Scientists believe that modern birds evolved from a large flesh-eating species of dinosaur that shrank in size over 50 million years.

An international team of scientists have described an extraordinary evolutionary process in which a lineage of carnivorous dinosaurs shrank steadily and acquired numerous traits that led to the first appearance of birds.

The researchers, using techniques developed by molecular biologists to reconstruct virus evolution, examined 1,500 anatomical traits in 120 different dinosaurs from the theropod group.

These bipedal meat-eaters included giants like the tyrannosaurus rex and giganotosaurus as well as the lineage that produced birds.

"Our study measured the rate of evolution of different groups of theropod dinosaurs," said lead researcher Michael Lee, a paleontologist at the University of Adelaide and the South Australian Museum.

"The fastest-evolving group also happened to be ancestral to birds. So, ultimately, the most adaptable dinosaurs proved to be the best long-term survivors, and surround us today in their feathered splendour."

The earliest known bird was the crow-sized archaeopteryx, which lived in Germany 150 million years ago.

It was characterised by primitive traits like teeth, a long bony tail and the absence of a bony, keeled sternum where flight muscles attach, as well as some attributes shared with modern birds.

"What was impressive was the consistency of the size change along the dinosaur-to-bird transition. Every descendent was smaller than its ancestor," Mr Lee said.

"The lineage was continually pushing the envelope of life at a smaller body size, little by little, over 50 million years."

The researchers completed a family tree of this dinosaur lineage and their bird descendants. The dinosaurs decreased in size from about 200kg to 0.8kg in 12 discernible steps.

Aside from sustained miniaturisation, this lineage also benefited from new traits such as feathers, wishbones, wings, shorter snouts and smaller teeth.

The study found that this lineage acquired evolutionary adaptations at a rate four times faster than other dinosaurs.

"The dinosaurs most closely related to birds are all small, and many of them - like the aptly named microraptor - had some ability to climb and glide," said study participant Gareth Dyke, a paleontologist at Britain's University of Southampton.

The decrease in body size may have helped dinosaurs in the lineage that evolved into birds to take advantage of certain ecological niches that would have been off-limits to their larger relatives and to experiment with unique body shapes.

"It would have permitted them to chase insects, climb trees, leap and glide, and eventually develop powered flight," Mr Lee said.

He said the changes may have helped these creatures to survive the cataclysm that doomed the other dinosaurs - an asteroid that struck Earth 65 million years ago.

Flight, for example, would have allowed them to cover vast territory in search of suitable habitat, and warm-bloodedness would have buffered them against climate changes.

Reuters