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Why was the beginning of the Quaternary Period set at 2.588 million years ago?

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Quaternary paleogeography — Quaternary Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Quaternary Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The beginning of the Quaternary Period was formally set at 2.588 million years ago by the International Union of Geological Sciences (IUGS) in 2009.1

This date was chosen because it corresponds to a significant turning point in Earth's history, marked by extensive evidence in rock strata of the widespread expansion of ice sheets across northern continents. This period also signals the onset of an era characterized by dramatic climatic and oceanographic shifts.1

Prior to this decision, the beginning of the Pleistocene Epoch and the Quaternary was often cited as 1.8 million years ago, based on earlier regional glaciation evidence. However, modern research revealed that large-scale glaciation occurred globally well before that time. By aligning the start of the Quaternary with the base of the Gelasian Stage, the scientific community established a more accurate, globally consistent boundary that reflects the true onset of these major environmental changes.123