Evolution  (Page 7/9)

Since Darwin’s time the fossil record has become much more extensive, and the evidence for this explanation has become much more well-supported. Gaps in the fossil record that were pointed out by Darwin’s contemporaries have been gradually filled in. Indeed, the sciences of geology and paleontology, in combination with biology, have allowed scientists to make predictions about where, exactly, particular fossils in particular gaps should be found.

The most recent (and spectacular) example of this was the discovery of fossilized remains of a creature that bridges the gap between fish and amphibians, which the first four-legged creatures (aka tetrapods) to move onto land. The fossil record, coupled with genetic evidence from modern-day amphibians and fish, indicated that this transition to land occurred about 375-400 million years ago. But only fish fossils, or amphibian fossils, had ever been found. Logic dictates that there should be a transitional creature, or “missing link” in popular jargon, which had characteristics of both fish and amphibians. It was reasoned that creatures such as this, if they existed, would probably live in shallow areas at the edge of seas or bays. Geologists knew which particular rock formations resulted from those sort of environments of that age, and so expeditions were dispatched to search for such fossils in one of those geological formations. These rocks were deposited in warm shallow tropical seas 375 million years ago, but are now, as a result of continental drift, located on Ellesmere Island in the Canadian Arctic. In 2004 fossils were found in those rocks that elegantly fit that prediction; the creature was named Tiktaalik roseae. (figure 3, below). The genus name for this “fishapod” comes from the name of a fish and was suggested by local Inuits on Ellesmere Island, and the specific epithet “roseae” honors an anonymous donor who helped fund these grueling expeditions to the high Arctic. Tiktaalik “fins” have basic wrist bones, but no digits, or fingers. It is truly a missing link, and its discovery stems directly from predictions made on the basis of previous scientific observations, in a classic example of the power of the explanatory framework known as the theory of evolution. Descriptions of the expeditions, and lots more about the incredible insights that have come from those fossils, can be found in a charming book called “Your Inner Fish”, written by Neil Shubin and published in 2008.

Comparative anatomy and embryology

At the same time that the fossil record was making some scientists scratch their heads and question the creation explanation for the diversity of life, other scientists were looking more closely at these fossils and at the bones of existing organisms. These comparative anatomists also made observations which were much more easily explained by the theory of evolution. The different bones in fossil skulls, for example, could be compared to the bones in modern skulls, allowing anatomists to discern that the fossil skulls and the modern skulls had remarkable similarities in the number and the position of the individual bones in the skull. Most of the bones in a fossil fish skull have counterparts not only in modern fish skulls, but in fossil and modern amphibian skulls, or fossil and modern reptile skulls, and even fossil and modern mammal skulls. Occasionally the fossil record shows us when a skull bone is added or one is lost, and also allows us to track progressive modifications in the positions of these bones on the skull surface. We can only understand these observations in the light of evolutionary theory – if we conclude that the bones reflect the fact that each kind of organism is descended from some other. Descent with modification is the most satisfying scientific explanation for these observations.