A recent and significant find, as reported by Tom Metcalfe for NBC News (hat tip: Paul Halsall):

Ancient stone tools suggest first people arrived in America earlier than thought

Three deliberately-shaped pieces of limestone — a pointed stone and two cutting flakes — may be the oldest human tools yet found in the Americas.

Pieces of limestone from a cave in Mexico may be the oldest human tools ever found in the Americas, and suggest people first entered the continent up to 33,000 years ago – much earlier than previously thought.

The findings, published Wednesday in two papers in the journal Nature, which include the discovery of the stone tools, challenge the idea that people first entered North America on a land bridge between Siberia and Alaska and an ice-free corridor to the interior of the continent.

Precise archaeological dating of early human sites throughout North America, including the cave in Mexico, suggests instead that they may have entered along the Pacific coast, according to the research.

Ciprian Ardelean, an archaeologist with the Autonomous University of Zacatecas in Mexico, the lead author of one of the papers, said the finds were the result of years of careful digging at the Chiquihuite Cave in north-central Mexico.

The steeply-inclined cave is high on a mountainside and filled with crumbling layers of gravel: “The deeper you go, the higher the risk for the walls to collapse,” he said.

The excavations paid off with the discovery of three deliberately-shaped pieces of limestone — a pointed stone and two cutting flakes — that may be the oldest human tools yet found in the Americas.

They date from a time when the continent seems to have been occupied by only a few groups of early humans – perhaps “lost migrations” that left little trace on the landscape and in the genetic record, Ardelean said.

More at the link. UPDATE: More at Nature.