THE EVOLUTION OF TERRESTRIAL VERTEBRATES

Let’s now turn our attention to the evolution of the first amphibians, the vertebrate group that first colonized the land, or partially did. The fossil record has given us a fair understanding of both the species involved in this transition and the time. A group of Devonian Period bony fish known as Rhipidistians appear to have been the ancestors of the first amphibians. These fish were dominant predators, and most or all appear to have been fresh water animals. This in itself is interesting and suggests that the bridge to land was first through freshwater. The same may have been true for the arthropods as well.

The Rhipidistians were seemingly preadapted to evolving limbs capable of providing locomotion on land by having fleshy lobes on their fins. The still-living coelacanth provides a glorious example of both a living fossil and a model for envisioning the kind of animal that did give rise to the amphibians. But another group of lobe-finned fish, the lungfish, also is useful in understanding the transition, not in terms of locomotion but in the all-important transition from gill to lung. The best limbs in the world were of no use if the amphibian-in-waiting could not breathe. There were thus two lineages of lobe-finned fishes, the crossopterygians (of which the coelacanth is a member) and the lungfish.

There is controversy about which of these groups was the real ancestor of the amphibians. Whichever it was, there is a record of the first “tetrapods,” animals with four legs, in the latter part of the Devonian, meaning that the crucial transition from a fish with lobed fins and gills to an animal with four legs took place prior to that time. But when? And just how terrestrial were those first tetrapods? Could they walk on land? More importantly, could they breathe in air without the help of water-breathing gills as well? Both genetic information and the fossil record are of use here. But in some ways we are very hampered. Not until we somehow find the earliest tetrapods with fossil soft parts preserved will we be able to answer the respiration question.

Happily we have extant representatives of the crossopterygians and lung fish, and some relatively primitive amphibians. Geneticist Blair Hedges has compared their genetic codes in an effort to discover the time that fish and amphibians diverged. The “molecular clock” discov-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement