Stone tools unearthed at a Brazilian rock-shelter may date from at least 22,000 years ago, adding to evidence from nearby sites challenging the longstanding view of Clovis people as the first Americans 13,000 years ago, say a team led by geochronologist Christelle Lahaye of the University of Bordeaux, and archaeologist Eric Boeda of the University of Paris.
     Among other South American locations proposed as pre-Clovis human settlements, the most controversial is Brazil's Pedra Furada rock-shelter. There, archaeologists unearthed burned wood and sharp-edged stones and dated them to more than 50,000 years ago. Pedra Furada's excavators regard the finds as evidence of ancient human hearths and stone tools. Critics say the Brazilian discoveries could have resulted from natural fires and rock slides.
     The new discovery, at Toca da Tira Peia rock-shelter, in the same national park as Pedra Furada, has also drawn skeptics. Dating hinges on calculations of how long ago objects were buried by soil, which depends on various environmental conditions, including fluctuations in soil moisture.
     However, archaeologist Tom Dillehay of Vanderbilt University in Nashville has seen some of the Toca da Tira Peia finds and regards them as human-made implements. Similar tools have been unearthed at sites in Chile and Peru, Dillehay says. His team previously estimated that people settled Chile's Monte Verde site by 14,000 years ago, and possibly as long as 33,000 years ago.
     An absence of burned wood or other finds suitable for radiocarbon dating at Toca da Tira Peia is a problem, because that's the standard method for estimating the age of sites up to around 40,000 years ago, Dillehay says. But if people reached South America by 20,000 years ago, "this is the type of archaeological record we might expect: ephemeral and lightly scattered material in local shelters."
     Lahaye and Boeda's team excavated 113 stone artefacts consisting of tools and tool debris in five soil layers, estimating that the last exposure of soil to sunlight ranged from about 4,000 years ago in the top layer to 22,000 years ago in the third layer.
     Lahaye says that 15 human-altered stones from the bottom two soil layers must be older than 22,000 years. The researchers plan to calculate when those artefacts might have been buried.

Edited from ScienceNews (13 March 2013)