Changes in the air: variations in atmospheric oxygen have affected evolution in big ways

Science News, Dec 17, 2005 by Sid Perkins

As any camper who's blown on a flickering ember can tell you, a campfire needs a steady source of air to stay alight. Without enough oxygen, the chemical reactions that release the energy stored in firewood falter and fail. When oxygen is plentiful, the release of energy proceeds apace. The same principles apply to metabolic processes: All animals require oxygen to extract energy from their food and to fuel their activity.

It's no surprise, then, that during geologic periods when atmospheric oxygen concentrations have been high, biological innovation has blazed brightly. At such times, insects grew to gargantuan proportions, reptiles took to the air, and the forerunners of mammals developed a warm-blooded metabolism. When oxygen concentrations fell precipitously, biodiversity was smothered. Some of the planet's mass extinctions occurred during or after geologically sudden drops in atmospheric oxygen.

Earth's atmosphere had little if any oxygen until about 2.5 billion years ago (SN: 1/24/04, p. 61), after organisms began using chlorophyll to convert sunlight into useful energy. Because this reaction releases oxygen, the gas slowly became more plentiful in the atmosphere. The conversion of vegetation into coal and the burial of plant matter also puts oxygen into the atmosphere.

These boosts have been tempered by geological processes that remove oxygen from the air. Widespread episodes of mountain building were inevitably followed by extended periods of erosion and chemical breakdown of the newly exposed rocks, a process that consumes oxygen.

Atmospheric oxygen concentrations have risen and fallen--sometimes gradually and sometimes rapidly--at various times. To investigate how these changing concentrations have affected the world's biota, scientists have most often looked to the fossil record. Now, lab experiments are fleshing out the ways in which animals respond to changes in oxygen abundance.

THE NEED TO BREATHE Imagine a world populated by meter-long millipedes, mayflies with the wingspans of today's robins, and dragonflies with wingspans rivaling those of hawks. This isn't science fiction--it's our world at the end of the Carboniferous period, about 300 million years ago. The atmospheric concentration of oxygen then probably was about 35 percent, an all-time high that far exceeds today's 21 percent figure.

The Carboniferous abundance of oxygen enabled insects and other arthropods, which get their oxygen via diffusion through holes in their exoskeleton, to grow to immense proportions, says Robert A. Berner, a geophysicist at Yale University. Models of insect physiology suggest that the higher atmospheric concentrations of oxygen, as well as the increased air pressure that resulted, would have increased the diffusion rate of oxygen into an insect's bloodstream as much as 67 percent.

The evolution of large-bodied arthropods occurred slowly but steadily as atmospheric oxygen gradually increased during the 60-million-year-long Carboniferous period. In laboratory experiments today, insects raised in oxygen-rich conditions can attain beefier proportions in just a few generations.

For example, fruit flies and mealworms raised in chambers for just one generation with twice the normal concentration of oxygen grew 3 percent larger bodies than those reared in standard conditions, says Jon F. Harrison of Arizona State University in Tempe. In other fruit fly-breeding experiments, low oxygen prevented the insects from becoming as large as other flies raised under normal or oxygen-rich conditions.

These findings, as well as evidence from the fossil record, suggest that the atmospheric concentration of oxygen tends to constrain the maximum size that an insect can attain, Harrison said in August at a Calgary, Alberta meeting of the Geological Society of America.

Experiments show that variations in oxygen abundance can also affect the development of reptiles, says John M. Vanden Brooks, avertebrate paleontologist at Yale University. Under normal conditions today, eggs of the American alligator (Alligator mississippiensis) hatch between 65 and 70 days after they're laid. In research that he reported at the meeting in Calgary, Vanden Brooks incubated alligator eggs in sealed, 20-liter aquaria in which he could control the oxygen concentration.

If the oxygen concentration was kept at 16 percent, embryonic development was delayed by almost a week. In experiments where the concentration was raised to 27 percent, development of the embryonic alligators was accelerated by almost a week.

At oxygen concentrations higher than 27 percent, mortality among the embryos was about twice that measured in eggs raised in a normal atmosphere, says Vanden Brooks. That higher death rate probably resulted from damage from oxidation of tissues.

Although the first reptiles evolved around 350 million years ago, and their descendants thus experienced oxygen concentrations above 27 percent, the first creatures resembling crocodiles appeared about 220 million years ago, when oxygen concentrations had dropped to about 16 percent.