The Ear's Missing Link

Ear in sight.
The middle ear evolved as primitive fish (top) became more advanced (bottom): The spiracle enlarged as the proportions of other bones (yellow, blue) changed.

Adapted from: M. Brazeau and P. Ahlberg, Nature, 439 (January 2006)

Whether a heart, a toe, or a nose, evolutionary biologists are keen to know where our body parts came from. Now they're getting a better idea of how our ears formed thanks to a 370-million-year-old fish, whose jawbone was beginning to resemble a bone found in our middle ear.

Before we used the middle ear to amplify and transmit sound, fish used its components to breathe. Over time, a tube called a spiracle, which connects the gills to the water outside, evolved into a chamber behind the eardrum. And a bony strut that connects a fish's jaw hinge to the brain case became one of three tiny bones in this chamber.

The early stages of this transition have now been studied by Martin Brazeau, a graduate student in evolutionary biology at Uppsala University, Sweden, and his advisor, paleontologist Per Ahlberg. The researchers analyzed a skull of Panderichthys--an ancient fish that evolved at about the same time as tetrapods (early four-legged land-dwellers) from a common ancestor. The team compared the fish's bones and head structure to fossils of a more primitive fish and an early tetrapod.

To their surprise, the researchers discovered that Panderichthys wasn't very fishlike when it came to the spiracle and the bony strut. Its strut was much smaller and finer than those of the other fish studied were. And the spiracle was much larger than expected. Thanks to these modifications, the bony strut began to resemble the stapes, the middle ear bone that derived from it. "Our work overturns the previously held notion that Panderichthys was primitively fishlike in [what became] the middle ear region," says Brazeau. The findings indicate that these stages of ear evolution were set 10 million years before tetrapods appeared, the team reports 19 January in Nature.

The work "is filling an evolutionary gap," says paleontologist Tom Rich from Museum Victoria in Melbourne, Australia. Zhe-Xi Luo, a vertebrate paleontologist from the Carnegie Museum in Pittsburgh, Pennsylvania, adds: "We now have a finer understanding about exactly where and when these features started to appear. They occurred earlier than we thought."

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