Life on Earth grew in spurts

By Robert S. Boyd, McClatchy Newspapers

Last update: October 27, 2009 - 10:08 AM

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WASHINGTON — Twice in the Earth's history, living creatures underwent astonishing growth spurts, and each time, new organisms emerged that were a million times larger than anything that had existed before.

Scientists say that's the way life on our planet expanded from tiny single-celled microbes billions of years ago to the ponderous whales and lofty sequoia trees that are today's biggest living things.

Rather than a gradual increase in maximum body size, as scientists used to think, they now think that growth was a two-step process. The first spurt happened about 1.85 billion years ago, and the second about 580 million years ago, long before dinosaurs walked the Earth.

Scientists say the main driver of each growth step was a massive increase in the supply of oxygen, which is needed to convert food to the additional energy required for larger, more complex life forms.

"The two most rapid increases in maximum size correspond closely with the two primary episodes of increase in the concentration of atmospheric oxygen," Jonathan Payne, a paleobiologist at Stanford University, reported in the Proceedings of the National Academy of Sciences.

"Size rules life," John Bonner, a retired professor of evolutionary biology at Princeton University, wrote in his book, "Why Size Matters." "It is the supreme and universal determinant of what any organism can be and can do."

Larger creatures are better able to capture prey, fight or escape predators and survive hard times. On the other hand, they need more food and water, depend on their mothers longer and are slower to adapt to environmental changes than their smaller cousins are.

Based on fossil evidence, biologists think that life on Earth began in the ocean at least 3.5 billion years ago, a billion years after the birth of our planet. The earliest microbes didn't need oxygen, but fed by scavenging molecules of carbon, iron, sulfur and other minerals they found in the sea.

After a billion or more years, pioneer organisms known as cyanobacteria — resembling today's pond scum — learned how to capture energy from the sun through photosynthesis. A byproduct was oxygen.

"All of the oxygen in the atmosphere ultimately exists because of the evolution of cyanobacteria," Payne said. "There is no other process on the planet that can generate oxygen in sufficient quantities."

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