PART.2

Tool trade

Farther south, César Méndez has followed similar clues in his search for late-Pleistocene sites along the Chilean coast. Beginning in 2004, Méndez, an anthropologist at the University of Chile in Santiago, and his colleagues excavated an ancient encampment, which they dated to around 13,000 years ago4.

Some of the stone tools at the site, called Quebrada Santa Julia, were made of translucent quartz that is not found in coastal deposits. Like Rademaker, Méndez mapped potential paths towards known quartz deposits inland. Sampling along those routes, his team found an outcrop of translucent quartz at a site where people had lived and quarried between 12,600 and 11,400 years ago. The similarity with Quebrada Santa Julia in terms of age and tool-making techniques suggests that the coastal tools came from these mountain outcrops.

“What we're seeing is that 12,000 years ago or more, these groups already had networks, knew the landscape and moved between the coast and the interior,” says Méndez.

Sites such as Quebrada Jaguay and Quebrada Santa Julia suggest that some early hunter-gatherers in South America might have travelled along the coast, taking advantage of the fish, shellfish, animals and plants found in wetlands and near river deltas, says Dillehay. He is finding more evidence beneath Huaca Prieta, a 32-metre-high mound on the coast of northern Peru.

The mound was first excavated in the 1940s, but Dillehay dug deeper and uncovered traces of Ice Age settlements in 2010. Radiocarbon dating indicates5 that humans had lived there as much as 14,200 years ago, when the area was surrounded by wetlands.

Coastal drift

If early people did migrate along the coast, some of the best evidence has probably been swallowed up by the ocean. At the end of the Pleistocene, melting ice sheets caused sea levels to rise by 70 metres, which would have flooded much of the former coastline. That effect would have been greatest in some regions of eastern South America, where the land is relatively flat and the ocean migrated well inland.

At the border between Uruguay and Argentina, for example, archaeologists suspect that ancient people might have hunted and camped on a broad delta that formerly existed at the mouth of the Uruguay River. But any such sites would have been drowned when the sea advanced by more than 120 kilometres, says Rafael Suárez, an archaeologist at the University of the Republic in Montevideo.

Suárez has looked for clues upriver, and has dated several residential sites to between 12,900 and 10,200 years ago. Some tools found at a site called Pay Paso are made of translucent agate, which apparently came from quarries near the border with Brazil about 150 kilometres away. And other tools from Uruguay have been found 500 kilometres to the south in Argentina's Buenos Aires province6, says Nora Flegenheimer, an archaeologist with the National Scientific and Technical Research Council (CONICET) in Necochea, Argentina. Such finds point to widespread trade or travel routes in eastern South America.

Some archaeologists wonder whether early residents of the continent might even have crossed the Andes. Bolivian archaeologist José Capriles of the University of Tarapacá in Arica, Chile, has raised that possibility after studying 12,800-year-old artefacts at Cueva Bautista, a rock shelter 3,930 metres above sea level in southwestern Bolivia. He notes that a similarly aged site exists at the same latitude in Chile on the western slope of the Andes. Future research could explore tools found at both sites to see whether people migrated from one side to the other or established trading routes.

But some of the best evidence for Pleistocene humans in South America may disappear soon, owing to rapid expansion in industrial-scale agriculture, road building and other forms of development. Those human threats come on top of the natural ones — wind erosion and changing watercourses — that constantly alter landscapes.

Suárez and his team had to call the navy to evacuate them from a site in Uruguay last December, when floodwaters rose dangerously in the lake behind a nearby hydroelectric dam. A proposed dam could also flood sites in the Ocoña River valley in Peru, which Rademaker thinks could have been an early route from the coast to the Andes.

In the highlands, the rapid expansion of mining can be both a bane and a blessing. Archaeologists discovered Bolivia's Cueva Bautista site during a survey for a road leading to a mine. But open-pit mines threaten many other sites, says Capriles.

Archaeological surveys must be carried out before development and infrastructure projects can go ahead, but the people who perform such studies do not always recognize the subtle signs of ancient human occupation, the researchers say. And even if the surveys do turn up important archaeological evidence, developing countries are often reluctant to let the past stand in the way of the future.

“I've never seen such destruction as you get in Peru,” says Dillehay. He has witnessed bulldozers ravage sites and landowners destroy evidence to avoid delaying construction work.

There are no signs yet of such activity reaching Rademaker's survey site in the high Peruvian Andes. Over the past decade, he and his colleagues have extensively explored the region on foot in an effort to determine whether the inhabitants of the Cuncaicha rock shelter traded for their exotic tools and whether they lived there year-round. The answers may lie in undiscovered occupation sites between the cave and the coast, so Rademaker is exploring likely avenues, mapping the routes that would have required the least energy expenditure while providing access to water and food.

The researchers have backpacked along dozens of streams and rivers, sometimes clambering up steep cliffs to avoid flash floods, always with an eye out for gashes in the rock face that signal a potential shelter. Early inhabitants probably would have explored the new landscape in the same way with the same targets in mind.

Rademaker surveyed four rock shelters this year but all of them were inhabited too recently — only 4,000 to 6,000 years ago. Still, he is convinced that there are more late-Pleistocene sites in the Andes. Early inhabitants must have found other places like the Pucuncho Basin and the Cuncaicha rock shelter. They might have followed rivers that flow from the highlands to the coast. Or perhaps they trailed the herds of wild guanacos that still descend along spurs of the Andes nearly to the ocean shore.

Each field season dangles more possibilities before Rademaker's team. “I went for a walk one night, found another confluence and found another cave,” he says. “It's never-ending.”

Nature  514,  24–26  (02 October 2014)  doi:10.1038/514024a