For more than a decade, evidence has been piling up that humans colonized the Americas thousands of years before the Clovis people. The Clovis, who are the early ancestors of today's Native Americans, left abundant evidence of their lives behind in the form of tools and graves. But the mysterious pre-Clovis humans, who likely arrived 17,000 to 15,000 years ago, have left only a few dozen sources of evidence for their existence across the Americas, mostly at campsites where they processed animals during hunting trips. Now a fresh examination of one such campsite, a 14,000-year-old hunter's rest stop outside the city of Tres Arroyos in Argentina, has given us a new understanding of how the pre-Clovis people might have lived.

Archaeologists are still uncertain how the pre-Clovis people arrived in the Americas. They came after the end of the ice age but at a time when glaciers and an icy, barren environment would still have blocked easy entrance into the Americas via Northern Canada. So it's extremely unlikely that they marched over a land bridge from Siberia and into the Americas through the middle of the continent—instead, they would have come from Asia via a coastal route, frequently using boats for transport. That would explain why many pre-Clovis sites are on the coast, on islands, or on rivers that meet the ocean.

These early settlers were hunter-gatherers who used stone tools for a wide range of activities, including hunting, butchery, scraping hides, preparing food, and making other tools out of bone and wood. Many of the pre-Clovis stone tools look fairly simple and were made by using one stone to flake pieces off the other, thus creating sharp edges. At the campsite in Argentina, known as the Arroyo Seco 2 site, archaeologists have found more than 50 such tools made from materials like chert and quartzite. They're scattered across an area that was once a grassy knoll above a deep lake, which is rich with thousands of animal bone fragments that have been carbon dated to as early as 14,000 years ago. There are even a couple-dozen human burials at the site, dated to a later period starting roughly 9,000 years ago. The spot has the characteristic look of a hunter's camp, used for processing animals, that was revisited seasonally for thousands of years.

The Pleistocene diet

Writing in PLoS One , the researchers describe a number of reasons why a bunch of sharp-edged rocks and broken animal bones point to a 14,000-year-old human occupation of Argentina. First of all, there are far too many animal bones from a diversity of species grouped in one place for it to be accidental. Yes, there are some natural traps where we find massive numbers of prehistoric bones, but those are almost always in holes or depressions in the ground—and this area was on a rather high hill during the Pleistocene. Second, the stones aren't just sharp-edged in a way that suggests flaking; many also show signs of wear and tear from scraping hide. "A large majority of the flaked edges were used transversely on dry skin," the researchers write. "Consequently, it is likely that the skins were brought to the site in a state of intermediate processing." Also, most of the stone used for the tools, including quartzite and chert, can only be found over 110km from Arroyo Seco. So that piece of evidence also points to human hunter-gatherers carrying tools with them over great distances.

One question remains. How can we be sure the tools at the site really are 14,000 years old? Archaeologists infer some of this from carbon dates on the animal bones, which have been tested by several labs around the world. The problem is that the site's stratigraphy, or historical layers, are difficult to read due to erosion at the site. So even if a tool appears right next to a bone in a given layer, it may have come from later and been moved around by wind and water. That said, there is evidence that some of the early bones were broken by stone tools. A 14,000-year-old bone from Equus neogeus, an extinct American horse, bears distinct marks from a hammerstone. "This bone was intentionally broken while still fresh," note the researchers.





Robert Horsfall

With a firm connection between the human tools and the animal bones found at Arroyo Seco, we can begin to piece together what everyday life was like for these people—at least at mealtime. Analysis of more than 600 bone fragments out of thousands found at the site revealed that a large amount of these people's meat came from animals that no longer exist. Various extinct horse species were a major part of the pre-Clovis diet, as were other extinct mammals like giant ground sloths, camels, mammoths, and giant armadillos. When these people arrived in South America, they found a land that no human had ever colonized. Many of these species would have been easy pickings for well-organized bands of hunters with sophisticated languages, tools, and tactics. Some paleoecologists hypothesize that these animals went extinct partly due to human hunting, and this campsite definitely provides evidence that extinct animals were part of the pre-Clovis diet for millennia. That said, Arroyo Seco contains far more bones from guanaco (a local relative of the camel) and rodents than it does from extinct mammals.

The absence of certain bones can tell us about how these people lived, too. Though there are bones from megafauna like the giant sloth Megatherium, we see no skulls, chest, or pelvic bones from the animal. The researchers speculate that's because hunters would have done an initial butchery at the site where they killed or scavenged the animal and then transported parts of it to be processed at camp:

Given the body mass of this species (between 4 and 5 tons), it would have been extremely difficult to transport the entire carcass and even challenging to transport complete hindquarters weighing between 600 and 750 kg, and forequarters weighing between 250 and 300 kg. Taking into consideration these values, the best hypothesis is that the Megatherium was hunted or scavenged near the site, the skeleton was butchered into smaller parts, and these units were then transported to their current location at the site. The larger bones were transported with portions of meat already removed, and the bone may have been used for other purposes such as bone quarrying.

Of the extinct mammals that humans processed at Arroyo Seco, the most common seems to be horse. When people arrived in the Americas, it was full of at least two species of extinct horses. But by the time of the Inca and other great civilizations of South America, those animals were long gone. It wasn't until Europeans arrived with their steeds that the continent was once again populated with horses.

Still, we can look back and imagine what it must have been like for those pre-Clovis people, entering a world where no human had ever gone before, full of animals that are legendary to us today. In many ways, they lived on a different planet than the one we inhabit now. At the edge of a now mostly vanished lake, on a knoll, those people fed their families, made tools, and strategized about how to hunt for game bigger than anything on land in the modern world. They returned year after year for centuries. Eventually, they buried their dead there among the animal bones left by their ancestors.

PLoS One, 2016. DOI: 10.1371/journal.pone.0162870