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There are significant obstacles to explaining the mass disappearance
of organisms at the end of the last "Ice Age".  One is that the
extinctions do not coincide well with the timing of major
environmental changes.   The graph below shows large mammal
extinctions (just for North America), yet the peak does not coincide
closely with the last glacial maximum, the Bering land bridge, the
arrival of humans, or development of societies.  

A second problem (not obvious in the graph below) is the selectivity
of extinctions; only large carnivorous and scavenging birds suffered
similar losses--few plants, smaller mammals, reptiles, fish, insects,
or smaller birds disappeared during the late Pleistocene extinctions.

[27]North America Pleistocene Mammal Extinctions 

adapted from [28]Piliou, E. C. (1991)  and others      [29][BACK]
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A third challenge to explanation is a geographic discrepancy between
Pleistocene extinction waves at separate locations.  The timing map
below indicates that apparent peaks of disappearance for large animals
differed between continents, which implies regional, biome, or
realm-centered rather than global triggers.

[30]World Pleistocene Extinction Timings 

[31][BACK]
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And as if any explanation is not elusive enough, there is further the
discrepancy between the origin of the survivors versus that of the
lost.  Among North American Pleistocene large mammals, the listing
below suggests an enormous inequity in likelihood for becoming
extinct, and high incidence appears strongly associated with North
American origin (or presence exceeding one million years).  Few Old
World origin newcomers" disappeared; many New World large mammals
did.  The selectivity is suggestive of differential resistance to some
pathogen, and there is at least some parallel to the catastrophic
declines of Native Americans following the arrival of Europeans during
the 15th Century. 

[32]Origin Extinction Disparity 

compiled from [33]Kurten, B. (1988) and others

For the present, the unsatisfying conclusion is that we do not truly
know--and we may never know--what caused the selective and
uncoordinated late Pleistocene extinction wave.  If there ever is an
"answer", it likely shall invoke some synergistic effect of several
(if not all) of the factors that various researchers have
proposed.