The Clovis People, named after the Clovis archaeological site in New Mexico are generally accepted as the earliest identifiable human population in the Americas. It was thought that they arrived on that continent around 9,000 BC. Now, however, at a site at Buttermilk Creek in Texas(a), archaeologists have found stone tools in thick sediments beneath what is accepted as typical Clovis material. It is believed that these artifacts may be as much as 15,500 years old, once again pushing back the date of the earliest Americans.
(a) http://www.nytimes.com/2011/03/25/science/25archeo.html