NANJING, Jan. 29 (Xinhua) -- While dinosaurs are generally believed to be the ancestors of modern birds, it is largely unknown when and how the evolution took place. However, a recent study by an international team of researchers has shed new light on the mystery.

The researchers conducted molecular analysis of feathers from fossilized Jurassic feathered dinosaurs, fossil birds and modern birds, and found the key proteins that make up feathers gradually became thinner and more flexible as flightless dinosaurs evolved into flying birds.

The study, published on the Proceedings of the National Academy of Sciences (PNAS) on Tuesday, was conducted by a group of Chinese and American scientists from 2016.

The team used electron microscopy and chemical analysis to examine and compare the molecular structure of flight feathers from Anchiornis, a feathered dinosaur dating back about 160 million years, and today's chicken.

They discovered chicken's feathers were mainly composed of β-keratin, a structural protein commonly found in the skins, claws and beaks of reptiles and modern birds. It provides birds with flexibility, elasticity, strength, and other biomechanical properties needed for flight.

Though β-keratin was also found in Anchiornis feathers, the feathers are predominated by a thicker and more rigid protein, α-keratin, making them unsuitable for flight.

"It suggests the feathers of early feathered dinosaurs were not used for flight, but merely for warmth and self-defense," said the study's leading scientist Pan Yanhong, a researcher from the Nanjing Institute of Geology and Palaeontology with the Chinese Academy of Sciences.

The researchers further examined the feathers of later bird fossils, including the 130-million-year-old Eoconfuciusornis and 125-million-year-old Yanornis, and concluded that the amount of β-keratin in the feathers increased over time as dinosaurs evolved into birds.

The molecular structure of feathers found in a 20 million-year-old bird fossil was discovered to be the same as in modern birds.

"The study demonstrates the molecular structure of feathers has evolved step by step," Pan said. "The transition from dinosaurs to birds was a complex process, and the change of the feathers' structural protein is a crucial step."