By FIONA MACRAE

Last updated at 14:38 17 April 2007

Palaeontologists have long accepted that birds are a form of dinosaur.

Now the theory that the most feared dinosaur of all, Tyrannosaurus Rex, evolved into the modern-day chicken has been given scientific backing with the discovery of some pre-historic collagen.

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The protein - responsible for giving skin its elasticity and bone its structure - is nowadays more readily associated with dinosaurs of the aging celebrity kind.

But scientists were surprised to discover traces in a 68million-year-old fossilised T-rex thighbone - supporting the belief expounded by Charles Darwin in his influential On The Origin Of Species By Means Of Natural Selection published nearly 150 years ago.

Analysis of the collagen has, they say, provided genetic proof of a link to chickens, with both sharing remarkably similar bone structure.

The collagen, from a bone unearthed in the U.S. state of Montana four years ago, also bore resemblance to that found in frogs and newts.

British palaeontologist Dr Angela Milner, of the National History Museum, said the unusual discovery had been made possible by the remarkable preservation of the particular fossil. She said: "This corroborates a huge body of evidence from the fossil record that demonstrates birds are made from meat-eating dinosaurs.

"The analysis shows that T-Rex collagen make-up is almost identical to that of a modern chicken.

"The fact that identifiable proteins can be removed has opened up an exciting new field of investigation that may tell us more about patterns and rates of evolution from the past to the present."

The U.S. researchers' findings, reported in the journal Science, provide the first hard genetic evidence for the theory that birds evolved from dinosaurs.

Researcher Dr Mary Schweitzer, of North Carolina State University, said: "For centuries it was believed that the process of fossilisation destroyed any original material (such as collagen).

"Consequently no one looked carefully at really old bones.

"This data will help us learn more about dinosaurs' evolutionary relationships, about how preservation happens, and about how molecules degrade over time, which could also have some important medical implications for treating disease."