Published online 5 November 2003 | Nature | doi:10.1038/news031103-7

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Long history mooted for low-oxygen survival trick.

Birds' breathing maximises oxygen, enabling them to fly at high altitudes. © GettyImages

Birds may owe their ability to soar over mountaintops to flightless dinosaur ancestors who adapted to survive with less oxygen.

Between 275 million and 175 million years ago the atmosphere's oxygen content was probably much lower than today. One group of dinosaurs, the Saurischians, seems to have evolved a more efficient breathing system to deal with the thinner air, says palaeontologist Peter Ward of the University of Washington in Seattle.

Today's birds may be distant relatives of these dinosaurs, Ward is suggesting.

Back then, the amount of oxygen available at sea level was similar to that at an altitude of 3,700 metres today. Twice during this time, oxygen concentrations plunged even further, possibly causing two of the biggest mass extinctions in history, Ward proposed at the annual meeting of the Geological Society of America in Seattle, Washington this week.

Others argue that the extinctions were caused by asteroid impacts such as the one 65 million years ago often linked to the dinosaurs' demise.

The first of these events, 250 million years ago, was the most catastrophic extinction ever. Some 90% of the planet's species disappear from the fossil record of the time. Life slowly recovered, but 50 million years later half of it vanishes once again. Most of the species that survived suffered big declines: "It was a massive hit on all vertebrates except the Saurischians," says Ward.

Saurischians may have flourished because they used oxygen more efficiently, says Ward. Their fossil bones contain large holes that could have held air sacs like those of modern birds.

Birds fly well above Everest climbers today because they breathe in air with their lungs and then pass it through a series of sacs, taking more oxygen out at each one, before finally exhaling it. "If you want a breathing system that maximizes oxygen use, be a bird," says Ward.