The earliest migrations, which served to gradually expand the range of human habitation, progressed in waves that paralleled evolutionary steps. As Homo erectus evolved into Homo sapiens, who in turn evolved into Homo sapiens sapiens, migratory movements into new territories became increasingly long-range. The first migrations of the ancient humanity took place in Africa between 1.8 million and 100,000 years ago, as Homo erectus spread throughout the African continent to areas of Madagascar, Europe, the Middle East, India, southern China, and Southeast Asia. Around 98,000 B.C.E., with the evolution of Homo sapiens, the human habitat expanded even farther. By 35,000-30,000 years ago humans had also adapted technologically and socially to the harsh boreal climates of the north, and were now found throughout northern Europe and Siberia. This is where we pick up the story of the peopling of the Americas, for it is at this point that migration into the North American continent became possible.

There are several theories regarding the earliest migrations into North America. The range of estimated dates extends from as early as 50,000 to as late as 15,000 years ago. The earlier estimates are based on assumptions of a long doubling time (one of perhaps 50 to 70 years), a short life span (approximately 20 years), and slow expansion of the migrant population, as well as on an earlier date of human occupation in Northeast Asia. Many of the later estimates are based on just the opposite - a fast doubling time (about 20 to 30 years), longer life span (up to 30 years), and very rapid expansion throughout the continent. Without clear information about the reproductive rate, health, and mobility of these early migrants it is extremely difficult to use estimates based on demographic assumptions.

Other approximated dates for the first migrations to the continent fall within this range and are based on dates derived from archaeological sites, or based on linguistic, physiological, and genetic differentiation between populations in northern Asia and in North and South America. One recent model for the peopling of North and South America is the "three wave" model, which proposes that the Americas were populated by three distinct waves of migrants moving into what is now Alaska via Siberia. This theory is based on the trio of linguistic groups that exist in the Americas: Amerind, Na-dene, and Eskimo-Aleut. The geneticists, Sandro L. Bonatto and Francisco M. Salzano have proffered another theory. By analysing mitochondrial DNA sequences of modern Native North and South Americans, these researchers believe they have found evidence which suggests a single migration to the New World, with linguistic differences developing later. Furthermore, they have extrapolated an estimated migration date of between 41,000-28,000 B.C.E., based on this genetic evidence. It is clear from these examples how complex and intricate genetic and linguistic analysis can be.

There is also controversy surrounding the method of travel used by these early migrants. Low sea levels during the last ice age may have created Beringia , a vast land bridge linking Siberia and North America. It would have been quite possible for people to simply walk across from Asia to Alaska. However, pollen analysis of the modern seabed and shorelines of Siberia and Alaska have given rise to a variety of interpretations about climatic conditions on the land bridge. Several theories which favour migration to the Americas at least in part by boat use evidence of extremely harsh conditions on the land bridge as support. Mammoth remains have been found on the now submerged land bridge, so it seems fair to assume that humans would have been able to find enough food to sustain a journey across Beringia. However, travel by foot does not necessarily mean that there was no travel by boat. Many anthropologists feel that the traditional seagoing cultures of the Arctic and the West Coast of modern-day British Columbia did not develop solely within the confines of North America, but stem from maritime technologies of these first migrants. Still other scholars prefer to consider a combination of the two forms of travel, with some seagoing and some land-based travel. As mentioned above, the sea levels rose after the last ice age, thus eliminating virtually all of the coastal evidence for early migrations.

Not all scholars feel that it is certain that humans first entered the Americas via Beringia. Some believe that it is equally possible that South America was peopled first, by seagoing peoples of south and East Asia who travelled across the Pacific to land on the West Coast. This is a highly controversial theory of early migration, and is not widely accepted amongst the archaeological and anthropological communities. Its opponents feel that humans did not possess the necessary deep-sea sailing technology at the time, while supporters have attempted to prove that even primitive vessels could cross oceans.

What caused these early migrations remains unknown. The first migrants to the Americas were big game hunters, constantly following the massive fauna of their time from place to place. It is possible that it was this movement in search of food that first led humans to North America. Equally probable is an argument based on population pressures. It has been estimated that early hunter-gatherer societies could support a population density of one person per ten square kilometres, and that in moving towards unoccupied lands early migrants were responding to demographic pressures. It is also possible that warfare and conflict may have pushed early migrants into the Americas. Unlike more modern migrations, there is no remaining testimony about the factors leading up to these migrations. Perhaps these migrants were not responding to the push factor of increasing population or warfare but to the pull factor of the unknown. Perhaps they travelled into the Americas in part simply to see what was there.

The most generally accepted theory of early migration into North and South America is perhaps necessarily vague. Most who have studied the topic accept the use of the Bering Straits land bridge, Beringia. The combination of travel by land and sea is also accepted, as there has been no compelling evidence presented which excludes one or the other. Dated archaeological sites and cautious demographic estimates as well as established genetic, linguistic, and physiological evidence have resulted in an approximate range of dates for first entry into North America. Most scholars now seem to accept somewhere between 25,000 and 15,000 years ago as an appropriate date. Although some more controversial theories are in some measure supported by evidence, this widely accepted model shall serve as our basis of understanding the earliest migrations into the Americas.

It is important to note that First Nations and American Indian groups often interpret their past differently than do the scientific community. Many believe that their ancestors were not involved in migrations from North Asia to North America at all, but rather that they evolved in the Americas independently of humans in Europe, Asia, and Africa.

Once humans were present in north-western North America they continued their migrations into uncharted territory. Either through coastal travel by seagoing cultures or through interior travel via an ice-free corridor which was open intermittently between 23,000 and 13,000 B.C.E., humans made their way south to lands free of the heavy glaciation of the ice age. Either route would have been dangerous and formidable during the ice age. The coastal route would have forced the settlers to move between ice-free pockets as the Cordilleran glacial ice would have expanded as far west as the Pacific Ocean. On the other hand, southward migration via the ice-free corridor would have involved surviving a harsh, windswept, barren valley between two massive continental glaciers. No archaeological evidence has yet been found to support either hypothesis, although glacial activity or rising sea levels could easily have destroyed it. Linguistic evidence tends to support a longer period of habitation on the coast as opposed to the interior, implying that the coastal route was used. It has also been established that people possessing a highly developed maritime culture lived on Heceta Island, off the coast of Oregon, as early as 9,000 years ago. Some scholars feel that this indicates the arrival of a partially developed maritime people several millennia before this date, which also supports the coastal travel hypothesis. The coastal route is the preferred theory for many modern scholars. They feel that seagoing cultures were sufficiently advanced to take advantage of the rapidity and ease of travel by boat, while the ice-free corridor, with its limited period of existence and extremely harsh conditions, would have been sporadically used at best.

Whether by land or by sea, humans moved south to ice-free areas of North America. Migrations continued throughout the continent so that by 12,000 years ago the Americas were populated (to varying degrees of density) from the Arctic to the tip of South America, Tierra del Fuego. Climatic changes influenced both the type and availability of food sources, which in turn influenced settlement and migration patterns throughout the Americas. As the glaciers retreated, humans returned north into the Great Plains and the boreal woodlands of present-day Canada. At the same time, the 'funnel-shape' of Mexico and Central America forced people migrating south into increasingly close contact, producing intricately developed societies. By at least 5,000 years ago, humans had migrated across the Gulf of Mexico to the Caribbean, peopling the islands of the gulf with the ancestors of the Arawak and the Carib, who met Columbus upon his arrival in the New World. In the Arctic, the Thule Inuit gradually moved eastwards, eventually crossing Smith Sound between Greenland and Ellesmere Island and migrating southwards to become the first Arctic people to have contact with European cultures, specifically the Norse Greenlanders.

Between 8,000 and 9,000 years ago the first recognisably cultivated forms of crops appeared in present-day Mexico. The development of agriculture occurred simultaneously in several regions of the world at this time, including Mesoamerica. This has been linked to several possible causes. One theory suggests that the movement into the New World and Australia meant that humans had settled all available territory. Thus, based on basic resources these early humans were able to extract from the land, Homo sapiens was unable to expand any further. The human population had peaked at its pre-agricultural high, with a density of one person per ten square kilometres throughout the world. This meant that the agricultural revolution helped support the increase of the human population, and was thus a function of demographic pressures. A second theory, which does not exclude the first, hypothesises that the agricultural revolution may have been related to a sudden and unexplained jump in carbon dioxide levels around 13,000 B.C.E. which made photosynthesis easier and more efficient for plants. This in turn increased the growth rate and size of plants, triggering domestication and the emergence of agriculture. Whatever combination of influences caused the agricultural revolution, its immediate result was a marked increase in the quality of life of early farmers, as compared to their non-agricultural predecessors. Agriculture meant that the land could support many more people in a small area. The population increased dramatically, as did the rate of population growth. Life expectancy also increased in affected cultures as agricultural technology moved from south to north.

Maize was usually not the first plant to be domesticated, although it was one of the most important and influential. In Mesoamerica it was preceded by cultivated gourds, squash, and avocados. In the north-eastern woodlands of North America agriculture was introduced with the squash, and was shortly followed by the domestication of the sunflower, a local plant. However, maize was the first crop to be domesticated in what is now southern Ontario when it reached the area around 500 C.E. Agriculture in the Americas depended heavily on the trio of maize, squash, and beans. These crops benefited the soil when sown together and also complemented one another as nutritional sources. Early farming was efficient and nutrient-rich, and an improved diet encouraged the growth of populations and with it the development of complex societies. Societies which continued to rely upon traditional hunting and gathering techniques, usually because the land they occupied was not suitable for agriculture, tended to remain loosely structured, with minimal hierarchy and smaller populations. This was because they were unable to support large, highly structured systems with an elite that did not participate in food collection. Even larger and more stationary societies in North America did not develop centralised empires, as the land could not support continued and intensive agriculture. In addition, population densities were generally not high enough to lead to the development of such societies.

In Mesoamerica, the increase in population density and permanent settlements that accompanied the development of agriculture encouraged the growth of increasingly complex and intricate cultures. The geography of the region also made much inter-cultural contact necessary. These highly stratified societies included the Olmecs , the Teotihiacans , the Toltecs, the Maya , and the Aztecs. These cultures developed highly advanced systems of agriculture, which led, over time, to impressive technological and cultural achievements. They possessed several calendrical systems, all incredibly accurate. Their societal structures were often elaborate and advanced.

Many migrations occurred in the history of Mesoamerican civilisations. The most notable of these were the movement of the Maya from southern Mesoamerica and Central America, and the later southwards push of the Aztecs towards Mexico valley, the site of present-day Mexico City. Between 600 C.E. and 900 C.E. the Maya expanded from their capital Tikal in what is now Belize north into the Yucatan Peninsula and the eastern regions of Mexico. They absorbed the latest stages of Teotihuacan culture and became one of the most significant and influential societies of Mesoamerica.

The Aztecs were also very migratory. In the thirteenth century they were the last of the groups from the north to enter the Mexico valley before the arrival of the Europeans. Their movements led them to the Valley of Mexico, where they first achieved acceptance from the Zapotecs. Later they rose to power and constructed a vast, hierarchical, and militant society led by priests and warriors. These two migrations had a great impact upon the civilisations of the areas into which they moved, advancing what they considered important and often dominating or integrating previous local systems and beliefs.

These migrations were the last major movements in North and Central America prior to the arrival of the Europeans. In them, there is evidence of development and growth, which was cut short by European disease and colonisation.