Linchpin of Flight (IMAGE) Brown University Caption Using computer modeling, treadmills and the fossil record, researchers have shown that the acrocoracohumeral ligament (AHL), a short band of tissue that connects the humerus to the shoulder joint in birds, was a critical element in the evolution of flight. Credit David Baier/Brown University Usage Restrictions Must credit David Baier and Brown University License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.