The Proterozoic ( /ˌproʊtərəˈzoʊɪk, prɒt-, -əroʊ-, -trə-, -troʊ-/) is a geological eon spanning the time interval from 2500 to 538.8 million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.