How Did Darwin Interpret The Fact That Animals Have Similar Bone Structures
Q&A: Morphological insights into evolution
Neal Anthwal
Section of Craniofacial Development and Stem Cell Biology, King's Higher London, Floor 27 Guy's Tower, Guy's Hospital, London Bridge, London, SE1 9RT UK
Abigail Due south. Tucker
Department of Craniofacial Evolution and Stalk Jail cell Biological science, King'due south College London, Floor 27 Guy'southward Tower, Guy's Hospital, London Bridge, London, SE1 9RT UK
Abstract
In this question and respond article we discuss how evolution shapes morphology (the shape and pattern of our bodies) just also how learning about morphology, and specifically how that morphology arises during development, can shed light on mechanisms that might allow modify during evolution. For this we concentrate on recent findings from our lab on how the middle ear has formed in mammals.
How does development help us empathize morphology?
Development is cardinal to understanding why we look like we do: it tin can explicate why humans take four limbs each with five digits, two forward facing camera eyes, and a mouth full of teeth of different shapes compared to why fruit flies have 6 limbs plus two wings, two compound eyes, and a proboscis for a mouth. Our anatomy has been slowly shaped over millions of years, and an understanding of evolutionary history tin can assistance explain the similar pattern of basic observed in vertebrate limbs. Humans, bats, reptiles and whales evolved from a common ancestor, and the developmental programme to make limbs is shared across these animals and is based on that of this common ancestor. Although the limbs of vertebrates have diverged functionally into the wings of bats, the arms of humans, the forelimbs of reptiles and the fins of whales, they are nevertheless homologous: the general skeletal structure is similar in each, despite large differences in individual bone size and shape (Fig.one). In contrast, the common ancestor of humans and fruit flies did non have any limbs, so our limbs and the limbs of the fly are independently evolved and not homologous.
Comparative beefcake of vertebrate limbs. The general skeletal structure of vertebrate limbs is similar in each species, despite large differences in individual bone size and shape reflecting the unlike functions
Information technology might be useful to be able to fly—why don't we evolve wings on our backs like the fly?
Information technology would exist useful, but unfortunately information technology'due south virtually on impossible. This is because the form of an organism is fabricated during its embryonic development by following developmental programmes encoded in our genes. These programmes need to be adjusted to change anatomy, and adjustments can simply be fabricated on what is already in that location. Any changes made in before developmental programs will have effects on all later programs. Therefore, the possibilities of grade (known as phenotype) encoded by the genes (or genotype) are not space and form tin simply modify past tinkering with what's already there. This is called developmental constraint [1]. Insect wings are likely to take evolved from appendages on the exoskeleton of their ancestors that are absent in our lineage, then they cannot be altered to course wings. Although in principle ane might evolve to wing in a unlike way—bats and birds have both independently evolved wings from their forelimbs, what we call convergence—in this case other constraints are in operation. To evolve wings like those of bats, we would have to lose the current role of our hands and arms, which seems an unlikely evolutionary path to take. Other constraints would besides be in operation—for case the ability required for flight, given the typical human's weight, would be more could exist generated past our pectoral muscles. When the bones of vertebrates that fly are studied it is clear that they have undergone adaptations to allow flight, with the development of hollow or very slender bones.
What can we learn most evolution by studying morphology?
Morphology is a very useful manner of understanding evolutionary processes. Charles Darwin famously noticed differences in neb morphology of Galapagos finches, which helped inform his theory of natural choice and the 'Origin of species'. Recently the developmental programs underlying shape variation in Darwin'due south finches have begun to be understood, with key gene networks—involving Bmp4, calmodulin, β-catenin, Tgfbr2 and Dkk—having been demonstrated to control the size and shape of the beak. Strikingly finch-like beaks could exist induced in chick embryos by manipulating these signaling pathways [2]. Understanding morphology, and how that morphology is created in the embryo (developmental biology), can illustrate how it is possible to modify structures and thereby suggest mechanisms that may underlie evolutionary change (evodevo).
Does this mean that understanding morphology can but tell us most small changes that make species different to each other within groups of animals?
No, while the above examples are compelling examples for the importance of morphological change at the micro level, morphology can exist very useful in understanding changes that gave rising to unlike groups of animals, i.due east. development at the macro level. For instance in our lab nosotros are interested in the morphological and developmental changes giving rise to the evolution of mammals. This work involves comparing embryonic development with the fossil record.
How can we study mammalian evolution through morphology?
To empathize mammalian evolution we demand to exist able to accurately identify what defines a mammal—but this is somewhat difficult, peculiarly in evolutionary history as observed in the fossil tape. Well-nigh of the specialisations mammals have are shared past other groups, and then are not on their own sufficient to identify a mammal. Mammals belong to the aminote clade—tetrapod vertebrates that protect their developing embryos—either in an egg or in the female parent—in a membrane called the amnion. Other amniotes include the birds and reptiles, and one needs to exist able to distinguish mammals from their amniote relatives. While nearly all mammals are warm-blooded (the naked mole rat is a possible exception) so are birds, and so this tin can't be used equally a defining characteristic. It is probable that the common antecedent of mammals and birds was cold blooded, so the presence of endothermy in these ii groups is another instance of convergent evolution. Most mammals have live births; however, some reptile species such as Zootoca vivipara and Pseudemoia entrecasteauxii also give birth to alive young, while the extant monotreme species (the platypus and 2 echidna species) lay eggs but are still mammals. All mammals produce milk and most have fur, but these features are not useful since they are not usually preserved in fossils. All the same, a useful defining feature to place mammals and distinguish them from other amniotes like reptiles and birds is a specialised heart ear and jaw joint—and this is oft easier to find in the fossil tape.
Y'all mentioned the middle ear—what's the difference betwixt the middle ear in reptiles, birds and mammals?
The ears of reptiles, birds and mammals are made up of three components. These are the outer ear through which sound in the form of vibrating air enters the head, the inner ear in which sound is converted into neuronal signals by vibration of hair cells lining the cochlea, and the eye ear that sits between the two structures.
The centre ear is an impedance matching apparatus that facilitates the transmission of sound from the air (low impedance) to the liquid filled inner ear (high impedance). The middle ear consists of the tympanic membrane (ear-drum) for sound capture that is continued to a membrane window into the inner ear via small-scale bones called ossicles. In birds and reptiles there is a single ossicle, called the stapes or columella, whereas mammals accept a chain of ossicles, the malleus, incus and stapes (Fig.ii) [three]. In both cases the centre ear ossicle or ossicles are in an air-filled cavity that allows for free vibration and transfer of sound to the inner ear. In whales and aquatic mammals, this air-filled cavity is notwithstanding present but in addition to sound transfer through the three ossicles, bone and soft tissue conduction occurs through the lower jaw to assistance with underwater hearing. A more extreme reliance on bone conduction is observed in snakes. Here the middle ear cavity has been lost and is filled with tissue that surrounds the stapes. The tympanic membrane and external ear are absent and instead sound is detected every bit vibrations by the lower jaw [three].
Schematics of a sauropsid (bird, cadger) and mammal ear. In sauropsids (a) sound is transmitted from the ear drum to the sensory cells of the cochlea via a single bone, the stapes (S) in the center ear cavity (MEC). Mammals (b) have two extra bones, the malleus (K) and incus (I). Reproduced from [3]
Why does the middle ear differ between mammals and other amniotes?
The extra ossicles of the mammalian middle ear accept a surprising origin. The common amniote'southward ancestor did not have a tympanic ear—that is to say they had no tympanic membrane or air filled centre ear—and audio was heard by the vibration of bones embedded in tissue connected to the inner ear. In the mammalian lineage of mammal-similar reptiles, changes in the jaw musculature and teeth resulted in the evolution of a new jaw joint (the temporomandibular articulation; TMJ) between the squamosal and dentary bones. This new jaw joint appears to have aided stabilisation of the jaw and initially worked together with the original primary jaw articulation, located between the quadrate in the cranial base of operations and articular in the mandible. The fossil record reveals examples of mammal-like reptiles, such as Morganucodon, which used both joints to clear its jaw. The increased efficiency of the new jaw joint allowed the master articulation to become less integrated into the jaw over fourth dimension, and as a upshot the bones of the jaw reduced in size and were freed up for a new role in hearing. Eventually the main jaw joint separated completely from the lower jaw. This last separation gave rise to the definitive mammalian centre ear, with the articular being homologous to the malleus, and the quadrate to the incus. The ii extra ossicles in the mammalian ear were therefore repurposed from the jaw joint of reptiles—a rather remarkable change in function.
What is the evidence for this?
The evidence for the transition of the primary jaw articulation into the middle ear takes iii main forms. Firstly, the fossil record of the transition is remarkably consummate and we are able to follow the formation of the TMJ and middle ear ossicles though a broad range of mammalian ancestors known every bit cynodonts. Secondly, embryology and developmental biology accept revealed the mandibular origins of the new parts of the middle ear in mammals. In fact, it was the embryology carried out by Reichert and Gaupp in the nineteenth and early 20th centuries that first demonstrated the homology between the mammalian centre ear and non-mammalian jaw joint. Thirdly, we can report marsupials. Marsupials, such every bit opossums and kangaroos, are born very early on in development, before the bones of the jaw are fully formed, yet the young pups need to suckle. They therefore use their center ear basic, which are still fastened to the jaw at this stage, to feed. Once the mammalian jaw joint has formed, the ossicles then revert to a office in hearing. The change from a role in feeding to hearing, mimicking the transformation observed during development, can therefore exist followed in a living fauna.
You said that the embryology was done over a century ago—what's the modern take on this trouble?
We have recently been using modern developmental biology techniques to effort and understand the mechanism of this evolutionary change. Specifically we looked at the cellular and molecular mechanism of the final separation of the ear from the jaw, a developmental procedure that mirrors evolution. In doing so nosotros were able do demonstrate that a group of cells called clasts are recruited to pause down a construction called Meckel'due south cartilage that joins the malleus in the ear to the mandible in the lower jaw. In mice the ear and jaw are still physically fastened to each other at birth but a wave of clast cell recruitment to this region a few days afterward birth leads to their separation. In mice with a mutation in cFos these clast cells fail to form, and as a outcome Meckel's cartilage does not break down, just instead ossifies, and thus forms a difficult connection between the jaw and ear. This is similar to the morphology of cynodonts, and and then this mutant copies the long extinct cynodont anatomy in a modernistic mammal [4]. The recruitment of clast cells to this part of Meckel's cartilage may therefore have been an important pace in the isolation of the ear from the jaw, to create the definitive mammalian ear. Nosotros were able to confirm that the Tgf-beta signalling pathway played an important role in the separation of the ear from the jaw [5]. Furthermore, our bear witness also suggests that placental mammals and marsupial mammals have slightly different Meckel'southward cartilage breakdown mechanisms, and then may have independently caused the final footstep of middle ear evolution.
Why should I intendance nearly evolution and morphology?
An evolutionary insight into morphology can offering ways of agreement some man disorders and diseases. For example i of the most mutual human developmental disorders is a limb with fewer than five digits. When the limb anatomy of these affected individuals was compared with birds and amphibians that naturally have fewer than v digits, a high degree of similarity was constitute in the arrangement of muscular attachment to the skeleton [6]. The development of organisms from these phylogenetic classes could therefore offer insights into the footing of the man conditions, and the genetics of the human atmospheric condition could inform the understanding of digit evolution.
A further case lies in the center ear, and the spread of middle ear infection (otitis media). In mammals the epithelium in the lower regions of the crenel is derived from a function of the early embryo chosen the endoderm, while the remainder, like the big part of the ossicles themselves, is formed by some other group of early embryonic cells chosen the neural crest [7]. This dual origin appears to be unique to mammals and allowed for the creation of an air-filled cavity around the 3-ossicles in the center ear. The endoderm-derived epithelium is complex and covered in cilia while the neural crest-derived epithelium is simpler and unciliated. The ii epithelia answer differently to damage, and regions side by side to the neural crest-lined part of the middle ear (the cochlea and mastoid) are more susceptible to complications due to the spread of eye ear infections, compared to parts of the crenel lined by endoderm. The blueprint of spread of ear injections therefore only makes sense in the context of how the ear develops and why information technology formed in that way during evolution. Understanding how a construction evolved, and how structures are linked during evolution and development, can therefore shed light on why and how abnormalities arise.
Authors' contributions
NA and AST wrote the article. Both authors read and approved the last manuscript.
Notes
Competing interests
The authors declare that they have no competing interests.
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References
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