Past climate change - Climate Change: Biological and Human Aspects - page 107

have been found in Turkey, India and Pakistan. They are thought to have survived
up to about 8 mya and are evolutionary cousins (as opposed to grandparents) of
H . sapiens .
There is other more tangential relevance of the drivers behind the earlier Cretaceous
diversity of ferns to the Miocene expansion of grasses. We have more recent evidence
that other factors were at work than the decline in atmospheric carbon dioxide, with
which C 4 plants are better able to cope than C 3 flora. Isotopic studies of microfossils
from the end of the last glacial 13 000-21 000 years ago from two Central-American
lake beds show that C 4 plants began to flourish around one in Guatemala but not
the other, in Northern Mexico, even though the atmospheric carbon dioxide would
have been the same for both. The reason was a climatic change at the end of the
last glacial and local environmental conditions pertaining to each lake. Consequently,
while earlier in the Miocene low carbon dioxide may have facilitated C 4 grass expan-
sion, other factors such as regional climate were critical in determining where this
happened. Where the conditions were right, C 4 species flourished (Huang et al.,
2001). This is exactly analogous to the Cretaceous fern diversification (in niches
created by the rise of angiosperms). In short, major evolutionary drivers are rarely
the result of single factors and a holistic approach to understanding the development
of ecosystems is required.
We will return to this theme of a holistic approach in later chapters when looking
at the biological consequences of future climate change, which is not just dependent
on the climate change itself but also the nature and distribution of ecosystems in the
landscape.
Other evolutionary developments associated with the rise of grasses and grasslands
is that equids (the Equidae, the family of horses, asses and zebras) and other mammals
developed hypsodont (high-crowned) teeth in the early Miocene (beginning 25 mya)
in North America. This perspective is supported in that fossilised grasses and grass
pollen appear in abundance in the Miocene and become increasingly common to the
present. Grasses are far tougher to eat (see below) compared to the berries and leaves
of tropical forests and woodlands that equids used to inhabit. However, since the late
1990s it has been noted that the increase in hypsodonty in mammals occurred in
the Oligocene (beginning 35 mya) in South America. It is now thought that grasses
were significantly present a little earlier in South America and maybe also in Africa.
Unfortunately for palaeontologists, grasses are most abundant in oxidising and well-
drained environments that do not encourage fossilisation. So, field evidence is literally
thin on the ground.
The Miocene increase in open grasslands in favour of closed forests meant that
equids faced new pressures. In forests, being short helped movement through the
undergrowth, where manoeuvrability was the key to escaping from predators. Con-
versely, in open grasslands it was speed that was essential. Those individuals with the
longest feet and the ability to use their toes to give them a little extra speed tended
to survive while slower individuals did not. Some equids became taller and evolved
into (hooved) ungulates.
Grasses belong to the family Poaceae (formerly known as the Graminae) of the
order Poales (which in turn is sometimes referred to as the Glumiflorae), in the
monocotyledon class (Monocotyledonae) of angiosperms (flowering plants). They