A new study of ancient reptile fossils has pushed back the date for the earliest known ear by 60 million years and generated a new hypothesis of why hearing evolved in the first place.
An ear capable of hearing airborne sounds evolved independently at least six times among terrestrial vertebrate groups, including mammals, lizards, frogs, turtles, crocodiles, and birds. Yet although these ears may differ in some details, they all share certain features: an eardrumlike membrane to capture sound vibrations and small bones--such as the stapes--to transmit the sounds to the inner ear. Based on the fossil record, the earliest known ears of this type date to 200 million years ago or later.
To further explore the evolutionary roots of terrestrial vertebrate hearing, paleobiologists Johannes Müller and Linda Tsuji of Humboldt University in Berlin, Germany, examined several dozen specimens of early reptiles that have been found since the 1930s in the Mezen River Basin of central Russia. The fossils, which date to about 260 million years ago, had previously received only limited study. The researchers were able to identify six apparently closely related species, all of which showed clear evidence of large, eardrumlike structures covering much of their cheeks. In the better preserved specimens, inner ear bones similar to those of modern ears were found, including a stapes. Moreover, Müller and Tsuji found, the relative sizes of the eardrum and the part of the stapes that communicates with the inner ear were similar to those in modern terrestrial vertebrates, a key test that this ancient ear had the acoustic properties necessary for hearing airborne sounds. The findings are published online today in PLoS One.
Martin Brazeau, an evolutionary biologist at Uppsala University in Sweden, says that the authors are on firm ground in concluding that they have found the earliest ears. "They have identified structures that sure look like the parts of an ear capable of detecting airborne vibrations," he says.
This much earlier evidence for a fully functional ear challenges a leading hypothesis for why ears evolved in the first place. Several years ago, paleontologist Jennifer Clack of the University of Cambridge in the U.K. proposed that hearing evolved to help vertebrates catch the buzzing insects that were undergoing an evolutionary explosion around the same time. But buzzing insects were not as prevalent 260 million years ago. So the new data suggest that the auditory sense might have arisen among vertebrates that lived in dimly lit niches. This jibes with the fact that many modern animals with particularly sharp ears, such as owls, cats, and geckos, are adapted to living in the dark.
Clack says the authors' hypothesis is a plausible alternative. But she adds, "I'm not sure how you would test it."