Long ago in the north, below from the Place of Emergence, everybody came out. Now when those who are everyone's chiefs came out they all went out. They went down south ... They went along coming from the north, and they began to make towns.
—Cochiti origin myth
During the 10th to 8th millennia BCE, the San Juan Basin was occupied by Paleo-Indians known as the Clovis culture (c. 9,300) and the Folsom tradition (8,500 to 7,500). Projectile points found in the vicinity of Chaco Canyon suggest that hunters may have been active in the region as early as 10,000. By 6,000 the Picosa culture had developed from within the Paleolithic population, as environmental changes caused the eastward movement of animals and people to the Southern Plains, bringing Southwestern and northern Mexican cultures into the San Juan Basin. Cynthia Irwin-Williams proposed that the first human presence in Chaco Canyon dates to a hunter-gatherer society that she named the Oshara Tradition, which developed within the local Archaic (c. 6,000 to 800) Picosa population. The Oshara occupied portions of northwestern New Mexico, northeastern Arizona, southeastern Utah, and central and southwestern Colorado. They harvested jackrabbits in the basin as early as 5500. Irwin-Williams divided the Oshara Tradition into six phases, and during the Armijo phase (1800 to 800) the Arroyo Cuervo area east of Chaco Canyon saw the introduction of maize and the use of rock shelters. She hypothesized that this period saw the beginning of seasonal gatherings of people from around the San Juan Basin, who eventually began to aggregate into larger social units.
The San Juan Basin (note: U.S. Route 666 has been renumbered Route 491
Area occupied by the Ancestral Puebloans
By 200 BCE, the Basketmaker culture had begun to develop from the Oshara Tradition. At least two groups of transitional Basketmaker II people inhabited the San Juan Basin during this period, as increased rainfall allowed for sustained agriculture and permanent settlements by 1 CE, when the water table rose and intermittent streams became more reliable. During the first four centuries CE, the Basketmaker II people established pit-houses at elevated locations near sources of water and arable land. Brian M. Fagan notes that the development of pottery in the area during the 4th century permitted the boiling of maize and beans for the first time, and "must have brought a revolution in cooking". This period also marked the introduction of the bow and arrow to the region.
Parts of the San Juan Basin saw plentiful rainfall during the 5th to 8th centuries, leading to significant expansion of pit-house communities. Population increases during the 6th century led to the settlement of the area's lowlands, including Chaco Canyon, as the Basketmaker II people changed from a primarily hunter-gatherer society to one based on farming. This culture is known as Basketmaker III, and by 500 at least two such settlements had been established in Chaco Canyon. An important phase of Basketmaker III people is known as the La Plata. One of the earliest La Plata phase sites, Shabik'eshchee Village, was continuously occupied until the early 8th century, when the canyon was home to a few hundred people. Several clusters of Basketmaker III sites have been identified in the vicinity of Chetro Ketl.
As the Basketmaker III people improved their farming techniques during the 8th century, the well-watered areas of the San Juan Basin became densely populated. Greater crop yields necessitated the construction of above-ground storage facilities, which were the first large-scale construction projects in the region. Fagan identifies this as the beginning of the first pueblos, "ushering in a period of profound social tension, population movements, and political change." In his opinion, "by 800, there was no going back. The people of Chaco and elsewhere were locked completely into economies based on maize and bean cultivation." Archeologists refer to the period starting c. 800 as the Pueblo I Period. By the early 10th century the large pit-house settlements had been supplanted by modular construction that later served as the foundation for the Ancestral Puebloan great houses.[a] This marks the beginning of the Bonito Phase. During the 10th century, Chaco's population was swelled by a steady influx of immigrants from the San Juan River, approximately 100 miles (160 km) north. Fagan notes, "Within a few centuries, as rainfall became more irregular and life less predictable, the Chacoans embarked on a cultural trajectory that melded ancient traditions with new ideas that were to crystalize into a brilliant and short-lived Southwestern society."