When our early ancestors made their way onto land, they may have heard very little. After all, their auditory system was adapted for hearing underwater. According to researchers studying lungfish—a living model for hearing in our ancient ancestors—the first terrestrial creatures may have used their lungs to help them detect sounds. Eardrums and middle ears didn’t show up for another 100 million years. The findings were published in the Journal of Experimental Biology.
“It is quite a different task to detect sound in water and air as the physical properties of these two media are very different,” Christian Christensen from Aarhus University explains in Inside JEB. In water, sound vibrations pass unhindered through the animal’s tissue to vibrate the tiny structures in their inner ear, producing the sensation of hearing. In the air, however, these vibrations are reflected when they reach the animal. But even without middle ear adaptions to transform aerial sound pressure waves into physical vibrations in the inner ear, these first land animals weren’t deaf.
The African lungfish (Protopterus annectens) is the closest living relative of all four-legged, land-living vertebrates (or tetrapods), which also includes snakes who lost some limbs and whales who went back into the water. These air-breathing fish used their air-filled lungs to convert pressure waves into vibrations. To test the fish's hearing, Christensen and colleagues filled a two-meter-long steel tube with water and placed a loud speaker at one end. The pressure was higher where water particles were still and lower around fast-moving particles. Then they placed an anaesthetized lungfish at various places throughout the tube and recorded electrical activity in its brainstem.
The lungfish were able to hear sound pressure at frequencies above 200 Hz and particle motion at lower frequencies, Inside JEB explains. Using CT scans to measure the volume of air in the fish's lungs, the team found that their lungs generated the strongest vibrations at frequencies where the fish can hear sound pressure. “This strongly suggests that pressure detection in lungfish is enabled through detection of the pressure-induced particle motion generated by the resonating air volumes in the lungs,” Christensen says. Their lungs act as pressure-to-particle motion transducers.
Out of water, they could hear loud airborne sounds at frequencies up to 200 Hz. “It was a surprise that the lungfish, being completely unadapted to hearing, were in fact able to hear airborne sound,” Christensen adds.
Furthermore, his team also studied hearing in salamanders called axolotls (Ambystoma mexicanum), often thought of as an intermediate stage between our aquatic ancestors and modern tetrapods. These amphibians detected sound pressure at 120 Hz underwater. Like their lungfish predecessors, they were able to hear airborne sound, and their lungs vibrated in the frequency range they were able to hear, Science reports. These findings were published in Proceedings of the Royal Society B this week.
Together, these studies suggest that the first tetrapods had rudimentary aerial hearing that may have led to the evolution of the middle ear that modern tetrapods now enjoy.