A new study has suggested that dinosaurs evolved from large, land-based meat-eaters to birds over the course of 50 million years.

According to BBC News, the researchers, whose work is published in the journal Science, said therapods shrunk 12 times - from nine pounds to less than two - to eventually evolve into modern birds. They were apparently the only dinosaurs to grow smaller over time, but their skeletons were also known to change rapidly for survival reasons.

While fluctuations in size is believed to have been common among dinosaurs, the research team set out to determine if the evolution into birds was exclusive to therapods. Study lead author Mike Lee, from the University of Adelaide in Australia, and his team examined 1,500 dinosaur attributes from 120 therapod and early bird species.

"Birds evolved through a unique phase of sustained miniaturisation in dinosaurs," Lee told 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.

"Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins."

In a recent study, scientists discovered it was likely that a wide variety of dinosaurs were covered in feathers, either short, bristly ones or long, decorative ones. Lee told the Washington Post this was likely a contributor to the evolution in modern birds. For example, as the dinosaurs became smaller, their feathers became more useful for keeping warm, among other things.

"Those proved to be incredibly useful for flight," Lee said.

The researchers soon saw a long-term evolutionary trend among therapods.

"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," Michael Benton, a professor of earth and sciences at the University of Bristol, told 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.

"So perhaps it's a long-term trend associated with deputation to a new set of habitats, in the trees, to avoid predation, and to exploit new food resources."