Climate-proxy records of the past 100,000 years show that the Earth＇s climate has varied significantly and continuously on timescales as short as a few thousand years (refs 1-7), Similar variability has also recently been observed for the interval 340-500 thousand years ago(8). These dramatic climate shifts, expressed most strongly in the North Atlantic region, may be linked to - and possibly amplified by - alterations in the mode of ocean thermohaline circulation(4-9). Here we use sediment records of past iceberg discharge and deep-water chemistry to show that such millennial-scale oscillations in climate occurred over one million years ago. This was a time of significantly different climate boundary conditions; not only was the early Pleistocene epoch generally warmer, but global climate variations were governed largely by changes in Earth＇s orbital obliguity, Our results suggest that such millennial-scale climate instability may be a pervasive and long-term characteristic of Earth＇s climate, rather than just a feature of the strong glacial-interglacial cycles of the past 800,000 years.