Page 14 - Spring 2018

Physics and the Mammalian Ear
A clear example for evolutionary compromise can be seen in snakes, which have no external ear canal, and it is, falsely, supposed that snakes are deaf. Snake ancestors possessed normal middle ears and the loss of the eardrum in early snake evolution resulted in much poorer hearing. They be- came much less sensitive and lost the perception of frequen- cies above 1 kHz (Manley, 2002).
This evolutionary change was therefore unlikely to be guid- ed by the physics of hearing, and there must be a biological explanation involving a different change that was of greater survival benefit for the snake ancestors than the disadvan- tages of hearing insensitivity. Snakes that were able to de- vour large prey survived on a single prey item for long pe- riods during which no energy was needed to seek prey, an enormous advantage. The ingestion of such large prey was facilitated by the newly evolved ability to reversibly disartic- ulate the jaw and greatly stretch the tissues between the then floating jaw joints. It was obviously impossible to maintain a delicate eardrum suspended precisely where the stretch was necessary. Once the middle ear had been partially decon- structed in this way, the auditory epithelium of the inner ear was also simplified (Manley, 2002).
The design of the mammalian cochlea also provides other clear examples of compromises that ensure survival of spe- cies in their respective environment. For example, it is pos- sible to genetically manipulate the protein structure of the tectorial membrane overlying the sensory cells and, through this, change the physics of the cochlea such as to enhance its frequency resolution (Russell et al., 2007). Why did evolu- tion not use this opportunity? One trade-off of highly selec- tive tuning is that the operation of the cochlea is brought close to the safety limits that ensure detection of weak sig- nals (Lukashkin et al., 2009). Survival of an animal near the base of the food chain has a much higher dependency on detecting the sounds of a predator than using fine frequency resolution to detect which type of predator it is.
Opportunism of Evolution and Deterministic Dominance of Physics Evolutionary compromise is not the only principle that is different from the deterministic laws of physics. Opportun- ism of evolution, which utilizes preexisting structures and genotypes, also makes it less providential. Any biological sensory system will be subject to evolutionary selective pres- sures. These pressures can be not only the direct result of the physics underlying the characteristics of the signal to be sensed and the properties of the medium transmitting that
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signal but also the result of the requirements underlying the biological building blocks sensing that signal. In the case of the ear of land vertebrates, these include the outer and middle ears and the large variety of cellular and extracellular components that contribute to the structure and physiology of the sensory organ itself.
The shape of the mammalian cochlea itself is a good example of opportunism in evolution (Figure 1). A proposed reason for the spiraling snail-like shape of the cochlea is that it pro- vides efficient packing of the cochlear duct within a single bone of the skull, a proposal supported by a recent study of the human cochlea (Pietsch et al., 2017).
This view, however, is counter to an elegant analysis of the physics of the spiral in a wide range of nonhuman, mamma- lian cochleae that revealed a correlation between the curva- ture gradient of the spiral (rate at which the spiral tightens) and low-frequency hearing limits (Manoussaki et al., 2008). The greater the rate of tightening of the spiral ratio, the greater the ability of the cochlear curvature to focus acoustic energy at the outer wall of the cochlear canal (whispering gallery effect) that enables sound energy to propagate toward the apex of the cochlea where low frequencies are detected. The human cochlea study (Pietsch et al., 2017) revealed that the spiral of the human cochlea varies markedly between individuals and that its shape is influenced by neighboring structures such as the large facial nerve. Does this mean the outcome of the physical-mathematical analysis on a wide va- riety of mammals is wrong? No. It could indicate that space preservation was paramount in determining the spiral shape of the mammalian cochlea and that selective pressure has led to further refinement of the spiral and enhanced the de- tection of low frequencies vital for survival of some mam- malian species but not, apparently, humans. Release from this design constraint may free the human cochlea to other selective pressures, including diversity itself, in a changing communication environment.
To summarize, evolution is a creative and yet highly con- servative phenomenon whose trajectories are greatly influ- enced by evolutionary coincidences rather than by any de- terministic dominance of physics.
Convergent Evolution
and “Optimal” Designs
Does the necessity of detecting the same physical modality inevitably lead to similar, convergent “optimal” designs of hearing organs? Of course, the hearing system of vertebrates provides remarkable examples of convergent evolution. For