When Darwin published his theory of evolution by natural selection, he was well aware that there would be skeptics. He tried to anticipate many of the criticisms and offered his candid thoughts about them in The Origin of Species. Still, he acknowledged that the evidence available, well compelling, wasn't conclusive. That has since changed. Science has progressed considerably since Darwin's lifetime, and the amount of evidence that has now accumulated makes his theory of evolution among the best supported theories in all of science. Among the most important predictions of Darwin's theory was that there should be evidence from the fossil record of one type of organism evolving into another. Where were these transitional forms that some people called missing links? Darwin was well aware that the fossil record is incomplete. He actually discovered some fossils himself during his Voyage of the Beagle. Quite a few new fossils were discovered during his lifetime. One particularly fascinating fossil was uncovered from a limestone quarry in Germany in 1861. It ended up in London where it was investigated by a biologist named Richard Owen. Owen had become famous for describing a new kind of extinct animal he called dinosaurs. He recognized that the German fossil represented a type of animal not previously known, but it didn't seem to fit with what he knew about dinosaurs. It had wings covered with feathers like a bird, but unlike a bird, the wings also had clause. It also had a feathery tail like a bird, but the tail was made of bones, unlike modern birds. Owen hadn't accepted Darwin's new theory of evolution and saw the fossil as a type of bird. Despite having features not present in any living birds, he called it Archaeopteryx, which means ancient wing. Additional specimens of Archaeopteryx were later found that had other unusual combinations of traits, including a beak filled with teeth. What Owen failed to recognize is that the unusual combination of traits in Archaeopteryx is exactly what was predicted by Darwin's theory of evolution. The Archaeopteryx fossil show in exquisite detail the transition from dinosaurs, which had teeth, claws, and bony tails, to birds which lost these traits. Fossils of other species spanning the transition from dinosaurs to birds would eventually be found, making this one of the best documented evolutionary transitions among all living things. In fact, a lot of the gaps that existed in the fossil record during Darwin's lifetime have since been filled in by subsequent discoveries. Let's take a look at another example. I'm here at the Houston Museum of Natural Science, where there's a cast of another great example of a transitional form from the fossil record called Tiktaalik. Tiktaalik was discovered by a team led by Neil Shubin. Shubin was interested in understanding the origin of the first tetrapods, the four-legged animals. The tetrapods include amphibians, reptiles, birds and mammals. The first tetrapods were at least partially terrestrial, meaning they could live at least part of their life on land similar to modern amphibians like frogs, but their ancestors were fish. The first tetrapods had to make a major transition from life in the water to life on land. Shubin and his colleagues were able to make some predictions about what the transitional forms between fish and tetrapods would probably look like. They expected it to have a flat head with eyes on top, much like alligators, since this is a good adaptation for living in shallow water. They expected it to have a neck which fish don't have, and they expected it to have bony fins that could move like limbs. Shubin knew that fossils of true tetrapods could be found in rocks that were about 360 million years old. But if you go back to 385 million year old rocks, you find only fish. That meant that the common ancestor of all tetrapods must have lived between 360 and 385 million years ago. Shubin also guessed that the transition to life on land, would have taken place along the coastline of an ancient lake or pond. He looked on geological maps to find places where rocks from that type of environment that date to that exact time interval could be found near the surface. One of the only places was a remote part of the Canadian Arctic in a place called Ellesmere Island. Shubin's teams spent six summers working there with very little success. Until one day in 2004, when they found exactly what they were looking for. Sticking out of a rock along a slope was the fossilized remains of an animal's snout. It had exactly the features they had expected. A flat head, eyes on top of its head, a neck attached to shoulders and bony fins that could move like limbs. Shubin and his colleagues named the new species Tiktaalik at the suggestion of the Inuit people who live in the region where it was discovered. The name means large freshwater fish. Tiktaalik is now one of several species that have been discovered that span the transition from fish to tetrapods. They reveal a gradual progression from an aquatic lifestyle to a terrestrial one. As such, they provide a very clear demonstration of how evolution works by taking existing structures like fins and modifying them to serve a new purpose, like being able to walk on land. The discovery of Tiktaalik also shows us how paleontology, as one branch of evolutionary biology, can be a predictive science. Neil Shubin and his colleagues made hypotheses about the types of features they expected to be present in an animal that no one had ever seen, but they predicted must have existed. They hypothesized that the animal should have lived in a particular habitat at a particular time in geological history. They went out to test their hypotheses by searching in a place that met their predictions. They found evidence that supported all of their hypotheses. It's a remarkable example, not only of the evidence for evolution, but also of the power of the scientific method.