Fossilized human footprints found in New Mexico reveal that people dwelled in the Americas during the last ice age’s peak — a discovery that researchers suggest is conclusive proof of early migration to the New World, a new study finds. Although the newfound footprints are not the oldest evidence of humans’ arrival in the Americas, they may be the first unequivocal proof that people were there during the last ice age, scientists noted. The arrival of the first people in the Americas was a key step in humanity’s expansion across the planet, but the precise timing of this milestone remains hotly contested. Based on stone tools dating back roughly 13,000 years, archaeologists had long suggested that people from the prehistoric culture known as the Clovis were the first to migrate to the Americas. However, researchers recently unearthed a great deal of evidence of pre-Clovis artifacts. For example, last year scientists revealed that stone artifacts discovered in Chiquihuite Cave in central Mexico were at least 26,500 years old; computer models found the cave’s location was so far inland in the Americas, and thus so distant from the Old World from which human migrants arrived, that it suggested that humans might have first entered the New World as early as 33,000 years ago. Still, the earliest archaeological evidence of human settlement of the Americas remains highly controversial. Even the scientists who excavated Chiquihuite Cave suggested that others might argue that the stone objects discovered there are not of human origin but are merely “geofacts,” or normal rocks that look artificial. Now, 60 footprints embedded in an ancient lake bed in what is now White Sands National Park in south central New Mexico are strong evidence that humans occupied the New World between about 21,000 and 23,000 years ago.