A Tale of Three Lobefins

Three paths diverged in the Sea, and we took the one more travelled. Our fellow lobefins, the coelocanths and lungfish split up about 425 million years ago, and we other tetrapods split from the lungfish very slightly later, about 417 mya, according to Richard Dawkins. Only a very few species of lungfish and coelocanths still survive, but, based on the fossil evidence, they appear to have changed very little over that vast time, while the rest of us lobefins have exploded to become amphibians, reptiles, birds and mammals.

Dawkins mentions that continuity of form doesn't necessarily imply genetic stasis. In fact, he says, modern coelocanths and lungfish are about as genetically different from each other as they are from us. If this seems as profoundly counterintuitive to you as it does to me, you might want to check the fine print.

It seems that the DNA analyzed to reach this conclusion was all mitochondrial DNA. Mitochondrial DNA is special for a variety of reasons, but most interestingly here because non-fatal mutations tend to be neutral or very nearly so. Neutral mutations, by definition, don't change the phenotype. Quite possibly, then, the actual proteins produced by modern coelocanths and lungfish might be very similar to the productions of their remote ancestors. Has anyone done that comparison? (For the modern species, I mean.)

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