New fossil gives insight into evolution of ear

Evolution works in mysterious ways, in methods analogous to cookie optimization. First, there were sugar, flour, eggs and butter. Fortunate thermodynamic processes involving high-speed mixing caused these elements to spontaneously assemble at high temperatures into the prehistoric cookie.

Man took this recipe and made four batches of cookies, adding chocolate chips, nuts, berries and rocks. The rocks tasted bad. Our ancestors thus moved forward with the chocolate.

And so on.

Chinese and American paleontologists, working with scientists at Pittsburgh's Carnegie Museum of Natural History, recently discovered a well-preserved fossil of a chipmunk-sized marsupial that lived 123 million years ago. They presume the animal lived in the Liaoning Province of China, where the fossil was found, and have named it Maotherium asiaticus.

The results of this excavation and the study following it, which have been published in Science earlier this month, are of particular importance because they offer important clues about the evolution of the structure of the middle ear.

Hearing is an essential evolutionary mechanism that is crucial to our continued existence, and the story of the ear is a long and convoluted one. Modern ear structure in mammals includes an outer ear, middle ear and inner ear. The middle ear, located just behind the ear drum, is comprised of three small bones. Damage to these bones results in sustained and often permanent hearing loss.

"With a tiny and intricate middle ear structure, mammals have more sensitive hearing in a wider range of sounds than other vertebrates," said H. Richard Lane, program director in the National Science Foundation's (NSF) Division of Earth Sciences, in a statement on the NSF's Web site.

"This sensitive hearing was crucial for mammals to develop nocturnal adaptations and to survive in dinosaur-dominated times."

The researchers determined that Maotherium had middle ear bones similar to those in modern mammals, as well as a unique connection between the middle ear structure and the lower jaw.

This connection, known as an ossified Meckel's cartilage, is similar to jaw structures found in pre-mammalian, almost certainly reptilian, ancestors of Maotherium.

Like all evolutionary steps, the interesting structure developed as a product of incremental gene mutations spanning the time of prehistoric mammalian evolution. The researchers concluded from the fossil that evolutionary change based on timing, as well as gene patterning, are major players in the development of the structure of the modern mammalian middle ear.

This study received support from the National Natural Science Foundation (China), the Ministry of Science and Technology (China) and the National Geographic Society.