The Macro Library(beta)
A collection of macroevolutionary examples for K-12 teachers, students, and publishers



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The Origin of Tetrapods

The invasion of land by vertebrates, the so-called "sea-land transition"[1], 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

tetrapods

  • Animals that were at some point four-footed, including humans

and the invasion of the land can be traced to the gradual alteration of fin structures seen in

sarcopterygians.

  • "Fleshy-limbed" vertebrates, including lungfish, coelocanths, and tetrapods

Click for larger image.
Figure 1. Cladogram representing relationships between actinopterygians, sarcopterygians, and basal tetrapods. Figure adapted from Kitzmiller v. Dover trial.Click for larger image.



   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[2]. 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[3]. 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

actinopterygian

  • Ray-finned fishes, including almost all modern finned fish such as tuna or salmon

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

distal elements

  • Parts at the end; in this case, bones that would later become digits

became less suited for propulsive action in a dense fluid (fins) and more adapted toward providing traction on a rigid substrate (more leg-like).

Click for larger image.
Figure 2. Graphical depiction of digital reduction and morphological changes in limb structure for derived sarcopterygians. Adapted from Kitzmiller v. DoverClick for larger image.



   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.

Click for larger image.
Figure 3. Illustration of hip-sacral attachment changes for derived sarcopterygians. Figure adapted from Kitzmiller v. Dover.Click for larger image.



   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[4] 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[5] on land, a diet still retained by

extant

  • Alive today, in contrast with "extinct"

amphibians. While a complete invasion of the land was not yet possible, due to desiccation problems of both the animals and their eggs[6], 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.

Philip Kahn
University of California, Berkeley
Submitted July 24 2008

  • [1] This is more accurately called the water-land transition, as it is not yet sure if the transition occured in saltwater or in freshwater
  • [2] Contrary to popular belief, an organism is not truly weightless in water. If life were made of lead, it would have had to overcome similar structural issues in water that life had to overcome moving onto land.
  • [3] More accurately, it must promote the spread of a piece of its genome in the population. Sometimes this distinction is important, though for the purposes of this article treating the organism as the beneficiary is sufficient.
  • [4] The reasoning behind the water generally being dirty and brackish is that it lowers the oxygen content of the water, thus prompting a greater reliance on atmospheric oxygen.
  • [5] The invertebrates were found on land due to a lack of competition themselves, both for preying on each other and for the plants which had by then widely colonized the land.
  • [6] Again, a trait still found in modern amphibians which are tied to water or moist environments.
  1. Kitzmiller v. Dover trial testimony.