Why do scientists describe the history of life as a branching tree?
Playing out over these immense time scales are two distinct phenomena. First, life was continually evolving, casting out new branches representing new lineages of organisms like a great tree. At the same time, however, many lineages went extinct as if nature were pruning the tree of life. The kinds of animals and plants that populate the Earth have been changing continually. Truly cataclysmic events often led to mass extinctions, some of them eliminating more than three quarters of all life on Earth. Recovery was slow, but it always occurred. Often the survivors of these extinction events were kinds of organisms that had not been especially important before the mass extinction. Part of the reason that Kansas fossils from the Pennsylvanian Period differ from those of the Permian Period is because a time of extinction separates these two intervals. In addition, these fossils differ because evolutionary change had occurred.
