Researchers first reported in 2007 that these particles were found at several archaeological sites in layers of sediment 12,900 years old. Now an independent study published in the (Sept.17) in the Proceedings of the National Academy of Sciences (PNAS) says those findings hold up.

A team led by Malcolm LeCompte, of Elizabeth City State University in North Carolina, studied sediment samples from three sites in the Unites States: Blackwater Draw in New Mexico, Topper in South Carolina and Paw Paw Cove in Maryland. The researchers said they found the same microscopic spherules in some of the same ancient layers.