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Nemesis Origin of the theory The Nemesis Theory was an outgrowth of the discovery of Alvarez et al that the impact of a large 10 km diameter comet or asteroid was responsible for the great mass extinction that took place 65 million years ago Studies of the fossil record by Dave Raup and Jack Sepkoski shows that this was not an isolated event but one of several mass extinctions that appear to occur on a regular 26 million year cycle Their original paper analyzed marine fossil families and was published in the Proceedings of the National Academy of Science USA vol 81 pages 801-805 1984 The original extinction data of Raup and Sepkoski are replotted in the following figure The vertical axis shows the extinction rate This was taken from the values given by Raup and Sepkoski for the percent family extinctions at each geologic boundary In order to take into account the uncertainty in the boundary ages each data point was plotted as a Gaussian with width equal to the uncertainty and area equal to the extinction rate This plot thus represents a statistical estimate of the extinction rate vs time The individual Gaussians for each stage boundary are shown as dotted lines The extinction 65 million years ago is indicated with the little dinosaur icon The peak near 11 Ma is real but exaggerated by the requirement that the plot go to zero at the present Arrows are plotted every 26 million years Note that many of these are close to the peaks in the extinction rate This is the apparant 26 million year periodicity discovered by Raup and Sepkoski There have been many statistical studies of these data Although several studies indicate the periodicity is significant not everyone agrees I suggest that you decide for yourself If you decide that the extinctions are not statistically significant then there is no need for the Nemesis theory Additional work by
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