Can you hear me now? The evolution of malleus, incus and stapes in early hominins

By Ellie Smith (Claremont McKenna College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only.]

Ears come in all shapes and sizes. Bird ears superficially appear as simple holes on the side of their head (Figure 1), and some insects even have ears on their legs (Figure 2). There is no external structure of an ear in fish yet they have intricate internal structures that detect sound vibrations and pass those signals to their brains. An animal’s ability to hear is determined by the shape and structure of their ear. Hearing is used in predator/prey relationships, mating, niche building etc., thus it is key to an animal’s survival and the propagation of species.

Mammal ears have three parts: the external, middle and internal. The external ear carries sounds from the environment to the tympanic membrane and protects the tympanic membrane. The middle ear transmits sound frequencies of different lengths from air to fluid from the tympanic membrane to the oval window, and the internal ear picks up these frequencies and transmits signals to neurons.

In mammals the middle ear is made up of three small ossicles: the incus, malleus and stapes. These ossicles transmit sound from air to fluid, and there are two neat mechanisms to mitigate the loss of sound pressure that occurs at the air-fluid transition. First, malleus and incus form a lever system, with the malleus (in-lever) being slightly longer than the incus (out-lever). This lever provides a mechanical advantage, i.e. the resulting force is higher. Second, the tympanum is much larger than the oval window, so the sound pressure (pressure=force/area) is much larger at the oval window — and that’s where the inner ear picks up the sound waves.

Not only do the shapes of the ossicles impact the sounds that animals pick up but the three ossicles are evolutionarily significant. The evolution of the malleus and incus from the lower jaw into the ear is a key trade mark in mammals and a determining factor of the evolution of hearing (Quam et al. 2013). Recently palaeoanthropologist Rolf Quam and his colleagues compared the shapes and sizes of the middle ear ossicles found in fossils of two ancient hominins the Australopithecus africanus (figure 4) and the Paranthropus robustus. The shape of the malleus in both taxa closely resembled the shape of the malleus in present day humans while the stapes and incus shapes more closely resembled African and Asian great apes (Quam et al. 2013).

Skull of Australopithecus africanus. [http://en.wikipedia.org/wiki/File:Mrs_Ples_Face.jpg]

Skull of Australopithecus africanus. [http://en.wikipedia.org/wiki/File:Mrs_Ples_Face.jpg]

Quam and colleagues compared the shapes and sizes of the malleus, incus and stapes from a 1.8 million year old fossil of P. robustus and an incomplete sample of bones from a fossil of A. africanus 3.3 million to 2.1 million years old to the shape of the same three bones taken from African apes, orangutans, and Homo sapiens through previously developed methods (Quam et al. 2013). Measurements were collected from pictures of the bones after they were extracted from the tympanic cavity. Angle measurements between the bones as well as size measurements were taken (Quam et al 2013). They found that the malleus in both specimens was much smaller than the malleus of African and Asian great apes and clearly resembled the malleus of humans. The shapes of the incus and stapes of the fossils on the other hand resembled those of the African and Asian apes more closely (Quam et al 2013).

Previous studies of the evolution of the ossicles in human ancestors are rare because the bones are not frequently found in the fossil record, yet, understanding trends in the evolution of the shapes of these bones may spread insight on the evolution of sensory systems in our ancestors. As was previously stated, the shape and relative lengths of the malleus and incus determine how sound vibrations are transmitted from air to fluid and ultimately picked up by neurons and transmitted to the brain. Quam and colleagues finding suggests that early hominins heard differently than humans of today. The lever ratio found in the hominins between the malleus and incus is intermediate between modern humans and other apes. Interesting, a short incus suggests that early hominins were less sensitive to midrange frequencies, similar to the mid-frequency notch in the audiograms of many primates (Quam et al 2013). This is consistent with past studies, which were centered on the shapes of the inner, outer, and middle ear. This finding can help us map the evolution of hearing in primates and the evolution of hearing in hominins in general because similar shaped ossicles in both P. robustus and A. africanus suggests “a deep and ancient origin” and could be a key feature of hominins (Quam et al. 2013). The fact that the malleus of the fossils resembles the malleus of humans suggests that the malleus evolved in a common ancestor of both groups. Future studies looking at the shape of the malleus of ancestors of early hominins and Homo sapiens could help clarify when this trait evolved. In contrast, the incus more closely resembling the incus of non-human hominoids suggests that the human-like incus evolved later. One could also compare the shape the incus of more derived hominins to Homo sapiens to determine when exactly this morphological transition occurred. Future studies could also improve our knowledge of the evolution of the ear by modeling the various hearing systems of P. robustus, A. africanus, humans and other hominoids to see the impact that the different shaped bones have on sound transmission (Cormier 2013).

Hearing is complex and involves many structures that transmit signals from the environment to the brain of an animal. Thus, it is hard to draw conclusions about hearing from simply comparing the shapes of three ossicles found in several hominin taxa as was done in this study (Cormier, 2013). Consistent with this, other studies have indicated that the outer and inner ear structures have a significant impact on an organism’s ability to hear. Callum Ross at the University of Chicago in Illinois has stated that Quam et al.’s finding was “a step in the right direction in the understanding of human hearing” (Comier 2013) but his own research has determined that the shape of the outer ear can have an even greater effect on hearing ability (Coleman et al 2004). He argues that differences in ossicle shape are not as evolutionarily significant as other key characteristics of hominins including bipedalism and the evolution of brain size (Coleman et al 2004).

References

Coleman, M. N. & Colbert, M. W. 2010 Correlations between auditory structures and hearing sensitivity in non-human primates. J. Morphol. 271, 511–532. (DOI: 10.1002/jmor.10814)

Coleman, M. N. & Ross, C. F. 2004 Auditory Morphology and Hearing Sensitivity in Fossil New World Monkeys. Anat. Rec. A Discov. Mol. Cell. Evol. Biol. 281, 1123–1137. (DOI: 10.1002/ar.21199)

Cormier, Zoe. 2013 Hearing Changes Could Be Ancient in the Human Line. Nature  (DOI:10.1038/nature.2013.12976)

Quam, R. M., De Ruiter, D. J. , Masali, M, Arsuaga, . J.-L. , Martinez, I. ,Moggi-Cecchi, J.. 2013 Early Hominin Auditory Ossicles from South Africa. Proc. Nat. Acad. Sci. 110, 8847-851. (DOI:10.1073/pnas.1303375110)

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