Recent scientific findings date their arrival earlier than ever thought, sparking hot debate among archaeologists For much of its length, the slow-moving Aucilla River in northern Florida flows underground, tunneling through bedrock limestone. But here and there it surfaces, and preserved in those inky ponds lie secrets of the first Americans.For years adventurous divers had hunted fossils and artifacts in the sinkholes of the Aucilla about an hour east of Tallahassee. They found stone arrowheads and the bones of extinct mammals such as mammoth, mastodon and the American ice age horse.Then, in the 1980s, archaeologists from the Florida Museum of...