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.
A Rube Goldberg contraption is built of a long sequence of many pieces that all perform some simple task in order to accomplish one larger goal. In the same way, each of the parts of the three layers of the ear perform simple functions in order to allow the ear as a whole to process sound waves and send a signal to the brain. Our ears may look different from animal ears on the outside, but Shubin again points out inner similarities.
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.
Shubin brings back the arches that form all the structures of the human head, this time looking with more nuance at the complex development of the middle ear bones. Gaupp builds off of Reichert’s work in another example of how scientists can work together to make more impactful discoveries than they could alone. Yet Gaupp’s theory alone is not enough to prove that mammals evolved from reptiles, because he did not have the intermediary forms that show the jaw bones becoming ear bones.
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.
Though Gaupp saw the similarity between reptilian jaw development and mammalian ear development, it took Owen’s specimens and Gregory’s insight to cement the theory that these jaw bones gradually became ear bones. This points to Shubin’s larger point that one scientist’s work is often not enough to make large claims, but that scientists can work together as they each provide a piece of the evidence. Gregory filled in a huge gap in the evolutionary path between reptiles and mammals, one that seemed insurmountable due to the many superficial differences between these species groups.
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.
There are many developmental similarities between the stapes and the jaw of fish, even though the bones themselves look radically different. Shubin again pushes past the surface to get at the fundamental similarities that connect these anatomical structures. As Tiktaalik’s primitive limb fills in a gap of the developmental path of appendages from fins to hands, the fossil record also holds intermediate versions of the jaw bone moving to the ear.
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.
Shubin gives the example of a flexible snow globe to explain the inner ear, because the inner ear is hard to visualize for most people who have never seen this bodily structure.
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.
Shubin builds from the simple version of the inner ear as a snow globe and adds nuance to adequately explain the complexity of this structure specifically in humans. Only then does he introduce the additional sensory capabilities of the human inner ear, so that each piece is easier to understand.
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.
As when the function of genes is easiest to understand based on mutations, the function of the inner ear is easiest to understand when it is misfiring. The same basic approaches can be used for many different experimental questions. Shubin then reduces the complex ear-eye connection to the key functions that allow the ear and eye to work together when both the are working properly.
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.
Shubin focuses on the neuromast of trout, not explaining that trout also have an inner ear that handle the trout’s sense of hearing. The unanswered question here is how the neuromast functions were enveloped into the inner ear in land animals, as Shubin offers no sense of when the first animal with an inner ear capable of sensing acceleration developed.
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.
Shubin seems to be suggesting that primitive fish might have had the capability to form both inner ears and neuromasts, from which modern animals refined these organs based on which mechanism was most useful to their environment. This is similar to the evidence that primitive fish have mechanisms for smelling molecules in air and water that was later refined for land animals. The ear is another example of a basic template that became more complex over time.
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.
Recall from Chapter 5 that amphioxus was the example of an animal with a primitive “front” that housed light-sensing organs as a precursor to animals with heads. The fixed orientation of this neuron might have contributed to amphioxus’ sense of forward direction, despite amphioxus not having a true ear.
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.
Genetic similarities tie together different species once again. Mice and humans are compared often because scientists have a large body of knowledge about the mouse genome and the human genome after years of using mice as test subjects in labs.
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.
The eyes and the inner ear share a developmental path based on this genetic evidence. As when copies were made of odor genes or the genes for color vision, it is likely that a descendent from the box jellyfish produced a copy of the jellyfish’s combined Pax 2 and Pax 6 genes that then underwent thousands of generations of mutations until Pax 2 was specialized for ears and Pax 6 for eyes.