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*5. The Pleistocene Extinctions* The extinction of about 200 genera,
most of which were megafauna (animals which weigh more than 44 kg),
characterises the late Pleistocene (Martin & Klein, 1984). These
extinctions took place in many different geographical locations, but
not with equal severity and not at the same time. North America lost
thirty-three genera of megafauna including two species of Mammoth
(/Mammuthus jeffersoni/ and /M/. /primigenius/), which represented the
extinction of the order Proboscidea from North America (Lundelius,
1990). These extinctions eradicated nearly half of the large mammals
in North America (Alroy, 2001). There are two causation theories for
this recent wave of extinctions. Some scientists believe that it was
due to climate change. There is no doubt that the climate was
changing. For example the end of an ice age actually marks the end of
the late Pleistocene just 11,000 years ago. However, many ice ages
came and went during the Pleistocene without causing mass extinctions
and there is no evidence to suggest that the last one was any
different from the others. There appears to be no particular reason
why large-scale extinctions should occur just as the glaciers retreat
for the last time. In addition, this extinction event was uniquely
restricted to terrestrial vertebrates; marine fauna was completely
unaffected. This evidence suggests that a more subtle force than
global climate change was at play, a force which caused a selective
extinction event by reducing the numbers of certain groups of animals
(mainly megafauna) at different times around the planet (Figure 5).
The other causation theory centres upon the timing of the arrival of a
new and efficient predator into these ecosystems. The species in
question originated in Africa and spread quickly through Eurasia to
Australia and the Americas. Its migration around the globe exactly
coincided with the Pleistocene extinctions. This species is a social
animal capable of efficient co-operative hunting and tool use, and is
called /Homo sapiens/; wise man- some would say a misnomer. Martin
(1967) termed the process Pleistocene overkill. It was he who noticed
that the magafaunal extinctions in North America coincided with the
first successful colonisation of North American by early humans, who
came across the exposed Bering land bridge from Asia (Lewin, 1987 and
Martin, 1973). These accomplished hunters, called Clovis people,
spread south through the continent reaching Mexico in just 350 years
(Martin, 1967). The North American animals had evolved in the absence
of humans and therefore may not have recognised them as potential
predators. It appears that the Clovis people made good use of this
easy food source and hunted many of them to extinction. caveman text
box Pleistocene extinctions Incredibly, Australia completely lost its
entire fauna weighing more than 100 kg. Six out of the seven megafauna
genera also became extinct in Australia, not only land mammals such as
giant kangaroos and wombats but also lizards and large flightless
birds (Roberts /et al./, 2001). It is difficult to accurately date the
Pleistocene extinctions in Australia, but there is growing evidence
that they happened shortly after the colonization Aboriginals,
probably around 55,000 years ago (Roberts /et al./, 2001). In this way
the evidence is mounting that human influence has been severely
deleterious to global diversity for quite some time, and that the sum
total of species loss may be considerable. Despite this evidence,
there are still those who choose not to accept this contentious
concept. However, as we look to more recent events in human history
during the Holocene Epoch (12,000 years ago to present), the evidence
for human induced extinctions on a massive scale becomes unequivocal.
Other Phanerozoic Extinctions <4.4OtherPhanerozoicExtinctions.htm>
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