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Too high. If the low atmospheric oxygen levels of the late Permian period prevailed today, few vertebrate animals could live much above an altitude of 500 meters (red).

Gasping for Air in the Permian

Thin air may have forced animals down from higher latitudes 250 million years ago, crowding them into the lowlands and possibly helping along the largest extinction in the history of the planet, according to a new study in today's issue of Science.

In 2002, geochemist Robert Berner of Yale University calculated that during the past 600 million years, atmospheric concentrations of oxygen were stable near present-day levels of 20% until about 400 million years ago, rose sharply to a peak above 30% about 300 million years ago, and then dove to 12% by 240 million years ago.

For ill prepared animals, losing more than half of their normal oxygen supply would have had far-reaching effects, say evolutionary physiologist Raymond Huey and paleontologist Peter Ward of the University of Washington, Seattle. Every animal has its own minimum oxygen requirement, they note. That's why each species has a particular altitude above which it doesn't live. For example, humans live and reproduce no higher than 5.1 kilometers, in the Peruvian Andes. So, "if oxygen is 12%, sea level would be like living at 5.3 kilometers," says Huey.

With oxygen at the mid-Permian's peak of 30%, animals probably could have breathed easily at any altitude on Earth, says Huey. But as oxygen levels dropped, animals capable of living at 6.0 kilometers in the mid-Permian would have been driven down to 300 meters. Perhaps half of the Permian land area would have been denied to animals. Species specialized to live in upland habitats would have perished, assuming they couldn't adapt their relatively unsophisticated breathing systems. Survivors would have been squeezed down into smaller, more isolated areas, where habitat fragmentation would have driven up extinctions and smaller habitats would have limited the number of species the land could support. "We can explain some big part of land extinction with this," says Ward.

Extinction by crowding into lowlands "is a very interesting idea," says biologist Robert Dudley of the University of California, Berkeley, but "it's pretty hypothetical. None of the assumptions is yet testable." Further studies of breathing physiology and geographical patterns in the fossil record should help size up just how bad life might have been.

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