First Author

How birds first got airborne remains an enigma. The puzzle is how a dinosaur forelimb could so completely change its function in the course of evolution. To try to answer this question, David Baier, a postdoc at Brown University in Providence, Rhode Island, and his colleagues used numerous lines of inquiry, including three-dimensional models of modern birds, fossil comparisons and force-balance metrics — modelling the forces acting on different animals' shoulder joints. Explaining the origin of flight eluded them, but they show on page 307 that a particular ligament in the shoulder was an important development in the evolution of modern bird flight.

Why is the origin of bird flight still not well understood?

Unfortunately, these are small animals with thin bones that are unlikely to be preserved. For much of the recent past, Archaeopteryx, a 150-million-year-old species, has been the only fossil available for analysis. Recently, a plethora of fossil information has come from China and around the world to drive further questions.

What hints do modern-day animals provide?

I had anticipated that once we had a better understanding of how the shoulder joint works in living birds, we'd need to examine modern birds' living relatives to understand how the joint evolved. As alligators are among modern birds' closest living kin, it made sense to use them to compare with and give context to the newer bird fossils found in China.

What were the key advances allowing you to better study living animals?

Using the force-balance approach was crucial. The muscles were insufficient to balance the joint, so I was very interested in knowing which forces — from ligaments, for example — were stabilizing. We modelled in three dimensions the structures that can balance the aerodynamic and muscle forces, and found that the acrocoracohumeral ligament is required to maintain stability at the joint and is a key structural component in flying birds.

What surprised you most about the findings?

It was a little surprising that the ligament was not associated with the origin of flight. It is a modification that developed later. This study became more of an investigation of the evolution of flight. We still don't know how the earliest birds flew and how they stabilized their forces — were they gliders or flappers?