Development of the Indigenous Tribes
At minimum, the intermittent travels by Eurasian groups to the Americas reinforces the idea of history as a continuum, with groups subdividing, migrating and re-subdividing when conditions warranted. This process brought the largest numbers of migrants to the Americas some 20-30,000 years ago by way of the land bridge then present between the Americas and Asia. By 15,000 B.C. various groups had penetrated deep into both the northern and southern hemispheres. Their small numbers and the large land area available to support a nomadic existence forestalled a settled existence and development of a hierarchical socio-political structure until almost 1,000 B.C. Amazingly, as late as the sixteenth century, when the Aztecs of Mexico and the Incas of Peru were overwhelmed by Spanish conquistadors (assisted by indigenous enemies of these powerful empires), neither group apparently had any knowledge of the other's existence. Distance, a mountainous terrain and dense forests certainly contributed to the isolation of these empire-builders of the southern hemisphere.
Hunter-gatherer groups had discovered the grassy uplands of the Peruvian region as early as 15,000 B.C. Although mountainous, they also found abundant sources of water and a wide variety of game animals. The Pacific coast of Peru, on the other hand, receives little rainfall; inland, there is desert. As a consequence, those who settled this part of Peru did so to harvest the sea rather than the soil, and permanent settlements appeared only around 2,500 B.C.
Civilization was advancing in the Americas on the Eurasian model but at a considerably slower pace. Whether or not one accepts the evidence of Atlantic migrations by the Vikings and earlier Eurasian groups, what is certain is that the dynamics of an increasing population and changing environment stimulated the indigenous tribes of the Americas establishment of hierarchical socio-political structures in the same way this occurred in more distant times for their Eurasian ancestors.
Certain technological discoveries by tribes in the Americas remained to be discovered hundreds or even thousands of years into the future, had they not been introduced by the invading Europeans of the sixteenth century. This was the case even down to the domestication of animals as sources of labor and food. With the exception of the dog, which was domesticated by tribes in the northwestern part of the northern hemisphere around 8,400 B.C., only the Peruvian tribes relied on domesticated work animals; these included the guinea pig (6,000 B.C.) and the llama (3,500 B.C.). Interestingly, the evidence strongly suggests that the knowledge gained by these early Peruvian settlers spread outward to other, less advanced groups. The Aztec, who came to dominate what is now Mexico, apparently acquired at least some of their agricultural practices from the Peruvians. Time and the experience of periodic natural disasters resulted in a re-isolation of these two great civilizations.