Steps have been around essentially as long as humans have been carving rock. It's not really possible to pinpoint any particular origin because steps appear around the same time as people start constructing permanent settlements. For instance, the early site of San Jose Mogote, one of the first major settlements in the valley of Oaxaca, uses steps quite extensively. The Nahua used steps rather extensively as well in their famous pyramids. Even the Puebloans of the early basketmaker period used ladders in their pit houses, a technology largely identical to steps.

The archaeological record on this note is roughly similar the world over: stairs and steps occur whenever people use difficult to work materials to change between altitudes. In fact, slopes themselves are rather uncommon. Excepting for a minute natural terrain, smoothed slopes are quite ancient. They're used rather early in mesopotamian water channels. For human transportation, slopes aren't particularly useful until you have the wheel, a technology that was never used extensively in the Americas.