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This study focuses on uncovering and synthesizing evidence which attempts to answer the research question of how fossil and genetic analysis of ancient creatures cumulatively contribute to the understanding of the evolution of limbs. This evolutionary mechanism is one of the most complexes and has been, until recently, largely unknown to researchers, which is why it was selected for investigation.
By combining morphology and genitive evolutionary biology, the transition from fish to tetrapods becomes clearer. The study adds to the existing knowledge by offering a comprehensive combined perspective on the issue and examining recent research on the topic. A comprehensive synthesis and analysis of scholarly literature were conducted to aid in this research. It was concluded that both fossil and genetic approaches offer unique insight into the evolutionary processes and it is necessary to use both to understand the evolution of limbs in tetrapods.
The evolution of limbs is one of the most critical aspects of biological evolution, occurring when marine creatures developed into tetrapods. Limbs provided a significant shift as life could now live on land. This transformation is one of the most mysterious but also well-studied concepts of evolutionary biology. The mechanisms responsible for the modification of components in fish that resulted in transformation into limbs are complex and new research continuously challenges previously held perceptions regarding the topic. Fossil and genetic information can contribute to the understanding of limb evolution by examining the change over time and gene expressions affecting the evolution.
Purpose of Study
The purpose of this study is to examine the fossil and genetic evidence which informs modern paleontologists about the evolution of limbs in biological life and the arrival of tetrapods. The research question is whether currently available fossil evidence and genetic research provide enough information to make finalized conclusions regarding the evolution of limbs. The evolutionary context and process of the transition from fins to limbs is so inherently complex that it poses numerous questions and potential challenges to the current understanding.
The new information provides insight and has profound implications for the analysis of the biological system which underwent a diversification event as significant as the transformation to tetrapods and variability of limb function (Ruta & Wills, 2016). The author of this paper presents a non-conclusive hypothesis that current fossil and genetic evidence offers sufficient but not a finalized perspective on the issue of the evolution of limbs, and more theories may emerge with further study and enhanced technology.
The methods used for this study were focused on conducting scholarly research. After the topic was defined, a scholarly database search using Google Scholar and others was conducted. Keywords and phrases were utilized to achieve more accurate results. Parameters were set for both studies and online books to have been published within the last five years. Several online and physical books were also examined for evidence on the topic. In addition, the author of this paper viewed interviews and lectures on the topic to identify prominent issues and breakthroughs regarding the development of limbs and tetrapod evolution.
Fossils can offer an insightful perspective into the morphological and skeletal structure which has evolved in the fin-to-limb transition. A fossil fish Rhizodus hibberti is considered to be one of the origins of tetrapods, demonstrating a unique skeletal pattern in its pelvic fin. Many fish contain a pattern of the femur, tibia, and fibula similar to tetrapods, reflecting morphological development. However, some creatures such as the Rhizodus hibbertihas a femur that articulates distally, having a distinct structure. This suggests that in the process of evolution of limb development, patterning of the embryonic fin and limb combination was not necessarily limited to the structure seen in modern tetrapods and could have had varied skeletal patterns (Jeffery, Storrs, Holland, Tabin, & Ahlberg, 2018)
One of the primary questions remains on how the newly developed limbs were able to support the body weight and movement of early tetrapods. Anatomical analysis by Molnar, Diogo, Hutchinson, and Pierce (2017) suggests that tetrapod shoulder muscles already existed in tetrapodomorph fish, while distal appendicular muscles appeared later from dorsal muscle masses. Based on this anatomical approach, the evolution from the pectoral fin to the tetrapod forelimb occurred through the acquisition of specific muscle group splits and fusions.
It is largely supportive of previous research but adds detail. Fossil information supports a homology hypothesis that abductors in lobe-finned fish, transitioned into distal limb muscles, with further evidence suggesting that the intersections in the muscles eventually gave an appearance to the shoulder and forearm muscle groups. This is congruent with fossil records which indicate the persistence of deep and outward muscle layers during the sarcopterygian era (Molnar et al., 2017).
In recent years, biological research has demonstrated how genetic alterations affect the patterns of skeletal structures which led to the evolution of limbs from fins. The anatomical structure of fins consists of three or more basal bones which are connected to the pectoral girdle, or the shoulder. In tetrapods, the basal bones disappeared on the anterior side, leaving the posterior humerus bone. In fish, particularly catsharks which Onimaru et al. (2015) studied, there is a balance between anterior and posterior bones. Meanwhile, in tetrapods, the balance of limb buds is controlled by the regulator protein Gli3.
The fish genomes differ from tetrapods due to the restriction of Gli3 expression that prevents the development of limb buds and loss of skeletal structure. It is direct evidence that in the fin-to-limb evolution, one of the key genetic events was a shift of balance between anterior and posterior skeletal elements with the role of the Gli3 regulator protein.
The diverse number of specialized limbs in tetrapods and crustaceans has been attributed to the Hox gene expression which has been theorized to play a significant role in limb development. The skeletal structure of a vertebrate body is defined by axial progenitors which operate through Hox-related mechanisms. The genes carry patterns of information that guide body structure formation. Gene expression regulation which guides the development of proximal and distal regions of the limb occurs in two specific domains, potentially leading to vertebrates having paired appendages. Furthermore, targeted inactivation allows limiting Hox-dependent function while maintaining others, which greatly increases the flexibility of evolutionary processes (Mallo, 2018).
SixHox genes are responsible for the development of various body parts of tetrapods, with certain expressions such as abd-A and abd-B directly influencing the type of limbs of the tetrapod, including thoracic type legs of an amphipod. Therefore, Hos genes define the type appendage and its modular nature based on input and expression. In the evolutionary context, it generates morphological differences between crustacean and tetrapod species, leading to diversity (Martin et al., 2016).
Based on the results, it can be concluded that both fossil anatomical evidence, as well as genetic information, can be used to conclude the evolution of limbs. While paleontology provides information about morphological and physical transformations, genetic evolutionary biology addresses the mechanisms which allow this to take place. Genetic evidence supplements paleontological fossil evidence by providing information on patterns of heterochrony, homology, characteristic definition and development, and evolvability of particular features(Thewissen, Cooper, & Behringer, 2012).
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Fossil evidence continues to be enriched by genetics which helps to greater understand evolution that neither specialization could achieve alone. In the context of the evolution of limbs, it can be seen in the morphological peculiarities. While certain creatures developed forelimbs, others such as snakes do not have them due to a cranial shift (physical occurrence) and as a result of Hox genes expression.
Organisms are continuously being shaped with morphologists dividing these changes into discrete partitions. Meanwhile, evolutionary biology through gene expression offers information regarding relevant partitions. For example, the Hox gene expression in the neck is directly correlated to the presence of a forelimb, and part of the same developmental module even though neck and limb are anatomically disconnected (Thewissen et al., 2012).
It is an evident conjecture that a unique connection between genetic and developmental changes exists regarding the evolution of limbs. As molecular mechanisms underlying vertebrate evolution are being discovered, it is becoming more evident at the extent of the diversity of tetrapod limbs. The zone of polarizing activity of regulatory sequence (ZRS) is a transcriptional genome enhancer that exists in animal life that ultimately defines the presence of limbs. Thus, when Villar and Odom (2016) replaced endogenous ZRS in mice with orthologous sequences from humans and snakes (which lack the skeletal limb structure), the results were substantial.
The human genome led to normal development while snake sequence resulted in limb truncations. This suggests that, despite mice having the skeletal and morphological structure for limb development, genetics play a key role in limb development. This principle can be applied to the evolutionary aspects as well. The results indicate that the fins of marine creatures had the structure for limb development. However, as presented in later evidence, it is ultimately the activation and modification of the Hox gene that led to the appearance of the first tetrapods.
The results of this paper’s research synthesis mostly agree with the findings of the decades of previous paleontological research. Fossil records have demonstrated that fish had an unpaired dorsal fit, while later tetrapods had similar structures which resulted in paired appendages and limbs. However, genetics allow for more accurate and competent tracing of evolutionary mechanisms to determine the exact origin and genetic blueprints which led to the appearance of proto-limbs and then fully developed tetrapods.
The sequences such as ZRS and Hox genes are ancient and seen to be preserved across all vertebrates (Wood, 2018). Understanding the origins and activity of the gene enhances can allow researchers to trace the evolution of limbs and, perhaps, other parts of the body from ancient fishapod species to modern humans. Therefore, when the study findings and its premise largely agree with the primary paradigm held in the paleontological and biological communities.
The weakness of this study is that it uses only secondary research in its attempt to answer the research question. The author of this paper realizes that they lack the specialization to conduct primary research or make concrete conclusions. Furthermore, the research question can be considered broad, requiring more specification regarding particular species of tetrapods or modern-day biological life. Weaknesses can be addressed through further research and an experimental study under the guidance of more experienced researchers
Conclusions and Recommendations
It is recommended to further focus the research on identifying the correlating processes between morphological features and gene expressions. While fossil paleontological records can be used to inform hypotheses, functional genomic assays can be utilized to develop models which enhance the understanding of evolutionary biology. Genomic states can be explored with greater technology and experimental assays in laboratories to explore organogenesis and its influence in both past and present on morphological diversity (Pieretti et al., 2015).
This paper found both fossil morphological evidence as well as genetic biology information that supports explanations regarding the process and mechanisms which led to the evolution of limbs from fins in ancient creatures. Fossil discoveries examine physical features such as skeletal changes and muscle group formation. Furthermore, it has been suggested that organisms had the neural layouts that enabled limb movements long before tetrapods emerged due to similar circuitry existing between fish and land-based organisms today. Meanwhile, genetic predispositions such as the expressions of the Hoxgene provide insight regarding evidence for limb development.
This paper contributes to the general knowledge of the discipline by offering a variety of expert and scholarly perspectives. It seeks to combine the two schools of thought in paleontology and biological sciences which are fossil study and genetic research, offering a more enriched viewpoint on the evolutionary processes of the past.
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