The latest research on the mighty predator Tyrannosaurus rex confirms the long-hypothesized theory that there is a relationship between it and modern birds, including chickens and ostriches, scientists say.

Previous fossil studies have already suggested modern birds were descended from T. rex based on their skeletons.

Now, analyzing bits of protein (samples of collagen) obtained from tissue in a T. rex fossil, scientists found that birds, not lizards or other reptiles, are the closest relatives of dinosaurs, according to a report in Friday’s edition of the journal Science.

“These results match predications made from skeletal anatomy, providing the first molecular evidence for the evolutionary relationships of a non-avian dinosaur,” Chris Organ, a postdoctoral researcher in biology at Harvard University said in a statement as quoted by the Associated Press. Dr. Organ is the lead author of the report.

“Our results at the genetic level basically agree with what has been seen in skeletal data. There is more than a 90 percent probability that the grouping of T rex with living birds is real,” John M. Asara of Harvard, who led the analysis, said.

The proteins for the analysis were processed from tissue recovered deep in the bones of a 68 million-year-old T. rex excavated in 2003 by John R. Horner of Montana State University. Mary H. Schweitzer of North Carolina State University discovered the preserved soft tissues in the bones.

Dr. Asara and Dr. Organ reached their conclusions after they compared the dinosaur protein with similar protein from several dozen species of modern birds, reptiles and other animals.

The researchers also studied material recovered from a mastodon fossil and concluded it was related to modern elephants.

Some scientists saying that protein preservation over tens of millions of years should not be possible contested the findings. They went even further questioning the origin of Dr. Asara’s sequences and saying they might not come from an ancient R. rex. In their theory, proteins from some other biological source could have somehow contaminated the dinosaur remains.

There is also another issue on which they built their skepticism, more specifically, the small size of the sequences included in this research.

“They have a very tiny bit of data relative to the size of the collagen molecule. What’s going to be really convincing is to actually see some more sequences. If [preservation of dinosaur proteins] is a ubiquitous occurrence, then that should be forthcoming,” Peggy Ostrom, a biologist at Michigan State University in East Lansing and an expert on fossil proteins said, according to National Geographic.

No matter how contested this study is, it still adds to the growing evidence that birds are descended directly from a group of dinosaurs that grew feathers, possibly as a form of thermal insulation, before they learnt to fly.

In China, scientists have found many examples of feathered dinosaurs, including several that may have used their wings to glide. One fossilized dinosaur had feathers growing in both pairs of limbs, suggesting that four-winged flight may have been possible among some species.

The National Institutes of Health, National Science Foundation, Paul F. Glenn Foundation and the David and Lucille Packard Foundation funded the research appearing in the journal Science.