Millennia before the arrival of Europeans, early Native Americans went on a construction binge, dotting the eastern side of the continent with thousands of vast earthen mounds. Now, as reported  in the current issue of Science, a finding in Louisiana has extended the tradition of mound building back in time by nearly 2000 years, to about 5400 years ago--predating the agriculture and trade networks thought necessary to build such structures. "It's rare that archaeologists ever find something that so totally changes our picture of what happened in the past, as is true for this case," comments anthropologist Vincas Steponaitis of the University of North Carolina, Chapel Hill.
The elaborate mound complex, called Watson Brake, first received scientific attention in the 1970s, after loggers clear-cut some of the area. Consisting of 11 mounds and connecting ridges that enclose nearly 9 hectares, it was originally thought to have been build by the Poverty Point people, who flourished in the region from 3700 to 2700 years ago and also constructed conical mounds and long ridges. But when archaeologist Joe Saunders of Northeast Louisiana University in Monroe walked the site, he found no evidence of characteristic Poverty Point artifacts.
Saunders then cored the tallest mound at Watson Brake by auger in 1993, working with colleague Thurman Allen, a soil scientist from the Natural Resources Conservation Service in Monroe. One meter down, the pair discovered a reddish, clay-enriched layer of soil, a so-called soil "horizon" that could only have been formed over several thousands of years. Radiocarbon dates on charcoal from horizons just underneath the base of the mounds showed that construction began at a startlingly early date: 5400 to 5300 years ago.
Building the complex earthworks would have required sophisticated organizational skills, suggesting that the Watson Brake people could not have been just the small, mobile bands of hunter-gatherers thought to inhabit the area then. But many mysteries remain about how they managed to build the mounds. Archaeologists once thought mound building was linked to practices such as agriculture, which created food surpluses and tended to lead to more permanent settlements and more complex societies, or to extensive trade, which could also foster more prosperous societies. But there is little evidence of either agriculture or extensive trading networks at Poverty Point. The area at the time was, however, rich in game and fish as well as undomesticated edible plants.
But even if there was ample food, there is another mystery. Saunders has so far unearthed few clues about the giant enclosure's purpose. Soil sampling inside the earthwork retrieved few artifacts, suggesting that the builders did not conduct ceremonies or other activities within the enclosure. Stranger still, there's little evidence that people occupied the area once the complex was completed. "I know it sounds awfully Zen-like," Saunders concludes, "but maybe the answer is that building them was the purpose."