Evolutionary Tree (IMAGE) The University of Hong Kong Caption Graphical abstract showing a simplified version of the revised evolutionary tree of birds (Avialae) and their closest relatives (non-avialan theropod dinosaurs) and the many ancestors that came close to powered flight potential (orange) but the few that achieved it (green; at least three times: once in early birds and twice among the dromaeosaurids ('raptors'). Credit Michael Pittman and co-authors Usage Restrictions None 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.