MONDAY, 5 JUNE 2023The land vertebrates, a lineage of animals to which we belong, evolved from bony fish, and underwent a number of adaptations for terrestrial life in doing so. Here, we will follow the evolutionary tale of the stapes, one of the bones in our middle ear that transduces sound waves from the air to the fluid-filled inner ear system. This is necessary because air has a much lower impedance than water, and without this, sound would just reflect uselessly off the inner ear. While we now know that the middle ear corresponds to parts of the gill arches in bony fish, a testament to the evolutionary link between us and our fishy ancestors, the journey scientists took to figure this out began in the 19th century, and involved multiple scientific breakthroughs and novel experiments.
Originating from a filter-feeding ancestor, all vertebrates have the genetic potential to produce seven identical pharyngeal slits. In the course of evolving to live on land, each of these have diverged in their structure and function. The most well known is the first, or most anterior pharyngeal slit, which became the jawbone, or mandibular arch that defines the mouth. It is probably the most significant evolutionary step in the history of vertebrate evolution, as it enabled the predation of other animals – especially as teeth evolved. The fate of the other arches were, however, more variable. Here, we will look at the fate of the second, or hyoid arch.
This is a tale told by the fossils of ancestral vertebrates, spanning the transition from sea to land. In fish, the monolithic hyoid arch comprises multiple fused bones, which help open and close the link between mouth and gill cavities, synchronising their movement to guide the unidirectional flow of water past the gills for optimum extraction efficiency of oxygen. The amphibious fish Panderichthys, however, separated the hyoid arch into an upper and a lower component. While the lower hyoid arch still functioned as before, the upper portion broke off and became vestigial. It is clearly non-functional, as there is nothing for this bone to transduce against. And later, in an early limbed land vertebrate called Acanthostega, we see proof that the upper hyoid arch would ultimately become the stapes, as we see a smaller stapedial bone resting against the auditory capsule.
Of course, the existence of transitional fossils has massively helped our understanding of the evolutionary course of the stapes, but these had yet to be discovered at the dawn of the field of evolutionary biology two hundred years ago. And, morphological evidence is by definition incomplete, as they are evolutionary snapshots at a random point in time, with no hint as to what happened in between. For example, the incus and malleus in the mammalian middle ear lack a counterpart in amphibians and reptiles. Thus, even though Reichert and his early 19th century peers were quick to agree that they arose from the posterior part of the first pharyngeal arch, there was disagreement about where the stapes arose from. Here, embryology has been helpful in filling in the blanks within the evolutionary history of the stapes. During embryogenesis, the same structure may change in radically different ways, allowing us to derive evolutionary homologies between apparently unrelated structures. It was one such painstaking sequential analysis of human embryos done at the turn of the 20th century that would conclusively prove that the stapes formed from a mass of undifferentiated tissue growing from the tip of the hyoid arch.
Beyond this, the discovery of ‘homeotic mutants’ was the ground for further breakthroughs in understanding the evolution of the stapes. These are genetic mutations that result in mutants with correctly formed structures but in the wrong places. For example, fruit flies with Bithorax mutations have two thorax segments. As each thorax forms intact wings, the resulting fly has two pairs of wings not unlike a dragonfly. For the first time, scientists had a mechanistic link between individual genes and morphological differences, and by extension the anatomical variation between animals.
As it turns out, these homeotic mutants have mutations in homeobox genes, which are genes that confer positional identity throughout the developing embryo. During embryogenesis, self-reinforcing gradients of expression form between different homeobox gene families, creating a coordinate system that endows each cell with the knowledge of its position within the embryo. This information is then accessible to genes controlling cell growth, specialisation, and death, enabling a mass of otherwise undifferentiated tissue to be sculpted into a functioning thing. In the case of the hyoid arch, it is the Dlx homeobox gene family that determines the fate of the upper and lower portions of the arch.
It was in this context that the concept of evolutionary developmental biology (Evo-Devo) arose, a new paradigm integrating molecular genetics, developmental biology, and evolutionary biology. Throughout the 21st century, homeobox and other genes have been extensively manipulated to help us understand how they help determine morphology. What do we gain from doing so? One key reason is that there is less room for subjectivity in determining homology when considering genetic information in the context of morphological comparisons, allowing greater confidence in our historical reconstructions of the evolutionary process. The fact that mutations in these genes are the root cause of morphological differences gives us testable hypotheses to prove the existence of deep homologies that are only tenuously supported by other types of evidence. We see this in how provably related variants of the same gene (PAX6) drive the formation of eyes of different shape and structure across multiple distantly related species.
And yet, knowing that the delicate interplay of these gradients uniquely define a positional coordinate system is one thing. A far harder task is to reconcile these gradients and how they work with the observed morphology. We do not yet fully understand the genetic changes underpinning the restructuring and subsequent repurposing of the hyoid arch. This indeed brings us to the larger question of how the mammalian ear evolved, as the outer, middle, and inner ear are homologous to various disparate structures in fish. In fact, the hair cells within the inner ear are homologous to the external sensory lateral line in fish, posing the obvious question of how an external structure became an internal structure embedded within the skull.
Even so, knowing what happened is half the battle. We do not yet know what evolutionary pressures could have conceivably reshaped a bony support into a tool of hearing, as there is no conceivable reason as to why the hyoid arch split into two in the first place. Perhaps, despite all appearances, the vestigial upper hyoid arch had a cryptic function in Panderichthys, or perhaps it was the inadvertent result of other evolutionary pressures on the complex interplay between homeobox genes. Moreover, the homeobox genes are extremely ancient, dating back to the unicellular last universal common ancestor (LUCA) and thus predating the origins of multicellularity. What gave them the power to control body shape? Are they innately special, or just a serendipitously useful gene from the annals of evolution? Whatever the reason, we at least know that we are on the right track to help us uncover even more answers about the how and why of evolution.
Hayoung Choi is a first-year undergraduate studying Natural Sciences at Peterhouse. Artwork by Mariadaria Ianni-Ravn.