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Why did geologists debate the starting date of the Pleistocene Epoch?

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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 debate over the starting date of the Pleistocene Epoch stemmed from differing scientific criteria used to define its lower boundary.12

Historically, many geologists advocated for a boundary based on climatic evidence, such as the onset of glaciation or the appearance of cold-climate fossils. In 1985, geological societies reached an agreement to set the start at approximately 1.8 million years ago, aligning with the onset of glaciation in Europe and North America. However, this date became controversial as modern research revealed that significant glaciers had formed in other regions of the world much earlier.12

Other proposed criteria included the appearance of humans, specific vertebrate fossils, or changes in microfossils found in deep-sea sediments. The debate was finally resolved in 2009 when the International Union of Geological Sciences (IUGS) ratified the decision to move the base of the Pleistocene to 2.588 million years ago, making it coincident with the base of the Gelasian Stage.13