The wind beneath their wings

On the wing: new research offers an answer to why birds began to fly

Why do birds fly? Controversy has always surrounded theories about how the ancestors of modern birds took to the skies. Now new American research suggests that changes in oxygen levels in the atmosphere made it possible for vertebrates to develop wings and launch themselves into the air.

Computer models of the atmosphere show that the concentration of oxygen in the atmosphere has varied over time, and peaked around the time that the first vertebrates started to develop wings.

The increased oxygen could have made flight easier by allowing creatures to produce more energy. It would also have helped by making the air easier to fly in.

According to Robert Dudley, a zoologist at the University of Texas: "If you increase oxygen in the atmosphere, you're increasing the total pressure of the atmosphere.

"If you increase total pressure, then by definition you are increasing air density. Air would have been heavier relative to what it is today, and forces on objects moving through air are proportional to the density of air.

"So a greater air density facilitates force production by proto-wings or early wings."

Soaring: Changes in the atmosphere may have enabled the ancestors of modern birds to start flying

Data show that oxygen levels reached 35%, much higher than the 21% of today. This was shortly after the first plants evolved over 250 million years ago.

Oxygen levels peaked again in the mid-Jurassic age, coinciding with the time that flight is thought to have evolved. Researchers initially investigated the links between atmospheric changes and the emergence of giant flying insects during the same period.

More recently, Mr Dudley's team started to explore how the increased oxygen levels made it possible for vertebrates to start flying as well.

Now that levels of gases in the atmosphere are regularly analysed in the study of global warming, Robert Dudley believes that models of past atmospheres will be easier to make.

As we begin to understand how the air has changed over hundreds of millions of years, new assessments of how other forms of life evolved might also be made.