Writing in NatureArdelean et al.one and Becerra-Valdivia and Highamtwo report evidence that the initial human settlement of the American continent occurred earlier than widely accepted, and some of this evidence suggests that expansion to the continent began at least 10,000 years earlier than was generally suspected. A radiocarbon dating study of the earliest archaeological sites by Becerra-Valdivia and Higham reveals that the interior regions of Alaska, Yukon in Canada, and the continental United States were already heavily populated before 13,000 years ago. For decades, that time frame was widely considered to mark the earliest possible date of initial entry, until data from sites over 13,000 years old in North and South America, first reported in the early 1990s. 1970, increased possibility of earlier arrivals3–5 5. Archaeological excavations in the Chiquihuite cave in northern Mexico by Ardelean and his colleagues provide evidence of human occupation some 26,500 years ago. This Mexican site now joins half a dozen other documented archaeological sites in northeast and central Brazil that have yielded evidence suggesting dates of human occupation between 20,000 and 30,000 years ago.6 6–12.
After discoveries in the 1930s on the Great American Plains of distinctive and well-designed stone spearheads of a type related to the Clovis culture, along with bones of mammoths, mastodons, and a now extinct bison species, they maintained archaeologists, for many of the After decades, that the first people in the Americas were specialized big game hunters that rapidly expanded in North and South America, within 1,000 years of initial entry13. This model became known as Clovis’s first theory. Later it was established that Clovis technology did not reach the southern continent. The timing of its entry from Alaska into what is now the continental United States was thought to coincide with the opening of an ice-free corridor (Fig. 1) some 13,000 years ago between the northern great continental ice sheet (called the Laurentide Ice Sheet ) and the northern rocky mountains covered in ice (the Cordilleran Ice Sheet) in western Canada.
Figure 1 | The early settlement of the Americas. During the last ice age, glaciers blocked overland entry to the Americas until an ice-free corridor was opened. It is unknown whether people first entered the Americas through this corridor, along a coastal route, or before glaciers blocked the path, and the timing of this initial entry is unknown. Becerra-Valdivia and Highamtwo report analysis of 42 archaeological sites in North America and Beringia (the land mass in the Bering Strait area that previously joined Alaska and Siberia) that provide evidence of earlier widespread human occupation of this region than previously reported. Some examples of sites associated with early human occupation are shown. Ardelean et al.one reports the discovery of a site associated with human occupation in the Chiquihuite cave in Mexico for 27,000 years. This is in addition to previous and debated evidence of the early occupation of South America at the Monte Verde II sites3,14Santa Elina12 and in the state of Piauí6 6–eleven in Toca do Boqueirão da Pedra Furada7 7Vale da Pedra Furada8, Toca do Sitio do Meio9, Toca da Tira Peia10 and Toca da Janela da Barra do Antonião-Norteeleven. Kyr, a thousand years ago.
However, starting in the mid-1970s, researchers identified archaeological sites in the Americas that date back more than 13,000 years, especially in South America. For example, the Monte Verde II site in south-central Chile, initially dating to 14,500 years ago3, is a well-preserved open settlement with wooden structures and artifacts indicating a lifestyle based primarily on a plant diet (later discoveries revealed earlier occupations of this site14) Other early archaeological sites in South America on the Pacific coast, in the northern and central Andes, on the Caribbean coast, in the highlands of Brazil, in the Amazon basin and in the Patagonian steppe in Argentina indicate that all The main environmental zones of the region were occupied by people with diverse adaptations and ecological technologies before approximately 13,000 years ago.4 4.
Becerra-Valdivia and Higham carried out a statistical analysis of the radiocarbon dates of the first widely distributed archaeological sites on the continent of North America and Beringia (the land that once joined Alaska and Siberia in the Strait of Bering). Their results now establish that, 15,000 years ago, North America was also widely settled, with some data suggesting little occupation before that; and several distinctive regional traditions in stone tool technology had been developed 13,000 years ago. Based on evidence from these earliest archaeological sites dating back more than 13,000 years, identified on both continents, the first Clovis model should be discarded. Clearly, people were in the Americas long before the development of Clovis technology in North America.
Instead, the key question is how much earlier the Americas were initially populated than previously thought. One aspect to consider is the route or routes people took when expanding to southern Alaska. This is the supposed entry point from Northeast Asia through the Bering Strait area. However, during a long interval during the last major glacial advance (dated between approximately 26,500 and 19,000 years agofifteen), the obvious overland route through the lowlands east of the Rocky Mountains was blocked by the fusion of the Cordilleran and Laurentide ice sheets. An alternative route along the Pacific coast by populations adapted to coastal life has gained strength as a possibility, as a result of increased archaeological research in coastal areas.sixteen. Another option to consider is an initial entry before the ice-free corridor closes during the last major glacial advance.
This is where the evidence from the Chiquihuite Cave comes in. After an initial test excavation suggested that the site was of great antiquity, Ardelean and his colleagues continued their research using a variety of scientific techniques. They recovered stone artifacts from a distinctive technology located in layers with dates corresponding to approximately 27,000 years ago in the lowest parts of the cave’s sedimentary deposits, and the authors discovered more artifacts in upper layers that date back as long as 13,000 years. Dating the layer with the earliest artifacts indicates that there were people in northern Mexico at a time corresponding to the beginning of, or early during, the last major stage of glacial advance in North America.
Ardelean and colleagues’ suggestion that the initial entry date was 33,000 years ago, which is more than double the currently popular date of some 16,000 years ago, will be very difficult to accept for most archaeologists specializing in the first United States. There will undoubtedly be challenges to this interpretation and close examination of the site data. The six Brazilian archaeological sites date back more than 20,000 years, five in the center of the state of Piauí.6 6–eleven and one in the center of Mato Grosso (the Santa Elina rock shelter)12Although excavated and analyzed by experts, they are commonly disputed or simply ignored by most archaeologists for being too old to be real. The findings in the Chiquihuite cave will bring new consideration to this topic.
An unanswered question is why no archeological site of equivalent age to the Chiquihuite Cave has been recognized in the continental United States, assuming that, with an entry point to the Bering Strait, the first people to expand south must having passed through that area. With the coastal entry model, it could be assumed that the earliest archaeological sites are now submerged in the high seas by rising sea levels at the end of the last ice age. For the continental interior, it could be a matter of carefully identifying and investigating geological or paleontological locations of the appropriate age, looking for traces of human presence, and reexamining previously discounted archaeological sites and collections to detect now recognizable evidence of human behavior. In light of these new discoveries, archaeological research in this period should be intensified.
