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How did the International Union of Geological Sciences settle the date for the beginning of the Quaternary Period?

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Map showing the extent of the Laurentide, Cordilleran, and Greenland ice sheets during the Wisconsin glaciation, roughly 100,000 to 11,000 years ago. The Laurentide Ice Sheet covers most of present-day Canada and parts of the northern United States in light blue, the Cordilleran Ice Sheet covers the Pacific Northwest in teal, and the Greenland Ice Sheet is marked in purple. Modern international and state or provincial boundaries are outlined for reference. — Pleistocene Epoch Ice sheets during the Wisconsin glacial episode penetrated deep into North America.

The International Union of Geological Sciences (IUGS) officially settled the beginning of the Quaternary Period in 2009 by ratifying a decision to set its start at 2,588,000 years ago.1

Prior to this, the status of the Quaternary was debated, and the Pleistocene Epoch was often understood to have begun approximately 1.8 million years ago. The 2009 decision resolved this by making the base of the Quaternary—and the base of the Pleistocene—coincident with the beginning of the Gelasian Stage.12

This date was selected because the rock strata from this time provide extensive evidence of the widespread expansion of ice sheets over northern continents and the onset of significant climatic and oceanographic changes. The formal boundary is defined by a Global Stratotype Section and Point (GSSP) located at Monte San Nicola near Gela, Sicily.12