Tiktaalik rosae is considered to be an evolutionary ______ . 【37】

Welcome everybody. My name is Derek Fisher and I’ll be taking you through this talk on a truly amazing discovery made by my team of researchers last year. As I’m sure you know, we discovered Tiktaalik rosae, a so-called "missing link" in the evolutionary process--a fish that walked on land. We made this extraordinary discovery in the Canadian Arctic. I should emphasise fight at the beginning that I do not have any evidence to suggest that this was the only such fish to make the leap from the sea to the land. I think that further discoveries may await researchers.
    Anyway, the fossil discovery illuminates a chapter in the history of life on Earth that was essential to the ultimate emergence of human beings. This is an impression of Tiktaalik roseae, which, we believe, lived about 375 million years ago. As you can see, it has features that blur the distinction between fish and terrestrial limbed creatures. The fossils that we found on Ellesmere Island, 600 miles from the North Pole, are a fine example of evolution in action. They have allowed us to freeze-frame a process of adaptation to land that took tens of millions of years, and which made possible the development of all the mammals, birds, reptiles and amphibians that have existed since.
    Without creatures such as Tiktaalik there would have been no dinosaurs, no primitive mammals and none of the hominids such as Australopithecus africanus and Homo erectus that started the human family tree. This animal represents the transition from water to land--the part of history that includes ourselves. It’s as much a part of our history as, say, Australopithecus africanus. Now, you can see those teeth. From that, it is clear to us that Tiktaalik roseae was a predator with sharp teeth and a head shaped like a crocodile’s that grew to between four feet and nine feet--that’s between 1.2 metres and 2.7 metres long for those of you using the metric system! We named it after consultation with elders from the Inuit people, who are native to the region, who suggested their word for "large shallow-water fish". The second part of the name honours the person who funded our research, but wishes to remain anonymous.
    It had several remarkable anatomical features that show it was capable not only of wading in shallow water, like slightly earlier fish on the cusp of the move to land, but also of supporting itself outside the water in the manner of four-limbed animals or tetrapods. This is where Tiktaalik truly blurs the boundary between fish and land animals. This animal is both fish and tetrapod. At first, we jokingly call it a "fishapod". Unlike fish, it had a clearly defined neck and a strong ribcage that would have enabled it to stand outside water. Its pectoral fins had a wrist joint, which enabled it to crawl on the ground. This wrist is sufficiently similar to that of later animals, including human beings, to suggest that Tiktaalik or something very like it was an ancestor of all subsequent land animals. However, we cannot be sure of that. When we tall about the fish’s wrist, we’re talking about the origin of parts of our own wrist. It is absolutely clear from Tiktaalik’s skeleton that it could support itself in shallow water or on land. This is why it represents a critical early phase in the evolution of all limbed animals, including humans.
    We found the Tiktaalik fossils in 2004 after a five-year search of a rock formation on Ellesmere Island, one of the large islands that comprise the north of Canada. This site was chosen because it was--or more exactly the rocks were--laid down during the late Devonian period, between 380 million and 365 million years ago, when the transition of fish from sea creatures to creatures that could survive on land is known to have taken place. It may surprise you to know that although the rocks are now within the Arctic Circle, in the late Devonian they lay close to the Equator. We, as individual humans, don’t notice plate movements because dramatic changes can only be seen over millions of years, but the continents as we know them today have moved considerably and will continue to do so. This exciting discovery is providing a much deeper understanding of this evolutionary milestone. Previous fossils representing this evolutionary event have really been fish with a few land characteristics, or land vertebrates with a few residual fish characteristics. These fossils show an animal that sits bang in the middle.
    Now, let’s move on to...