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E. F. Legner

Professor Emeritus, University of California



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       The magnitude of the dynamic forces that began in the Western Hemisphere about 10,000 B.C. is truly astounding.  Prior to that period many portions of the Americas resembled some of the African scene of today, but with even more species present.  There were large herds of herbivores and correspondingly smaller numbers of carnivores.  Pleistocene animals in great abundance were camels, guanacos, horses, mammoths, mammoths, mastodons, bovines, ground sloths, saber-tooth cats, tigers, lions, etc.  The recent construction of a reservoir in western Riverside County, California turned up great quantities of the remains of the herbivorous species, and provided good evidence of their large population size. However, by 9,000 B.C. most of them had disappeared!  Only the guanacos and tree sloths remained in South America, and the bison in North America.  There were few herds of bison west of the Rocky Mountains.


       Three hypotheses attempt to explain this mass extinction.  (1) The first hypothesis assumes that Amerindians slaughtered them all:  an imperceptive suggestion!  An argument against this notion is that in Southern California, Amerindians did not inhabit the region earlier than about 7,000 B.C.  In other words, people were present there no earlier than 2,000 years AFTER the animals had disappeared!  Also when Europeans first ventured en masse to America in the 15th Century, the natives were found to live amidst vast herds of bison and other herbivores, upon which they depended for food.  Similarly, the Pleistocene mammals became extinct inland from the 16,000 B.C. Monte Verde site in Chile, although humans had rarely ventured far inland from the coast by 10,000 B.C.  Nevertheless, Jared Diamond seems to support human involvement in the extinction vicariously by suggesting that it did not occur in Africa because of a longer period of co evolution with humans in the sub Saharan region.   (2)  A second hypothesis points out that the ice age had just ended abruptly and climates changed to a warmer, drier cycle by 9,000 B.C., but this does not account for the vastness of the extinction (from both North & South America).  At the same time, climates became warmer and drier in the Eastern Hemisphere (Europe-Africa-Asia), but horses and camels, both of which originated in the Americas, diversified and flourished.  (3)  A third hypothesis proposes, without evidence, that some devastating widespread disease decimated the animals, something uncharacteristic of pathogens and unprecedented in all of the world’s history.  Some of the extinctions in America seem obviously associated with the reduction in plant cover with a drying of the landscape.  An example is that of the saber-tooth cat that is believed to have stalked its prey from the cover of tall grass and brush.  When such cover was reduced in the drier climate this hulking and heavy animal was less efficient in surprising and capturing its primarily ungulate prey.  There is evidence from the La Brea tar pits in California that many of the saber-tooth cats had sustained great physical injuries resulting from difficulties in capturing their prey. The prey, such as bison and horses, also diminished in size and gained fleetness at the same time.  It has also been suggested that dietary limitations on horses caused them to become extinct under the drier conditions.  However, when horses were reintroduced during the European colonization, they flourished in large wild herds, which have remained abundant to the 21st Century, even under the increased pressure of human settlement. 


       Dr. Dee Simpson, recently deceased, believed that Homo erectus could have made it to the Americas (personal communication), and she suspected that the Calico, California dig site was possibly a H. erectus camp.  Could there have been large numbers of a separate race of humanlike people present in Pleistocene America that stemmed from Homo erectus stock, which then eliminated itself as well as the animals?  Well, that argument is weak also as there is no archeological evidence for such a large population around the time that the animals’ extinction occurred (10,000-9,000 B.C.).  Finally, it is noteworthy that Plato mentions the disappearance of a great civilization on the earth around 9,000 B.C.!


       As of 2009, available evidence to explain the mass extinction has pointed to the crash of a massive asteroid in northeastern Canada, which gave rise to widespread dust clouds and changes in Atlantic Ocean currents.  This in turn led to excessive droughts and other ecological disruptions that greatly restricted the available food supply for many large mammals.  It also led to emigrations of humans from the Eastern portions of North America to points west and southwest.