Ancient animals got a rise out of oxygen.

If today's insects scare you, consider the Goliath flies that ruled the skies during Earth's Carboniferous period 300 million years ago. Some souped up dragonflies had wingspans rivaling those of crows. Mayflies grew to sparrow size. On the ground, silverfish, scorpions, and other arthropods reached epic dimensions.

Blame it on the air, says a controversial new theory. A team of physiologists and biomechanics experts hypothesizes that elevated concentrations of oxygen in the Carboniferous atmosphere helped some invertebrates evolve bodies much larger than those seen today. "Oxygen was an enabling factor, just like fertilizer in your garden," explains Jeffrey B. Graham, a marine biologist at the Scripps Institution of Oceanography in La Jolla, Calif. Graham developed the theory along with Nancy M. Aguilar of Scripps, Robert Dudley of the University of Texas at Austin, and Carl Gans of the University of Michigan in Ann Arbor.

Geochemists have long maintained that the proportion of oxygen in the atmosphere has varied on a geologic timescale, but few biologists have considered how such changes would have affected life. In the May 11 Nature, Graham and his coworkers propose that enhanced oxygen concentrations profoundly altered evolution.

"I've been waiting for some paleontologists to worry about oxygen. Somebody finally has," says geochemist Robert A. Berner of Yale University. Six years ago, Berner suggested the idea of a Carboniferous atmosphere with oxygen concentrations of as much as 35 percent, compared to today's 21 percent. Oxygen accumulated in the atmosphere then, according to Berner, because much of the dead vegetation sank into the abundant swamps instead of decomposing--a process that pulls oxygen from the air. At the start of the Permian period, about 286 million years ago, oxygen values started falling, eventually dipping to 15 percent.

Berner's model remains uncertain because scientists lack direct evidence about ancient air. But Graham and his coworkers note that if oxygen amounts did indeed increase during the Carboniferous, it may explain why giant dragonflies appeared at that time. Unlike vertebrates, whose circulatory system delivers oxygen to inner cells, insects absorb atmospheric gases through a network of branching tubes that carries air into the body. Interior cells receive oxygen through diffusion, a system that limits the size of invertebrates, the researchers maintain. With more oxygen in the air, invertebrates could have developed bigger bodies.

The additional oxygen would also have helped early flying insects, the scientists suggest. Although the first flyers lacked aerodynamic wings, they would have received extra lift from the denser atmosphere.

Our own ancestors may have benefited as well. During the Carboniferous, four-limbed amphibians clambered out of the swamps and started colonizing the continents. The extra oxygen in the atmosphere may have boosted the efficiency of their primitive lungs, suggest Graham and his colleagues. With little evidence to support such contentions, the new theory will draw fire from many researchers.

"The fossil record of terrestrial vertebrates does not show convincing correlation with the postulated increase and subsequent decrease in atmospheric oxygen," comments vertebrate paleontologist Robert L. Carroll of McGill University in Montreal.

But Conrad C. Labandeira, who studies insect evolution at the Smithsonian Institution in Washington, D.C., calls the new idea intriguing and worthy of testing through laboratory experiments. Graham and his colleagues hope to examine how insects function in chambers with extra oxygen.