Large meat-eating, land-roaming dinosaurs evolved into birds by shrinking over the course of 50 million years, scientists have confirmed.
Theropods shrunk 12 times from 163kg (25st 9lb) to 0.8kg (1.8lb), before becoming modern birds, according to a new study published in a recent edition of the journal Science.
Their skeletons also changed four times faster than other dinosaurs, which helped them survive.
Previous studies showed that theropod dinosaurs, which includes Tyrannosaurus rex and Velociraptors, gave rise to modern birds.
"This study means we can't see the origin of birds as a sudden or dramatic event, with a dinosaur becoming a powered flyer overnight," Professor Michael Benton, from the University of Bristol's school of earth sciences, said according to BBC News.
"The functions of each special feature of birds changed over time - feathers first for insulation, and later co-opted for flight; early reductions in body size perhaps for other reasons, and later they were small enough for powered flight; improvements in sense of sight and enlargement of brain - even a small improvement in these is advantageous," Benton added.
Size changes happened a lot in dinosaur evolution, so a team of researchers, led by Mike Lee, from the University of Adelaide, Australia, set out to find if the dramatic size reduction associated with the origin of birds was unique, according to BBC News.
They also set out to measure the rate of evolution in dinosaurs using a large data set.
By using analytical tools, developed by molecular biologists trying to understand virus evolution, to study over 1,500 dinosaur body traits coded from 120 well documented species of theropod and early birds.
From this analysis, they were able to make a detailed family tree mapping out the transformation of theropods to their bird descendants.
They found that the dinosaur group related to birds shrank rapidly from 200 million years ago.
It also showed a decrease in body mass of 162.2kg (25st 7lb) from the largest average body size to Archaeopteryx, which is considered the earliest known bird.
These ancestors evolved new adaptations like feathers, winds, wishbones, four times faster than other dinos, according to the study.
Researchers concluded that the evolution of the dinosaurs leading to birds was more innovative than other lineages.
"Birds evolved through a unique phase of sustained miniaturization in dinosaurs," Lee said, according to BBC News. "Being smaller and lighter in the land of giants, with rapidly evolving anatomical adaptations, provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly.
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