Macroevolution is the study of long-term evolutionary change. Life has been present on earth for approximately 3500 million years and in that time has undergone extensive changes. That the first organisms, simple single-celled prokaryotes, have given rise to the current complexity and diversity of over 1 million known species, with an estimated 10 billion more unknown species, is almost unimaginable. The patterns by which groups of organisms arise and become extinct are the subject of the study of Macroevolution.
There are two competing theories as to how species evolve. The older theory is that of phyletic gradualism. This theory, supported by Darwin, states that evolution is a gradual process that proceeds slowly but constantly through a series of small changes. A newer theory that has received a lot of attention is that of punctuated equilibrium. Proposed by Niles Eldredge and Stephen J. Gould, this theory states that evolution is not gradual, but rather proceeds by means of bursts of change separated by long periods of relative stasis. Unfortunately, the fossil record cannot disprove either of these theories, so the debate over which is more accurate will likely continue for some time.
Though limited in many ways, the fossil record is able to provide us with a lot of information. From fossil remains we know that life began on earth about 3500 million years ago. We can also see that a series of mass extinctions has shaped the organismic landscape, allowing the fall and rise of several different dominant groups, including dinosaurs and, most recently, mammals.
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