Ancient Increase in Oxygen Tied to Surge of Giant Insects

A sharp increase in oxygen in the ancient atmosphere may have led to giant insects and helped animals with backbones make the evolutionary switch from water to land, researchers say.

The changes in the ancient animals support the hypothesis that oxygen concentration started rising about 400 million years ago and peaked 290 million years ago before declining, said biology professor Carl Gans of the University of Michigan.

The hypothesis says the oxygen reached an estimated 35% of the atmosphere, compared with the present level of 21%, Gans and co-authors noted in the journal Nature.

Insects get oxygen by diffusion through tiny tubes in their bodies, and the increase in atmospheric oxygen would have let ancient insects take in more, the researchers said. That in turn would let them get bigger.

Known examples of ancient giant insects include dragonflies with wing spans of more than two feet.

The rise in oxygen plus the simultaneous drop in carbon dioxide also would have made the primitive lungs of water-dwelling backbone animals more effective, researchers said. That may have helped the creatures make the transition to living on land.