The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 178

sees the greatest rapid climatic change. However, there are other times during glacials
where sudden, albeit smaller, climatic change takes place and extinctions (especially
locally) can result.
One key question is why was the end of the last glacial associated with a particu-
larly major extinction event compared to those in other glacial cycles? One answer,
mentioned previously, concerns humans. As noted (see Chapter 3 and also section
4.3) the late Cenozoic spurred hominid evolution. Indeed, the repeated series of gla-
cials and interglacials would favour hominids that were generalists and could adapt
to different environments, and especially those that could artificially modify their
own personal environment, such as through using fire. Although there is evidence of
an earlier minor human presence in North America before 13 000 years ago (which
we will discuss shortly) - there was the hunting of megafauna in the south west
of the present USA (perhaps as early as 18 000 years ago) - the first major incur-
sion of humans towards the glacial's end 13 000 years ago had crossed the Beringia
land bridge from Asia into North America. To establish themselves in that continent
they probably kept close to the coast and away from the Laurentide ice sheet that
still dominated inland North America. Prominent among these were people belong-
ing to what is termed the Clovis culture. The Clovis were big-game hunters, which
is typical of nomadic people as opposed to agrarian cultures that necessitate fixed
abodes.
Note that there are also clear signs of a pre-Clovis human presence. Among the
pre-Clovis signs are those at Monte Verde in Chile that date from around 12 500
years ago. Further, human footprints have been found fossilised in volcanic ash in the
Valsequillo Basin, Mexico, which have been dated to before 40 000 years ago. So,
while the Clovis migration was an important event, they were clearly not the first on
that continent.
The timing of the Clovis migration itself into North America is confined by the
melting of the Laurentide ice cap and around the time of the Younger Dryas (section
4.6.2). The only likely terrestrial route they could have taken would have been a
narrow gap between the ice sheet and the glaciers of the coastal Pacific mountains
in the Yukon. From carbon dating we know that this passage was not available
much before 13 000 years ago as the Laurentide ice sheet would have been too
big. Conversely, the problem with considering the pre-Clovis cultures' route on to the
continent is how did they overcome the glacial obstacles? One answer may be because
the sea was lower and they could have skirted along the coast. The coastal termination
of glaciers would have made this difficult but, especially with primitive boats, not
impossible. Unfortunately, evidence for such a passage is now under water (Marshall,
2001).
Earlier, in the Old World, humans had moved through Africa and into Eurasia.
There are signs of tool making going back as far as 125 000 years and earlier, back to
the interglacial before the last glacial. In short, humans had the means and opportunity
to provide added pressure on species already stressed by climate change.
Yet this situation is at first perplexing. If humans were in Africa well before the
American continents, why has the New World seen a greater extinction of large
animals compared to Africa? (see Table 4.1). The answer is likely to be co-evolution.