A Fisheye View of the Tree of Life
From water to land
A coelacanth swimming near Sulawesi, Indonesia
Like other sarcopterygians, the coelacanth has "lobe-fins," muscular fins with a single bone that articulates with the rest of the body. Most fishes (the Actinopterygii, or ray-finned fishes) have several bones at the bases of their pectoral fins, and their fins are composed of a set of webbed rays, not muscle- and skin-covered bone.
Tetrapods evolved from a group of organisms that, if they were alive today, we would call fish. They were aquatic and had scales and fleshy fins. However, they also had lungs that they used to breathe oxygen. Between 390 and 360 million years ago, the descendents of these organisms began to live in shallower waters, and eventually moved to land. As they did, they experienced natural selection that shaped many adaptations for a terrestrial way of life. Like other terrestrial sarcopterygians, modern humans still carry the evidence of our aquatic past in the way our arms and legs attach to our bodies, as well as in the many other features that link us to our fishy origins.
Which came first, the gill or the lung?
Since fish appear in the fossil record earlier than the clade we call tetrapods does, it's tempting to assume that modern fishes bear the same traits that their and our common ancestor did. This line of reasoning is intuitive, but it is not correct. Though it is true that both modern ray-finned fishes and the ancestor we tetrapods have in common with them are finned and aquatic, the same pattern of reasoning does not hold water when it comes to lungs.
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Coelacanth photo courtesy of Mark V. Erdmann; lungfish photo from Wikipedia user Haplochromis and licensed under Creative Commons
Understanding Evolution © 2018 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California