How Did Mammals Evolve Sensitive Hearing? Study Challenges Current Beliefs

Unlike reptiles, whose jaws are connected to parts of their ears, mammals possess three super tiny bones tucked away in the auditory canal, located just on the inner side of the eardrum. The bones equip mammals with a keen sense of hearing – but how did the mammalian middle ear evolve?

In the latest study, an international team of scientists, led by the University of Queensland (UQ), has shed some light on the matter, challenging current beliefs and calling for further fossil evidence. 

"One of the problems with earlier studies on mammalian development is that scientists saw a relative shrinking in size of middle ear bones, as well as a movement away from the jaw joint, possibly under the influence of a rapidly expanding brain," Dr. Vera Weisbecker, from UQ's School of Biological Sciences, explained in a news release, adding that the development of the mammalian middle ear represents an "extreme transformation" in the evolution of mammals from reptile-like ancestors. "Because scientists look to such development processes to find out about evolution, these processes were interpreted to reflect the evolution over time of the mammalian middle ear." 

Existing scientific theories about how and why mammals evolved their unique middle ear bone are based on insufficient data, Weisbecker says. In fact, most "patchy fossil records" do not allow scientists to trace the details of this relatively complex 320-million-year-long evolutionary process.

What is known, however, is that over the course of mammalian evolution, three ancestral reptile-like jaw bones, initially designed for feeding, shrank and were "retooled" for a new purpose of conducting sound more sensitively in the inner ear.

Using data collected from CT scans of marsupials and monotremes, researchers confirmed there must be something missing: The scans did not support existing theories about mammalian middle ear development. To the contrary, researchers suggest there is a need for additional fossil evidence. 

"It's not known why this change occurred, but it is thought that by extending their range of hearing to include high-pitched sounds, mammals could improve their detection of prey, such as small insects in the dark," Weisbecker concluded.

The study was recently published in the Proceedings of the Royal Society B.  

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