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Two rheas is a long way from the estimated 10 million species living today but it doesnt take many splits to get there. After only four splits there are sixteen species and it grows exponentially. It takes only 23 splits to create almost 10 million unique species 223 8388608. And species can diverge rapidly through splitting. In the diagram below the small rhea and large rhea are not very different after one split but how different are species X and Y after four splits How different would X and Y be on a tree with ten million species across the top Even if each difference were tiny there would be dramatic differences across the entire group. Small changes add up. In some cases the apparent evolutionary leap appears to be too big to explain through small incremental adaptations. One critical transition for example was from fish to tetrapods four-legged animals which led to land-based animals. But such dramatic leaps did not necessarily occur all at once-what appears to be one step can be the sum of several smaller steps. In 2004 researchers found a fossil in Northern Canada called Tiktaalik Inuit for big freshwater fish. It is clearly fish-like yet it had developed certain features that would have enabled it to push its head up out of shallow watera primitive wrist that would have allowed its fins to bear weightand shoulder blades that were not attached to its skull unlike other fish thereby allowing greater movement of the head. These adaptations may have helped Tiktaalik exploit new sources of food like insects found along the shore by pushing its head up and out of the water to snatch them. Later as these adaptations continued to evolve they may have enabled descendants to crawl onto land at first for short periods of time but eventually enabling a permanent transition to new land-based life forms that led to the evolution of the tetrapods. Thus even small adaptations-made for one reason-can result in significant evolutionary change by opening up new possibilities in completely unrelated ways. Although the fossil record is imperfect it does in fact support the theory of evolution and the principle of common descent. Through transitional forms like Tiktaalik and others it is possible to connect all living things through the Tree of Life. 44 Large rheaSmall rhea Large rhea X Y