ANN ARBOR—A sudden surge of oxygen in the Earth’s atmosphere occurring millions of years ago may have triggered a period of dramatic and unprecedented evolutionary change, including the appearance of the first creatures to crawl out of the ocean and begin living on land, according to Carl Gans, University of Michigan professor of biology.

In an article published in the May 11 issue of Nature, Gans and his colleagues explain how a denser, oxygen-rich atmosphere could have encouraged development of the extraordinary number and type of new species documented in fossil records from 280 million to 345 million years ago. Collaborators on the article are Robert Dudley of the University of Texas at Austin and Jeffrey B. Graham and Nancy M. Aguilar of the Scripps Institution of Oceanography.

” We are saying that the fossil record supports the hypothesis proposed by several geologists for an oxygen pulse during the late Paleozoic,” Gans said. ” The fossil record has many examples of animals living during this period which could not have existed without an oxygen-rich atmosphere. ”

Most scientists believe the relative amount of oxygen in our atmosphere has remained fairly constant since the evolution of green plants, according to Gans. Recently, however, geochemists published preliminary but intriguing evidence of an increase in atmospheric oxygen concentrations from 15 percent to as much as 35 percent beginning at the end of the Paleozoic Era nearly 350 million years ago and lasting for about 100 million years before oxygen concentrations returned to original levels. Earth’s atmosphere today contains about 21 percent oxygen.

During the late Paleozoic, dragonflies with 20-inch wingspans filled the skies, dense forests of enormous ferns covered the continents, and primitive four-footed creatures-part fish and part reptile-were staking their claim on the shorelines of lakes and oceans.

” A denser atmosphere would have favorably influenced the evolution of insect flight by providing greater lift and facilitating the higher sustained metabolic levels needed to power flight,” said Dudley.

Increased oxygen levels also would have made it easier for water animals to overcome gravity and adapt to life on land. ” A rise in atmospheric oxygen coupled with the reduction in carbon dioxide would have improved primitive lung effectiveness,” said Gans.

Gans emphasizes that other factors, such as access to new food sources and increased ability to escape from predators, also were important in driving late Paleozoic evolutionary development. But he believes the oxygen pulse may have provided the initial impetus for species diversification by creating a fundamental change in the environment.

” More oxygen provided more opportunities,” he said. “Any species equipped to take advantage of it would have had increased reproductive success. “

University of Texas at AustinScripps Institution of Oceanography