What is the Overkill Hypothesis?
The overkill hypothesis argues that humans were responsible for the Late Pleistocene extinction of megafauna in northern Eurasia and North and South America. Paul Martin of the University of Arizona and others see a chronological and causal link between the appearance of humans and the disappearance of many species of large mammals.
According to the overkill hypothesis, when ancestors of native Americans entered North America about 14,000 calendar years ago, they encountered a large number of species that had no experience with humans. As a result, they did not recognize humans as a threat. The ancestral Indians (or Paleo-Indians, as they are sometimes called) were able to take advantage of this fact and were able to hunt the large mammals with great ease. The Paleo-Indians became specialist big game hunters concentrating on game like mammoths, giant bison, ground sloths, and other species of large size. They hunted dozens of species to the point of extinction, and indirectly caused the extinction of many smaller species as a consequence of ecological disruption.
Because the evidence is mounting that the North American extinctions occurred very rapidly--perhaps in less than 1,000 years--Martin has dubbed the hypothesized overkill event as a "Blitzkrieg" or "lightning war." (Blitzkrieg refers to a style of warfare practiced by the Germans in World War II, in which areas were attacked or invaded with great speed so that the enemy had little or no time in which to react.) With the availability of so much meat human populations were able to increase rapidly and spread southward. Within 1,000 years, according to the available records, humans had migrated to the tip of South America. Actually, humans may have arrived in South America much earlier by following a coastal route. It is not necessary to infer that human only moved southwards.