowed oxygen to rise and get dissolved into sea water, leading eventually to the Cambrian explosion.

NASA and the U.S. National Science Foundation funded the research.