Your Inner Fish

Human ears are rather boring on the surface, but the mechanisms that funnel sound into the inner ear act like a complicated Rube Goldberg contraption with multiple differently shaped muscles and bones. There are three main parts to the ear: the external ear visible outside the body, the middle ear with three ear bones, and the inner ear of sensory cells, fluid, and a tissue cushion. The external ear is a relatively new evolutionary development, but the middle and inner ear is connected to the bone structure of sharks.

Starting with the ear bones, Shubin recalls from Chapter 5 that two of the ear bones (the malleus and the incus) develop from the first arch in the head and the third bone (the stapes) develops from the second arch. In 1827, German anatomist Karl Reichert studied the gill arches to find that two of the ear bones in mammals corresponded to two jaw bones in reptiles. Ernst Gaupp continued this study in 1910 to interpret Reichart’s conclusion to mean that mammals evolved from reptiles over time.

Gaupp worked only with living creatures, and so had no proof that the malleus and incus bones gradually moved from the jaw in reptiles to the ear in mammals. Richard Owen, the anatomist, then appears again, this time cataloguing small dog-sized reptiles found in South Africa that had oddly mammal-like teeth. In 1913, W.K. Gregory, a paleontologist at the American Museum of Natural History, connected Gaupp’s theory to Owen’s mammal-like reptiles to find that the reptiles that had the closest to mammal-like features in their teeth also had very small bones in their jaw that shifted back toward the ear. Gregory thus proved that the malleus and incus evolved from reptilian jaw bones.

If the malleus and the incus evolved from the reptilian jaw, Shubin now turns to the development of the stapes. This tiny bone in the middle ear of mammals comes from the second arch, just like the huge bone in the upper jaw of fish and sharks. These two incredibly different bones are even served by the same second-arch nerve in both mammals and fish. The fossil record shows a progression of fish to amphibians that have smaller and smaller jaw bones as these animals began to live on land and needed a way to hear higher frequency sounds in air instead of in water.

The Inner Ear – Gels Moving and Hairs Bending. The mammalian inner ear has different parts for functions of both hearing and balance. Special cells send hair-like bristles into the gel that fills the inner ear. If the gel moves, the bristles bend and send a signal to the brain that is interpreted as sound, position, or acceleration. Shubin imagines the inner ear like a snow globe with a flexible case that also moves when the snow globe is tipped upside down.

Human inner ears are even more sensitive because there are rock-like structures on top of the gel membrane that move when the head is tilted. Humans also perceive acceleration through three gel-filled tubes inside the ear that move when the human body accelerates or stops. Both of these position and acceleration mechanisms are also connected to the eye muscles that help keep humans looking in the same direction even when our head tilts or moves.

The easiest way to understand this eye-balance connection is to mess with it. If a person drinks too much, the alcohol makes the fluid inside the inner ear tubes less dense and convinces the inner ear that the person is moving. The brain then sends this “we’re moving” message to the eyes, causing the eye muscles to twitch. Hangovers are also an effect of the inner ear. Even if the liver removes alcohol from the bloodstream, there is still alcohol in the inner ear that convinces the inner ear that the person is moving even when they are standing still.

Fish like trout have a primitive version of the human inner ear. Trout hang out in quickly moving eddies in streams, and need a mechanism to sense the motion of the current around their bodies. Small sensory lines run under the trout’s skin and send hair-like projections into jelly-filled sacs called neuromasts. When water flows around the fish, the neuromasts change shape and the hairs send an impulse to the fish’s brain that tells the fish how fast the water is moving.

It’s hard to tell whether neuromasts or inner ears developed first, as the inner ear is almost never preserved with fossils. Due to the similarity between neuromasts and inner ears, it is likely that one evolved from the other. What is clear is that animals have developed a better sense of hearing over time, creating a bigger inner ear in mammals than in amphibians and reptiles. The sense of acceleration became more sensitive as well, with only one inner ear tube in ancient jawless fish and three ear tube canals in modern fish and other vertebrates.

The neurons inside the gel of the ear have an even more ancient history. Neurons in the ear are different from any other neurons in the body, as they have a long “hair” on the outside and remain in a fixed orientation in the body. These neurons have been found in animals that have no heads at all, like the worm amphioxus from Chapter 5.

Genetic information also tells the long history of the ear. A gene called Pax 2 seems to control ear formation in both mice and humans, as a mutation in Pax 2 creates an animal with a faulty inner ear. Pax 2 is also active in the neuromasts of fish.

Jellyfish and the Origins of Eyes and Ears. There is a link between the Pax 2 gene for ear formation and the Pax 6 gene we saw in Chapter 9 for eye formation. Box jellyfish are a fairly primitive animal that have over 20 eyes with a full cornea and lens spread all over their bodies. The gene that forms these eyes seems like a primitive version of sequences from both Pax 2 and Pax 6. This link helps explain why many human birth defects affect both the eyes and the inner ear.