To date, much controversy surrounds the expansion of H. sapiens out of Africa. When did humans spread to the different parts of the globe? How did they get there? Why did they come? In this post, the “why” will be addressed in simple terms.
This snippet from Coming to America (1988) seems applicable.
Lisa: So why did you come here?
Akeem: To find something special.
Lisa: It’s a long way to travel.
Akeem: No journey is too great when one finds what he seeks.
In other words, humans came to the New World to find a better life free from the pressures being placed upon them in their current environments, whatever they may be. The most likely candidates for expansion are climate change negatively impacting resident groups and population size approaching the carrying capacity of the then current area of habitation. Both would result in increased competition for resources, which would inevitably lead to stress and conflict. The need for food and raw materials as well as the need to escape potentially violent competition likely drove groups to move eastward in search of greater opportunity.
However, this may be oversimplifying the situation. A Post-Processual interpretation is likely to deliver some unique insight regarding the complexity of human behavior. For example, one should not discount the curiosity inherent in our species. Imagine a migrant group of hunter-gatherers or a lone hunting party wandering through the wilderness into new areas, not because they have to, but because they want to know what lies on the other side of that hill or copse of trees. The discovery of fecund, virgin lands, by this line of reasoning, could conceivably lead to the migration of groups not under resource pressures.
In reality, a combination of these factors as well as some, which were possibly overlooked, contributed to human migration throughout the world. However, it will likely be some time before technological advances in genetics and, hopefully, watershed discoveries in archaeology, climatology, etc. allow for a more fine grained understanding of prehistoric human migration patterns.