The Olmec population declined sharply between 400 and 350 BCE, though it is unclear why. It may have been caused by environmental changes. These changes may have been triggered by the silting up of rivers due to agricultural practices.

Another theory for the population drop relates to tectonic upheavals. Volcanic eruptions could have blanketed the lands and forced the Olmec to move their settlements.

The Smithsonian's Matthew Stirling kneeling beside a giant Olmec head, Mexico - 1938.