According to modern evolutionary theory, all populations of organisms are in transition. A "transitional form" is a species that is intermediate between two different species. However, due to the special circumstances required for preservation of living things, only a very small percentage of all life forms that ever have existed will have their remains unearthed for study. Earth's crust is continually being recycled, so fossils that may be contained in various rock layers get destroyed through natural processes. What's more, many potential fossil hiding grounds are inaccessible, for example, those deep undersea.

Nevertheless, scientists have uncovered enough fossils to illustrate many important evolutionary transitions. In this video, we learn about one recent find—the skeletal remains of a part-fish, part-tetrapod, or "fishapod"—that represents an intermediate form between fish with fins and land animals with four limbs.

Tiktaalik, from the Inuit word meaning "large shallow-water fish," had a flexible neck. This is significant because in modern tetrapods like amphibians, the head is separated from the body, whereas in fish it is not. Although Tiktaalik still had gills, it had lost the bony gill coverings that fish use to fan water and promote oxygen intake. This suggests that the "fishapod" may have been at least partly air-breathing, like modern tetrapods. Further, although Tiktaalik clearly had fins, the well-preserved bony, wrist-like arrangements near the tips of the fins suggested that the animal could push its body up off the ground, offering support much like a foot does. These fins, though still used for swimming, were clearly precursors to legs.

This important find reinforces the idea that the move from water to land was a very gradual process, and the evolution of limbs was neither a simple nor spontaneous adaptation. In fact, the new picture of this transition shows that most of the changes needed for life on dry land happened in creatures that were still living in the water. Tiktaalik illustrates yet another important evolutionary transition, much like the primitive bird Archaeopteryx links reptiles and birds, the "walking whale" Ambulocetus links land mammals to aquatic mammals, and hominids, including Australopithecus, link apes to humans.

To learn more about what scientists mean by "theory," check out What Is a Theory?.

To learn more about evolutionary relationships, check out All in the Family and Deep Time.

To learn more about the fossilization process, check out How a Dinosaur Became a Fossil and Fossils.

To learn more about the discovery of other transitional forms, check out Ted Daeschler and Neil Shubin: Early Tetrapod Fossils, Evolving Ideas: How Do We Know Evolution Happens?, Fish with Fingers, and Whales in the Making.

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