The Macro Library(beta)
The Origin of Tetrapods
The invasion of land by vertebrates, the so-called "sea-land transition", was an incredibly important event, but also very gradual, occurring over several million years. Contrary to many popular depictions, there was no clear point when fins became limbs, at which point the creature hauled itself onto the land; instead, the ancestry of
and the invasion of the land can be traced to the gradual alteration of fin structures seen in
The invasion of the land had to be slow because finned fishes of any sort are poorly adapted to life on land. The physical properties of water require significantly different appendages and support structures in an organism than those required on land. Fluids, including both air and water, exert forces evenly in all directions on anything in it. However, critically, water is about 1,000 times denser than air, and thus is much more "buoyant" — that is, it supports things much better. Then, since organisms are primarily composed of water (and thus not much denser in it), organisms will not sink very readily in water and experience the illusion of weightlessness. The immediate problem this presents to the invasion onto land is that this means that most fishes lack load-bearing structures — that is, are physically incapable of supporting their own bodies in such a lightly buoyant medium like air, and are thus unable to move. Finally, the exaptation of primitive swim bladder/lung structures to be the primary source of oxygen intake would enable actual excursions onto the land.
For any of these novel structures or uses of existing structures to become evolutionarily favorable, it is important to recognize that there must be a benefit to the organism. The first precursor structures that would ultimately be adapted to land living can be found in the sarcopterygian line of fishes. The sarcopterygian line split from the
line approximately 400 million years ago1. As can be seen in Figure 2, as the sarcopterygian-tetrapod line became more derived, the radius and ulna became more suitable for load-bearing and propulsive movement against a rigid medium, and the
became less suited for propulsive action in a dense fluid (fins) and more adapted toward providing traction on a rigid substrate (more leg-like).
Additionally, there were further adaptations associated with transitioning to a land-based existence that are somewhat more subtle. While the evolution of more robust limbs are associated with moving into shallower, brackish waters in which fins are no longer useful, there are other associated adaptations that are evolutionarily favorable under the same circumstances. Shallower water gives organisms less room to maneuver, which means if the head is attached such that it has a definite orientation with respect to the body, there can be positions where it is difficult to eat food, to submerge gills, or to breathe air. All three of these issues are addressed if the shoulder structures decouples from the head (permitting the head some motion with respect to the body trunk), and a neck that permits independent head movement allows more movement still. As these organisms became capable of moving into shallower waters, buoyant forces on the body decrease, forcing modifications of the rib cage to provide better support structures for the internal organs. Similarly, less buoyant support required the limbs to be more firmly attached to the backbone for locomotor purposes, as shown in Figure 3.
While the physiological changes associated with the land-sea transition are locked in the fossil record, the behavioral changes prompting or prompted by these changes are, necessarily, harder to tease from the rocks. Current theories postulate that these physiological changes were driven by the progressive movement of these creatures into shallower, more brackish water. In turn, this progression was prompted by the availability of an unfilled niche — there was simply little or no competition for food at the water's edge, as nothing had yet moved to occupy that position. Further, the actual excursions beyond the water would be prompted by a lack of competition for consuming invertebrates on land, a diet still retained by
amphibians. While a complete invasion of the land was not yet possible, due to desiccation problems of both the animals and their eggs, these were the critical issues which enabled movement about the land. The final innovations which enabled complete colonization were the evolution of scales to prevent desiccation, and the shell-covered amniotic egg to allow eggs to be lay away from water.