The Anasazi's Ancestors from 3,000 to 1,000 B.C.

The beginning of what is known as the "Medithermal" correlates with the start of the Late Archaic period (3,000 to 1,000 B.C.). Unlike the severe conditions of the preceding Altithermal, the beginning of the Medithermal was characterized by a rapid increase in effective moisture and climatic stability. These changes are reflected in the pollen samples from this period, which show an increase in open woodland and grassland communities, while geological studies indicate that many desiccated lakes were filling. Rodent middens also suggest wetter and warmer conditions for the upland portions of the Southwest.

The stability of the regional climate disappeared after 2000 B.C. Periods of lower moisture, floodplain degradation, higher temperatures, and marked seasonal variability alternated with periods of moderate rainfall, soil aggregation, moderate temperatures, and less extreme seasonal change. However, these environmental fluctuations were not as severe and rapidly changing as those in the Altithermal, and the climate was generally as wet as the preceding period. Sometime after 2000 B.C., the general climatic patterns and vegetation and animal communities found in the Southwest today were gradually established. Resources were generally divided vertically and seasonally, with succulents and wild game available in the lowlands during the winter and spring, while an abundance of nut-bearing trees, seed- and berry-producing plants, and animals were found in the highlands during the summer and fall.

Throughout the Southwest, the rapid environmental changes of the Late Archaic were followed by the expansion of people both into upland areas as well as down into the valleys and basins. Many groups appear to have responded to the unstable environment and the complex mosaic of habitats by adopting a biseasonal pattern of mobility. For example, Brad Vierra analyzed pre-agricultural Archaic settlement patterns in the San Juan Basin and concluded that a foraging strategy was most likely employed during the spring, summer, and fall, while stored foods and logistical collecting trips were utilized during the inhospitable winters. The most convincing evidence for extended seasonal occupations is the identification of more permanent and larger sites exhibiting small pithouses. For example, a pithouse on the Chama River that was accompanied by groundstone and pit hearths may be as old as 3,200 B.C. Other examples include a group of pithouses found near El Paso, Texas, that date to as early as 2200 B.C. and two possible structures at Moquino, New Mexico, that date to around 2700 B.C. Evidence for early forms of storage also appear during this period. For example, the El Paso pithouse site contained four possible storage pits, while at Bat Cave, most of the plant remains were recovered from very small side chambers that were probably used for storage. These early storage features may have alleviated food stresses caused by the unstable environment.

Artifacts recovered from Late Archaic sites throughout the Southwest are characteristic of an economy based on a diversified, nonspecialized diet. Digging sticks, basketry, snares, and atlatl fragments all attest to the diverse hunting and foraging strategies employed during this time. Projectile points are more crudely made and readily modifiable, and W. Wills contends that this increasingly efficient technology is suggestive of a logistical procurement system in which very specific resources are targeted. Wills also notes an increasing use of foreign lithic materials that likely reflects frequent exchange between Late Archaic groups.

On the Colorado Plateau, the Late Archaic time period is represented by a continuation of the lengthy Oshara Tradition, which is further divided into the San Jose and Armijo Phases. The San Jose Phase (3,000 - 1,800 B.C.) is represented by a large number of canyon-head base camps that appear to be larger and more intensively occupied than in preceding phases. This suggests that people are spending more time in favorable locations, and post holes at these sites may be the remains of simple structures. There is an increasing use of groundstone and heavy choppers for processing plant foods, and cobble-filled earth ovens may represent the first use of stone -boiling in this area. Medium- and small-sized animals are the focus of hunting activities, and points are now small and serrated. A study of Late Archaic coprolites from Cowboy Cave revealed that recent meals included dropseed, sunflower seeds, cheno-ams and other grasses, and cacti pads.

The San Jose Phase is well known for the quantity of basketry recovered from archaeological sites. Split-twig figurines representing various animals are found throughout the Colorado Plateau, and the frequency of these and other "ceremonial" items increases throughout the Late Archaic.

The Armijo Phase begins in 1,800 B.C. and lasts to 800 B.C. Many of the same trends that characterize the preceding San Jose Phase continue into the Armijo Phase, including increasing site size, a pattern of biseasonal mobility, and an increasing use of groundstone. There also appears to be a greater emphasis on gathering than before, and now mainly smaller animals are hunted.

During the Late Archaic, domesticated plants from the Upper Sonoran Agricultural Complex begin to be used in areas south of the Colorado Plateau. This is marked by the cultivation of Chapalote 12- to 14-row popcorn maize in the Sonora Desert around 1,200 B.C. and at places such as Bat Cave by 1,100 B.C., where it was integrated into the seasonal rounds of Chiricahua people. This early maize was not very productive and probably did not make up much of the diet until much later. None of the early domesticates appear on the Colorado Plateau until the Terminal Archaic period, which begins in 1,000 B.C.