The Origin and Evolution of Mammals - Moodle

agreement about exactly when within that time
frame (Brook and Bowman 2002), nor whether it
occurred rapidly or spread over as much as 10,000
years.
The arguments in favour of human overkill as the
cause of the megafaunal extinctions are twofold
(Martin 1984; Alroy 1999a). The first is the claim
that the timing of the extinction in each region coincides
with the timing of the arrival of human
populations. This applies particularly well to North
America, where the Clovis people evidently
crossed the Bering land-bridge from Asia, migrated
down an ice-free corridor along the western side of
Alaska and Canada, and entered the southern part
of the continent around 12–10,000 years ago. Details
of human entry into South America are less clear,
but probably occurred as a continuation of the
southerly migration through North America. In
northern Eurasia the evidence is more equivocal.
Neanderthals persisted there until about 30,000
years ago, at which time they were replaced by
modern humans (Stuart 1999). This was as much as
20,000 years before many of the megafaunal extinctions
actually occurred. The time of arrival of
humans into Australia is disputed and estimates
vary from as long as 70,000 to as recently as 40,000
years ago (Brook and Bowman 2002). Thus, while
the possibility of a close overlap between human
arrival and the Australian extinction certainly exists
(Miller et al. 1999), it cannot be claimed to be substantiated
by the evidence. More limited support
for the overkill hypothesis is also provided by the
much more accurately measured correlation
between human appearance and mammal extinctions
on various islands, notably Madagascar
where most of the extinction of giant lemurs, hippopotami,
the mysterious Plesiorycteropus, and also
the elephant bird Aepyornis occurred soon after the
arrival of humans about 1,500 years ago (Dewar
1984; Burney 1999). The reason offered by overkill
enthusiasts for the low level of extinction of large
mammals in Africa is attributed paradoxically to
the fact that humans had been there for so long that
they and the mammals had co-evolved behavioural
strategies, allowing them to coexist.
The second kind of argument for the overkill
hypothesis is the nature of the extinction itself. The
end-Pleistocene was a time of net increase in habit-
LIVING AND FOSSIL PLACENTALS 289
able area, as the ice retreated, and so a diversification
rather than an extinction phase would be expected if
climate change alone were affecting the fauna.
Furthermore, none of the numerous earlier episodes
of mammalian extinction had ever had such a
marked discriminatory effect against large body
size. Human hunting activity, however, would be
expected to focus more on larger mammals because
of the greater ease of finding and killing them, and
because of the greater resource each one represents.
The large mammals that did survive also supports
the hypothesis. They occupy habitats that are particularly
hostile to humans, such as the musk ox and
caribou of the tundra, and mountain dwelling
bovids, (Alroy 1999a), or else inaccessible such as the
tropical forests of southern Asia or arboreal and noctumal
species (Johnson 2002).
The arguments in favour of climatic change as
the cause of the extinction are first that a climatic
change of the same order of magnitude as those
that had caused earlier severe extinctions did
indeed occur (Webb 1985a), and second that small
populations of humans possessing very limited
hunting technologies could not plausibly have had
such a large effect. A number of authors have proposed
more specific theories of how the climate
change might have resulted in the end-Pleistocene
extinction, although the picture is so complex that it
is most unlikely a single, climatic factor could ever
be identified as the cause (Graham 1997). Guthrie
(1984) argued that the end-Pleistocene saw a continuation
of the increasing seasonality that had
been developing. The non-growing cold season in
the temperate latitudes, and the non-growing dry
season in the tropics, both increased in length. This,
he proposed, caused reduced plant diversity and
increased plant zonation, resulting in the change
from highly mosaic habitats to much more homogeneous
ones dominated by large, continuous areas
of, respectively, grassland, broad-leafed forest, and
coniferous forest, as is seen today. Large herbivores
would be the most directly affected by such a
shift, followed inevitably by their large carnivore
predators. Graham and Lundelius (1984) pointed
to evidence of the breaking up of co-adapted biotas
at this time, which they claim would have led
to a variety of possible kinds of environmental
stresses on individual species, such as the need for