A hint at how hearing evolved

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Early four-legged vertebrates may have been able to hear sounds on land, even though they lacked key ear structures.

Tom McHugh/SPL

Christian Christensen at Aarhus University in Denmark and his colleagues studied the hearing of the African lungfish (Protopterus annectens; pictured), the closest living relative of early tetrapods that began moving onto land around 350 million years ago.

The middle ear, which senses changes in air pressure caused by sound, is missing in lungfish. The researchers found that low-frequency sounds in air caused the lungfish's head to vibrate. Its brain responded to these frequencies, suggesting that the animal detects airborne sounds using the vibrations.

Another study by some of the same authors looked at salamanders, which also lack middle ear structures and live in water and on land. The team showed that even juvenile salamanders, which are fully aquatic, can detect sound in air.

The findings suggest that early tetrapods were pre-equipped to hear sounds in air, which probably helped them to adapt to life on land and eventually led to more-modern middle ear structures.

J. Exp. Biol. 218, 381387 (2015) ; Proc R. Soc. B 282, 20141943 (2015)

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