Fossils of several shrew-like mammals that lived some 120 million years ago have revealed  the earliest evidence of the middle ear bones separating from the jaw, a key step in the evolution of hearing.

Three tiny bones in the middle ear, known as the incus, malleus and stapes (or hammer, anvil and stirrup), are responsible for the exceptional hearing found in mammals such as dolphins and bats.

Biologists think that this complex architecture gradually evolved as the bones behind the back teeth of the lower jaw shrank and were pushed back. But fossil evidence of how and when this transition happened is rare.

Now Jin Meng at the American Museum of Natural History and his colleagues have discovered proof of this missing link in several nearly complete skeletons of a previously unknown creature, Origolestes lii , found in the Yixian Formation in Liaoning province, China.

The middle ear bones in this mole-sized creature sit behind and at the base of its jawbone, but like in modern mammals, are completely separate from the jaw.

In particular, O. lii ’s middle ear bones weren’t connected to the jaw through a bridge called Meckel’s cartilage – a feature found in a relative that lived around the same time.

Meng says this is “a snapshot of the moment when the two organs separated in evolutionary time”.

The evolution of small and loose bones allowed mammals to hear at higher frequencies, and this may have helped them catch insects, says Meng.

“On the other hand, the selection pressure for eating different food, such as vegetables and meat, require strong jaw movement, which would be constrained if the hearing organ was attached to the lower jaw,” he says.

Once the two were separate, hearing and chewing were both able to evolve rapidly without each impairing the other.

All living mammals have a separate middle ear, but reptiles, birds and frogs don’t – which means they lose hearing when eating. In fact, the middle ear is so advantageous that biologists think that it evolved at least three separate times in mammalian history.

It is possible that these findings represent the precursor to human hearing, because O. lii is part of the lineage that led to all living marsupials and mammals with placentas.

The animals were found curled up in a resting position, and the team believe that they may have been killed by toxic volcanic gas while they slept.

Journal reference: Science , DOI: 10.1126/science.aay9220