The Pleistocene Overkill Hypothesis is a theory championed by Paul S. Martin of the University of Arizona in Tucson.  A definition of "Overkill" was offered by Martin (1984) as meaning "the human destruction of native fauna either by gradual attrition over many thousands of years, or suddenly in as little as a few hundred years or less".  His hypothesis uses the fact that extinctions were most numerous and sudden on continents humans invaded and where they had not developed their hunting skills.  North America, South America, and Australia, which were invaded by humans, all experienced large extinctions, whereas in Africa and Eurasia, where humans evolved their hunting techniques, fewer extinctions occurred.  The large mammals that became extinct in North America were endemic to that continent (Pielou, 1991), whereas the mammals that are extant, namely the moose, Alces alces,  had migrated into North America from Asia along with the human colonizers (Guthrie, 1995).  This suggests that the  mammals that had migrated into North America had some advantage over the endemic mammals, which may have resulted in an increased ability to avoid the human predators (Pielou, 1991).  Thus, the endemic mammals did not coexist long enough with humans to develop an evasive mechanism to escape from hunting. This idea is analogous to the extinction of the dodo bird in the Americas.

   Evidence of climate change at the end of the Pleistocene is abundant, detailed in oxygen isotope ratios and terrestrial fossil records that show major migrations of plant and animal species, such as the expansion of the boreal forests.  However, Martin (1984) maintains that the climate change resulting from deglaciation was not unique to the late Pleistocene, but repeatedly occurred throughout the Ice Age, without triggering large megafaunal extinctions.  Because of this, climate change, which is used as the explanation for many biogeographic events, does not apply to this particular case (Martin, 1984).  Instead, the uniqueness of human migration into North America at the end of the Pleistocene, and the evidence of their hunting cultures, should be highlighted as the cause of the extinctions.

    The Overkill Hypothesis is dependent on time-transgressive extinctions from Northwest to Southeast, following the direction of human migration, as shown in the figure on the left, below.  The figure on the right, below, shows the percentage of mammalian genera over the last 3 million years that have gone extinct in North America.  Graph A shows a relatively straight line, which represents a relatively constant rate of extinctions of small mammals.  Graphs B and C show that for large mammals, there is a strong break in slope, which is indicative of a period of higher extinctions.  Graph C shows that the Wisconsin (Late Pleistocene) extinctions between 10,000 and 12,000 years BP created a relatively greater break in slope than the extinctions that occurred prior to 1.7 million years BP.
 

Figure above from Martin, 1984. Figure on the left from Matsch, 1976.

 
    Martin (1984) focuses on the diachronous nature of regional extinctions throughout the world during the Pleistocene (mainly in the last 100,000 years).  Because large extinctions did not occur at the same time on different continents, nor with the same intensity, climate change alone seems to be an unlikely cause of such largely time-transgressive extinctions.  Extinctions of Autralian megafauna occured 15-30,000 years ago, with 86.4% of the megafauna going extinct.  The large-bodied mammal extinctions occurred between 10-12,000 years ago in North and South America, with 73.3 and 79.6%, respectively, of megafauna becoming extinct.  Africa, on the other hand, which is the site of human evolution and dispersal, experienced a 14.3% extinction of megafauna within the last 100,000 years.  This sequence of extinction follows the the spread of people from Africa into Australia, Eurasia, and North and South America (Martin, 1984).