Scientists Discover “Missing Link” Between Fish and Land Animals!

How did early fish evolve to survive on land?

One of the greatest fulfilled predictions of evolutionary biology is the discovery, in 2004, of a transitional form [“missing link”] between fish and amphibians. The discovery of Tiktaalik roseae is a stunning vindication of the theory of evolution.

[The fossil record indicates that] until about 390 million years ago, the only vertebrates were fish. But 30 million years later , we find [in the fossil record] creatures that were clearly tetrapods: four-footed vertebrates that walked on land. These early tetrapods were like modern amphibians in several ways: they had flat heads and bodies, a distinct neck, and well-developed legs and limb girdles. Yet they also show strong links with earlier fishes, particularly the group known as “lobe-finned fishes,” so called because of their large bony fins that enabled them to prop themselves up on the bottom of shallow lakes and streams. The fish-like structures of early tetrapods include scales, limb bones, and head bones.

If there were lobe-finned fishes but no terrestrial vertebrates 390 million years ago, and clearly terrestrial vertebrates 360 million years ago, where would you expect to find the transitional forms? Somewhere in between. Following this logic, University of Chicago scientist, Neil Shubin predicted that if transitional forms [“missing links”] existed, their fossils should be found in [sediment] strata around 375 million years ago. Moreover, the rocks would have to be from freshwater rather than marine sediments, because lobe-finned fish and early amphibians both lived in fresh water.

Searching his college geology textbook for a map of exposed freshwater sediments of the right age, Shubin and his colleagues zeroed in on a paleontologically unexplored region of the Canadian Arctic: Ellesmere Island, which sits in the Arctic Ocean north of Canada. And after five long years of fruitless and expensive searching, they finally hit pay dirt: a group of fossil skeletons stacked one atop another in sedimentary rock from an ancient stream. When Shubin first saw the fossil face poking out of the rock, he knew that he had at last found his transitional form [“missing link”].

Reconstruction of Tiktaalik roseae

[The fossil he found] Tiktaalik roseae has features that make it a direct link between the earlier lobe-finned fish and the later amphibians. With gills, scales, and fins, it was clearly a fish that lived its life in water. But it also has amphibian-like features. For one thing, its head is flattened like that of a salamander, with the eyes and nostrils on top rather than on the sides of the skull. This suggests that it lived in shallow water and could peer, and probably breath, above the surface. The fins had become more robust, allowing the animal to flex upward to help survey its surroundings. And, like the early amphibians, Tiktaalik has a neck. Fish don’t have necks—their skull joins directly to their shoulders.

Most important, Tiktaalik has two novel traits that were to prove useful in helping its descendants invade the land. The first is a set of sturdy ribs that helped the animal pump air into its lungs and move oxygen from its gills (Tiktaalik could breath both ways). And instead of the many tiny bones in the fins of lobe-finned fish, Tiktaalik had fewer and sturdier bones in the limbs—bones similar in number and position to those of every land creature that came later, including ourselves. In fact, its limbs are best described as part fin, part leg.

Tiktaalik…is a fossil that marvelously connects fish with amphibians. And equally marvelous is that its discovery was not only anticipated, but predicted to occur in rocks of certain age and in a certain place.

—biologist Jerry Coyne, Why Evolution is True, pp 35-38

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