Radiocarbon dating on artifacts in a gypsum cave in Nevada shows the site was used by humans over 4,000 years ago.
Harrington’s surprise discovery was darts and other human artifacts below the ground sloth layer. Since the Shasta ground sloth became extinct about 9,000 years ago, the evidence seemed to indicate human habitation before that,
But Gilreath said when radiocarbon dating technology became available, tests showed the human evidence was not much older than 4,000 years. When she and her colleagues were studying the cave, they noticed a number of packrats scurrying through the jumbled rock, and concluded the rodents moved material from the upper layers to lower layers.