These late Pleistocene migrations coincided with the end of the earth's most recent period of glaciations (Ice Ages). The accumulation of vast amounts of ice in the glaciers resulted in punctuated drops in the sea level of up to 100m allowing easier access to the previously uninhabited continents.
When the Homo Sapiens left Africa they are believed to have migrated east toward India and then south east along the coast of Asia until they reached Australia between 45,000 to 42,000 years ago (O'Connella & Allen 2004). At that time, due to the much lower sea level, most of martime Southeast Asia formed one land mass - known as the lost continent of Sunda. Following the coastal route southeast they would have reached the channel between Sunda and Sahul (present day Australia and New Guinea). This channel, between the Sahul and Sunda (known as the Wallace Line) must have been traversed by the technologically more advanced Homo Sapiens whereas the earlier Homo Erectus never traversed it (O'Connella & Allen 2004). It is presumed that they used rafts of some sort to traverse the channel.
The close chronological coincidence of the arrival of humans capable of hunting megafauna and the Pleistocene mass extinction around 40,000 years ago in the Sahul lends support to the hypothesis that humans were in some way a causal factor. These megafauna, having evolved in the absence of human predation are thought to have been particularly vulnerable to the arrival of humans in the form of Homo Sapiens. The Megafauna had few other predators and because they evolved largely without significant predators.
Models of migration to the New World are more divided. The reliable evidence currently available suggests a north western migration of people who crossed from Siberia into Alaska. At the time, due to lower sea levels, the lost continent of Beringia connected North America to Eurasia. Whilst the bulk of evidence points towards findings related to the Clovis culture (circa up to 13,500 years ago), there are also the controversial findings in Monte Verde, Chile where human presence as early as 33,000 years ago is indicated - this earlier date has not yet been widely accepted by the academic community. The mass extinction of megafauna around the time of the Clovis findings suggests that, as earlier in Australia, that these people played a role in the extinction of the megafauna. The Clovis were Homo Sapiens thought to be capable of hunting large prey.
The "Overkill Hypothesis" was proposed in 1969 by Paul S. Martin and was primarily concerned with the New world evidence of megafauna extinction. While the extinction of the megafauna had earlier been explained by the "Hunting Hypothesis", Martin's new "Overkill Hypothesis" sought to explain this phenomenon over relatively short period of time. The extinction of 80% of the North American megafauna over 1000 years shortly after the arrival of humans is the core of the hypothesis. The Clovis culture was thought to be big game hunters who used their stone fluted projectiles for hunting.
While the hypothesis enjoys much support, the evidence to support it is somewhat tenuous. What has thrown doubt on the hypothesis has been linked to the actual processes which would have made the