The vertebrate forelimb is one of the most advance evolutionary hallmarks of an advanced life form. The evolution of the forelimb allowed our primitive aquatic ancestors to explore and take advantage of a totally new environment, ushering in the era of land dwelling vertebrates. Possessing this new evolutionary tool, land dwelling vertebrates developed an advantage that allowed them to evolve a great variety of locomotive adaptations such as running, jumping, flying, climbing, and swimming.
Additionally, it allowed evolved adaptations for grasping prey, gathering fruit and nuts, digging, and performing other functions which have become essential survival tools (Museum note). Ultimately, the diverse modifications and adaptations of the forelimb allowed for the diversification of vertebrate species which eventually lead to whatever animal life form – humans included – we have today.
This is why the subject of forelimb evolution is an important, widely debated and controversial focus of study among biologists. Understanding how our aquatic ancestors emerged onto land is an important subject for biologists since not only does it gives insight into the evolutionary process but also provides insights as to how evolutionary thinking has evolved in the past 100 years (Bowler, 7). There are several schools of thought regarding the evolution of the forelimb and which organism they evolved from.
One such prominent school of thought belongs under Gigenbaur and Haeckel, who both believed that the class Dipnoi (lung fishes) were the transitional class between fishes and amphibians – ergo the first class to evolve forelimbs and hence considered as ancestral tetrapods (Bowler, 10). However, some theories suggest that forelimbs evolved through two completely different routes: one involving the dipnoi that eventually became the amphibians and the second involving the crossopterygian fishes which eventually gave way to anurans and reptiles (Bowler, 12).
For the rest of the paper, discussion of the evolution of forelimbs would mostly refer to the second route and the examples cited would also be pertaining to the latter thought. Moving out of the water and onto land required the development of a skeleton capable of supporting the weight of the creature and the musculature to operate the appendages in coordinated manner (Lab notes).
This is why most studies regarding the evolution of forelimbs focused on homology of structures – especially since any data regarding this area of thought can only be supported by morphological studies and comparisons of fossil records and traceable modern equivalents. After all, the thought is that changes in the pectoral girdle and appendages was instrumental in the transition from water to land – with lobed finned fishes (aside from the Dipnois) being the first to exploit this feature.
It is thought that lobed-finned fishes were capable of crawling out of the water using their paired anterior fins to make temporary excursions on land and it was this evolutionary trait that gave way to the development of forelimbs. However, new discoveries of fossils such as Acanthostega suggest that limbs evolved before tetrapods became primarily land dwellers. (Museum notes). Regardless, the limb bone structure of a lobe-finned fish clearly demonstrates the homology of the bones that comprises limbs (Lab notes).
Modern tetrapod limbs are divided into three sections: a single, heavy bone (the femur or humerus) that articulates with the body and forms the foundation of the limbs; two bones (tibia and fibula; radius and ulna) that are lighter and combine both support and maneuverability and; the section farthest from the body which consists of many smaller bones (wrist, ankle, digits) and provide considerable flexibility for locomotion and food gathering (Museum notes).
Fossil records show that the limb bone structure of a lobe-finned fish clearly demonstrates the homology with the bones that comprise the tetrapod limbs (Lab notes). For example, the pectoral fin of Eusthenoteron – a crossopterygium and an extinct fish – demonstrates this homology through the presence of an ulna and radius homologous to the bones in the forearm, as the humerus is to the upper arm bone. The phalanges are the digits, with this case showing the primitive state with eight (Lab notes).
These same bones are identifiable and comparable to those found in Acanthostega and Sauripteris, the earliest known member of an extinct family of predatory fishes known as rhizodontids. The species were large predators common in many parts of the world during the late Paleozoic era. In the fossil, a single bone, the equivalent of the humerus, connect with the shoulder girdle and with two more bones, the radius and ulna. Notably, these two bones are unequal length while in most tetrapods, the radius and ulna are of equal length indicating that the latter is an advanced feature (Museum notes).
Lastly, these same traits are also present in Eryops, an early tetrapod that has fully developed limbs with an internal bony skeleton and movable joints surrounded by muscles grouped for specific functions. Eryops is advanced over primitive tetrapods like Acanthostega in having limbs bones with more complex joints and fewer fingers. Acanthostega has eight fingers while Eryops and other tetrapods have five or less (Museum notes).
Though opinions widely vary, to most evolutionary biologists, the presence of these homologous structures is enough to at least consider ancestry. However, non-Darwinian palaeontologists do not subscribe to similar thoughts. This is why studies are still ongoing in determining the exact evolutionary route that lead to the emergence of tetrapods.
Bowler, Peter J. “Fins and Limbs and Fins into Limbs: The Historical Context”. Fins into Limbs: Evolution, Development and Transformation editted by Brian Keith Hall (Illustrated). University of Chicago Press 2007.
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