Washington (AFP)

Modern mammals, including humans, owe their fine hearing to three middle ear ossicles that their reptile ancestors did not have, but until now it was unclear how far the transformation had begun.

Scientists report in a study published Thursday in the journal Science found de facto the missing link: a species that lived 125 million years ago in what is now north-eastern China, and which they have analyzed in detail fossils discovered since the mid-2000s.

"These are fantastic proofs," said evolution biologist Guillermo Rougier of the University of Louisville, who was not involved in the study. The fossils analyzed by the research team are "breathtaking", he told AFP.

The fossils analyzed by the team, mainly Chinese and which includes Jin Meng of the Museum of Natural History in New York, are six in number and are proto-mammals of the lower Cretaceous. They named the species "Origolestes lii". These animals rubbed dinosaurs and had the size and appearance of rodents as mice.

Reptiles use their jaws to chew and transmit external sounds through vibrations to their brains, which differs from the mammalian auditory system which involves the three knuckle hammer, anvil and stirrup and allows humans to listen to a symphony and to dolphins to locate objects by echolocation.

According to the prevailing scientific hypothesis, the progressive separation of the two functions - to chew and to hear - would have made it possible to alleviate the mutual constraints on the jaws, and the mammals would thus have been able both to diversify their diet and to improve their hearing. .

The research team used high-resolution imaging techniques to describe in detail the structure of the ossicles and auditory cartilages of fossilized animals.

"We have provided the fossil evidence in the history of evolution that illustrates this hypothesis," says Jin Meng.

Fossils are a true paleontological treasure, according to Guillermo Rougier. "It's almost too much," he says.

The study suggests new questions, he adds, such as: has this evolution occurred in all or only a few mammals?

"Did it happen once?" In several groups, we can ask more questions, "says the researcher.

In addition to the auditory system, Jin Meng and his colleagues are analyzing other parts of the fossils, including the brain cavity, which may reveal other secrets about mammalian evolution.

© 2019 AFP