Ear Evolution

Summary: Written as part of a capstone course, focusing on human evolution. I was asked to develop a topic, write, and present my research.


Tracing the evolution of the ear through models of ontogeny and biological anthropology

Audrey M. Williams

Introduction

All life on earth derived from a single origin. The magnificence of this concept is amplified when considering the extreme physical and functional diversity of species. As humans, one response to diversity is to categorize species according to physical similarities, such as the vertebral column and structures responsible for locomotion, respiration, and reproduction. Qualifications for membership in the family Hominidae, of which humans and other great apes are members, reflect the value we have assigned to certain traits. Features commonly used to distinguish great apes from other species are what generate the interest of researchers; the comparatively small size our eyes and nose among primates serving as an example. Consequently, this bias limits the amount and quality of research on traits that aren’t perceived as unique to humans or the Hominidae family.

The ear is often neglected when discussing human evolution; vision, olfaction, and means of locomotion are frequented the most. However, as I discuss in the body of this paper, ear structure and function is one of the more significant adaptations in our evolutionary history.

The lack of individual control over hearing is suggestive of a primitive need for constant auditory surveillance. Without the assistance of modern technology, our ability to block sound from entering the ear canal is very limited. Vision is capable of being controlled by the eyelids, taste and scent by utilizing the mouth instead of the nose and touch by limiting or increasing contact with an external stimulus (as well as the historic and present use of controlled substances). There is little research existing on auditory systems and what does exist tends to focus on mechanisms of hearing in modern species, with little consideration to morphology. In this paper, I explore the evolutionary course leading to the development of the ear seen in an anatomically modern human.

Methods

The ear is comprised mainly of cartilage, with three fragile bones: the incus, malleus, and stapes. Due to the ear’s composition, it is poorly represented in the fossil record; even in more recent specimens. In order to fill the gap where physical evidence is lacking, I referenced the fields of human ontogeny and embryology and biological anthropology.

For the second model, I have established the general environments of Australopithecus afarensis and Homo erectus through analysis of botanical and faunal artifacts within a shared archaeological context. Establishing the environment of hominids is necessary in order to find the most appropriate non-human primate groups to act as comparative models. At this point, it is important to clarify that I am not suggesting that every aspect of the interactions between early hominids and their environment will be present in the selected primate groups. However, due to the relatively recent addition of external stimulus brought on by human culture, the relationship between non-human primates and the environment is better suited as a model to understand the role of the ear in early hominids.

By incorporating a model of ontogeny with primate comparative models I hope to establish a realistic course for the evolution of the human ear. When discussing the relevance of this understanding I attempt to identify some of the effects an increased awareness may have on a variety of species.

Characteristics of sound

Sound waves; pressure; amplitude; intensity. The vibrations of molecules vary by speed and pressure, which is influenced by forces experienced through transport. The transport of sound waves may differ according to the interaction between sources, medium and particular interferences found in the medium. For the purpose of this paper, I define ‘source’ as the biotic or abiotic production of sound, ‘medium’ as the environment sound is being transported through and ‘interference’ as abiotic and biotic factors that change the initial properties of the sound.

Sources can be both biotic and abiotic. An example of an abiotic source is the sound produced by rustling leaves, while biotic sources include animal vocalizations. Each source has been researched and assigned its own intensity level, represented on a decibel scale. For example, ‘rustling leaves’ is assigned a level of 10dB while ‘military jet takeoff’ is measured at 140dB Physics 1996).

The type of medium a sound is traveling through determines the distance and speed that sound travels. The three basic mediums are air (gas), water (liquid) and solids (Physics 1996). The inertial and elastic property of each medium influences the speed of sound. Since elasticity is most influential on speed, solids promote the fastest travel, followed by water then air (Physics 1996).

An additional determinant within the medium is the amount biotic and abiotic interference experienced through transport. Since sound waves operate through pressure, the same properties that apply to a sound source also apply to sound interference during travel. Just as the source of sound is either abiotic or biotic, so too are the forces acting on interruption. These important aspects of sound are why an understanding of an organism’s environment is important to understand auditory evolution. If the noise associated with a particular environment is understood, accurate predictions of why the ear developed will become more apparent.

The audio receptor system (Figures omitted)

Human ontogeny

“The evolution of the cell can be regarded as the ‘big bang’ of biological evolution even though it took a very long time. The origin of embryonic development from cells can be regarded as the ‘little bang’ since the cell was already there.” (Wolpert 1994: 79)

The ontogeny of a species is sometimes referenced in evolutionary biology to compensate for a lack of material evidence in the fossil record. One could argue that this method is unsuitable for establishing evolutionary adaptations, but research has provided a link to cellular development over time and the stages undergone in embryonic development that cannot be ignored (Minelli

2003:1). To track early stages in human evolution, the anatomy and physiology of pre-mammalian species must be considered. These stages represent drastic change, replicated in the appearance and disappearance of similar structures throughout embryonic development.

Approximately two billion years ago marked the beginning of eukaryotic cells. Contained in their nucleus, these cells carried ‘instructions’ for self-replication to ensure survival. Following the release of oxygen into the atmosphere, multicellular organisms began to form external and internal skeletons. An event termed the Cambrian explosion was a turning point for the structure of many water-dwelling organisms. Primitive fish began to develop a jaw from bones that formerly supported the gills. These formations provide a foundation for additional adaptations, eventually leading to the structures of our modern form.

Mammaliaformes

In the human embryo, Meckel’s cartilage connects the middle ear bones to the jaw during the first weeks of development. Shortly after, the cartilage surrounding the ear disappears, replaced with a “fibrous membrane” (Junqueira 2003). The first known signs of this separation are found in the Mesozoic mammaliaform, Yanoconodon allini (Y-a allini)(Luo,2007). The ~125 mya fossil collection of Y- allini shows a permanent connection between Meckel’s cartilage and the mandible, however the future site of the malleus and incus are partially separated from the mandible (Luo 2007), representing an evolutionary intermediate stage in the structure of the modern mammalian ear.

The appearance of Hadrocodiu- wui in the fossil record corresponds with the disappearance of Meckel’s cartilage and complete separation of the middle ear from the mandible, visible in the early months of fetal development. The removal of cartilage freed the bones of the upper jaw in order to create the three bones of the middle ear, providing mammals with a heightened sense of hearing as well as stronger jaws.   

More sensitive hearing is posed as one of the causes leading to the large brains of mammals, opposed to their dinosaur competitors. According to various theories involving the evolution of intelligent life (Jastrow, 2008)(), the increased size of the mammalian brain was a consequence of a nocturnal lifestyle that depended on “the senses of smell and hearing for survival, rather than the sense of sight (Jastrow, 2008:343)” utilized by the dinosaurs. The use of multiple senses for information naturally led to a more complex mental process.

Early tree dwellers (prosimians) (Jastrow, 2008) advantaged from their superior auditory and vestibular function. The balance provided by the vestibular system supported nesting and travel through trees. An increased sensitivity to sound resulted in an earlier detection of predators, increasing the likelihood of survival.

Fossil hominids

By the time of our hominid ancestors, approximately 6 mya, the structural and anatomical position of the ear was very much like that of a modern human. The migration of the ear to the side of our head was complete, freeing the eyes to move towards the front of the face supporting stereoscopic vision. Equilibrium, like that of early tree dwellers, improved hominid ability to swiftly move arboreally and terrestrially in order to capture moving prey or to escape predation. Fossil records indicate that hominids such as Australopithecus-afarensis did indeed frequent closed and open wooded areas (Sourcebook 2006:79). This mix of locomotive capabilities is assumed to be the result of a partially frugivorous diet as well as the utilization of trees as sleep sites.

Other hominids, up until anatomically modern human, lived similarly. There was of course variation in diet and habitation as well as the use of tools and art by several hominid species, including homo-erectus, homo-habilis and neanderthal. However, little is known about the life and social structure of early hominids, besides occasional burial containing multiple individuals. Fossils such as a more developed Broca’s area in the skull of Homo-habilis provide support for a complex social organization. This is furthered when comparing early hominids to modern non-human primates such as species from our sister taxa homo; the common Chimpanzee (Pan-troglodytes) and Bonobo ( Pan-piniscus).

Implications for humans and global diversity

Treating the ear as a vital sensory system is necessary in order to accurately assess risks to the health of humans as well as other species. Describing the structure and mechanics of the human ear is relatively easy with the help of modern technology. However, measuring the effects of acoustic interaction on human hearing is much more difficult. Unlike Australopithecus-afarensis and Homo-erectus, modern humans have a different quality and quantity to the sounds they encounter on daily basis. This increase in acoustic complexity is brought on by cultural elements such as music, extensive vocal communication and anthropogenic noise created and furthered by technology.

Technology involving amplification is one instance of developments that result in human deafness. In the past nerve fibers on our outer and middle ear filled the role of sound amplification (Carlson 2009). This natural adaptation of the ear aligned with the frequencies that were safe for an individual to receive; frequencies that were less likely to damage the ear drums.

Now, due to the introduction of the ear to unnaturally high frequencies (through mechanical amplification) nerve fibers are being destroyed leading to increased stress on the eardrum and in some cases, deafness.

In every ecosystem, vocal signaling is proven to be a significant aspect of communication among species. Although there has been acoustic research in the past, few studies have explored the way mating calls, as well as other biotic and abiotically produced sounds, interact within ecosystems as a whole. Understanding the role of hearing within specific mating systems dependent on vocal communication is necessary in order to predict threats that may be accelerating the destruction of global biodiversity.

The effects of the acoustic environment on population size are present in a small-scale study involving multiple frog species in Thailand. Differences in call structure and timing were observed to be dependent on the type of acoustic interference present before and during periods of male calling. The most significant change in call structure and length were in response to anthropogenic interference. When airplane flyovers occurred, call amplitude and frequency in some species increased while significantly decreasing in others (Sun 05:424). The cause of this unequal response is attributed to the masking effect produced when a sound frequency is shared between certain species and engines of passing planes. In order to have call success the intended recipient (i.e. female frog) must be able to distinguish the sound of her caller among all other noise. When the frequency is the same, masking is increased and mating success is lowered in certain species, which could lead to extinction.

If anthropogenic acoustic threats are considered in future conservation measures global diversity could experience increased sustainability. However, the present amount of research on acoustic interactions within ecosystems is insufficient for implementing such policies.

Conclusions

From a time shortly following the appearance of an internal skeleton in primitive fish, the human ear has experienced drastic evolutionary change. Equilibrium and auditory sensitivity allowed early tree dwellers to survive despite the physical disadvantage of small stature and absence of external protection. Without the auditory and vestibular superiority provided by the ear, it is unlikely that humans would have advanced to their present form, if at all. The ear may not distinguish humans from other species, but its role in human evolution warrants increased attention. Understanding the way certain features influence our evolutionary history is important for our concept of self, but is vital for the continued health of our species as well as global biodiversity.


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