Apparently paleontologists have puzzled for some time over why all the mammals die off every 2,500,000 years or so. With a finer temporal analysis of mammalian remains, Jan van Dam of Utrecht University linked the mass extinctions to long-term cycles in the Earth's climate caused by perturbations in the Earth's orbit and the tilt of its axis of rotation. Here is a discussion of his findings.
The theory that orbital variations cause climate fluctuations dates back to the 19th century Scottish scientist James Croll, who taught himself physics and astronomy. At one time he took a job as a museum janitor to have access to better books for developing his ideas. By the end of the 19th century his theory had fallen into disfavor but it was taken up again in the 1920s by the Serbian civil engineer and geophysicist Milutin Milankovitch. By the 1970s the study of sediments in the deep seas brought a wide acceptance of the Croll/Milankovitch theory.
