The Mid-Pleistocene Transition

Before 1.2 million years ago, glacial cycles had a 40,000 year periodicity. But in the past 700,000 years the ice-age cycle has been around 100,000 years. There was a transitional period in between. See Atmospheric Carbon Dioxide Concentration Across the Mid-Pleistocene Transition from 2009 in Science magazine. There is a 41,000 year periodicity in the earth's orbit which could explain the earlier periodicity (see Milankovitch cycles), but the the transition to the more recent 100,000 year cycle is currently rather mysterious.