A comprehensive review of feathered dinosaurs in the journal Current Biology has concluded that the potential for flight actually evolved at least three times within theropods, twice before early birds came into the picture. The collaborative project took several years but the findings reveal that a revision of the evolutionary tree is needed to give a truer representation of how this mode of locomotion emerged among dinosaurs as well as birds.

The mammoth project was undertaken by an international team of researchers from five different countries, led by Professors Michael Pittman and Rui Pei at Hong Kong University. Together they pored over fossils looking for species whose anatomy might meet the threshold for flight. These include smaller body size, accelerated evolutionary rates, early feathers with complex coloration, flapping-based movement, and avian-like sleeping positions.

With this information, they developed a new evolutionary tree focusing on the emergence of physical features that had the potential for powered flight. They found that many avialan relatives came close but only on three occasions did species cross the threshold: once in birds and twice in dromaeosaurids. "The capability for gliding flight in some dromaeosaurids is well established so us finding at least two origins of powered flight potential among dromaeosaurids is really exciting," said Pittman in a statement.

The team’s revised tree still supports the close relation of dromaeosaurid ("raptors") and troodontid theropods to birds, but controversially indicates that anchiornithine theropods were actually the earliest birds.

"This was a fun collaboration over several years", said McGill University Professor Hans Larsson, who was a member of the international team of researchers. "For the first time, we have a well resolved evolutionary tree of these small, feathered dinosaurs to ask questions about how birds originated.

“We were able to map biomechanical limits to all these species and propose a picture of experimentation within a spectrum of near-flight to fully-flighted capabilities in these wonderful little carnivores. This goes against the simple, linear stepping forward through evolution model of bird origins and instead presents one where an explosive radiation of feathered dinosaurs were experimenting with many kinds of wing-assisted locomotion. I think this is the most realistic view of bird origins to date."