Enormous and fearsome dinosaurs no longer exist today but they are believed to have evolved into small and lightweight creatures that are now commonly seen in the skies: birds.

Scientists once thought that the earliest bird, the Archaeopteryx, which existed 150 million year ago, was a great evolutionary leap of dinosaurs becoming flying creatures. Findings of a new research, however, suggest that the evolution of the gigantic dinosaurs to birds was not abrupt as the process appears to have involved millions of years.

New discoveries made over the past two decades, for instance, revealed that dinosaurs already evolved avian traits as soon as they emerged about 230 million years ago, long before the Archaeopteryx lived during the Late Jurassic period.

For the new study published in the journal Current Biology on Sept. 25, Stephen Brusatte from the School of GeoSciences at the University of Edinburgh in the U.K., and colleagues examined hundreds of body features of 150 extinct species of bird and their closest known dinosaur relatives. The researchers then used the gathered data to construct a complete dinosaur family tree, which revealed that the emergence of birds about 150 million years ago was not sudden but rather involved lengthy and gradual process.

Over time, some of the dinosaurs in the lineage were found to have eventually acquired bird-like traits making it difficult to separate the dinosaurs and birds in the family tree because of their intersecting borders of characteristics. The development of avian features such as feathers and wings were also observed to have gradually taken place over tens of millions of years.

"This is statistical confirmation of a view about bird evolution that paleontologists have described for a while," said Roger Benson from the University of Oxford in the U.K. "Scientifically, it would have been crazier if they had shown birds appearing from dinosaurs all of a sudden out of nowhere."

The researchers said that while the progression from dinosaur to bird took very long, a burst of evolution took place once the dinosaurs assumed the bird body structure and this has led to the advent of thousands of species.

"Our results demonstrate that the rise of birds was a complex process: birds are a continuum of millions of years of theropod evolution, and there was no great jump between nonbirds and birds in morphospace," the researchers wrote. "[B]ut once the avian body plan was gradually assembled, birds experienced an early burst of rapid anatomical evolution."