Sudden civilized: new finds push back Americas' first society.

The earliest known civilization in the Americas emerged about 5,000 years ago in what's now Peru, a team of archaeologists finds. Until now, it wasn't clear that the Peruvian sites examined were older than about 3,800 years or that they had been part of the same society.

New excavations and radiocarbon dates indicate that more than 20 large settlements, which cover a 700-square-mile area in four river valleys of the Andes, belonged to a culture that lasted from about 5,000 to 3,800 years ago.

"It's stunning that so many sites were organized at the same time around terraced pyramids, sunken plazas, and irrigated fields," says Jonathan Haas of the Field Museum in Chicago. "That kick-started the growth of hierarchical societies in the Andes, culminating in the Inca [around A.D. 1200]."

In the Dec. 23/30, 2004 Nature, Haas and Winifred Creamer and Alvaro Ruiz, both of Northern Illinois University in De Kalb, present 95 new radiocarbon dates, some obtained from bits of reeds and wild cane that had been woven into mesh bags for hauling rocks to prehistoric construction sites. The researchers had directed new work at 13 of the previously discovered sites and argue that the findings apply to others nearby.

The ancient settlements range in size from 25 to more than 250 acres. Each contains the remains of pyramids, ceremonial structures, and residential areas. Remnants of domesticated plants include cotton, squash, beans, and avocados. Seafood, such as anchovies and shellfish, was also consumed at the sites.

Haas proposes that, for some unknown reason, coastal hunter-gatherers suddenly transformed their way of life to found the inland agricultural society.

Researchers now need to determine whether other Andean societies arose around 5,000 years ago, comments Charles Stanish of the University of California, Los Angeles, who works at prehistoric sites in that region.