The dinosaur lineage, which evolved into birds, shrank in body size continuously for more than 50 million years. Video screen shot, The Conversation

Modern-day birds were once massive, meat-eating, ground-dwelling dinosaurs, according to a new study, published in the journal Science on Friday.

Large terrestrial carnivorous dinosaurs, known as theropods, evolved more rapidly than their counterparts to shrink steadily for more than 50 million years, and acquired the necessary characteristics to evolve into today's birds of flight, researchers said in the study, which examined a detailed family tree of these dinosaurs and their avian descendants to determine how this unlikely transformation occurred.

“Birds arose from the most evolvable lineage of dinosaurs,” Michael Lee of the University of Adelaide and the study’s lead author said, in a statement. “The dinosaurs which eventually evolved into birds acquired new adaptations – such as wings and flight feathers – at a faster rate than other dinosaurs. They were also the only lineage of dinosaurs to continually shrink in size.”

Five successive ancestors leading from dinosaurs to modern birds. From left to right: the ancestral neotheropod (220 million years old), the ancestral tetanuran (200 myo), the ancestral coelurosaur (175 myo), the ancestral paravian (165 myo) and the ancestral avialan (150 myo). Davide Bonnadonna

As part of the study, the researchers examined more than 1,500 anatomical traits of dinosaurs to reconstruct their family tree with the help of a sophisticated mathematical model, which traced how the dinosaurs picked up new traits across the dinosaur family tree.

According to the scientists, these dinosaurs decreased in weight from about 440 pounds to 1.7 pounds in 12 noticeable steps, Reuters reported.

“Studies of bipedal carnivorous dinosaurs − such as Tyrannosaurus rex and Velociraptor − keep finding more and more bird-like traits, such as feathers, wishbones, hollow skeletons and a three-fingered hand,” Lee said. “Birds out-shrank and out-evolved their dinosaurian ancestors, surviving where their larger, less evolvable relatives could not.”

During their evolutionary process, these bird-like ancestors also acquired new traits, such as the ability to climb trees, glide and fly, which helped pre-historic birds survive the deadly asteroid impact that wiped out “all their dinosaurian cousins” nearly 66 million years ago.

“Our study sheds new light the seemingly unlikely transformation from bulky ground-dwelling dinosaurs into agile flying birds. Birds are just a branch of the dinosaur family tree, just like humans are a twig in the primate tree,” Lee said.