Past climate change - Climate Change: Biological and Human Aspects - page 108

include the C 4 species of maize, wheat and rice and so today are of central importance
to agrarian and technological humans. (Remember, C 4 plants form carbohydrates
through photosynthesis using a different metabolic carbon pathway from C 3 plants.)
Grasses, particularly C 4 grasses, have several properties marking them as different
from C 3 dicotyledons. (Dicotyledons have two cotyledons - leaves emerging from
the seed - as opposed to the one produced by grass monocotyledons.) First, grasses
usually have a lower nitrogen content, which makes them intrinsically less nutritious.
(Note: as food plants humans use the seed part of monocotyledons.) Second, C 4
and C 3 grasses rarely have secondary toxins that conversely are common to many
C 3 dicotyledons. Third, protein in C 4 plants is protected by a bundle of sheath
cells making them harder to digest (hence the agricultural pre-processing of such
crops). And finally, grasses have a higher silica content than dicotyledons. This
makes them more abrasive and this abrasiveness during mastication provides the link
with hypsodonty (teeth with high crowns good for grazing) that enabled mammals to
take advantage of the new food source.
Herbivorous animals' food can be determined one way through 13 C analysis. C 3
plants (including some grasses) have 13 C values of 26‰ whereas C 4 plants (which are
mostly grasses) have 13 C values of 12‰; consequently fossil teeth provide a record
of the proportion of C 4 and C 3 plants in herbivore diets and palaeosol (fossilised
soil) carbonates. However, there is a problem: C 3 (monocotyledonous) grasses have
the same 13 C content as C 4 dicotyledonous plants. This means that 13 C analysis only
reveals the expansion in C 4 grasses. Conversely, most C 4 plants are grasses and the C 4
grasses are the dominant grasses at low latitudes (around the tropics). C 4 plants can
have a carbon dioxide photosynthetic advantage over their C 3 counterparts. Carbon
dioxide (on an annual basis) is effectively the same the planet over and so at low
latitudes C 4 plants are better placed to make use of the higher-intensity insolation
(the solar radiation received at the Earth's surface) and temperature. C 4 grasses are
therefore the dominant grasses in the tropics whereas C 3 grasses are found at higher
latitudes (or altitudes, if in the tropics).
The 13 C record of North American equids shows an abrupt increase starting about
7 mya, although Pacific coast and Canadian equids' 13 C values suggest that this
increase occurred later. A similar change was noted at the end of the Miocene
(approximately 7-5 mya) in Pakistan, Kenya and South America.
Just as ferns did in the Cretaceous, over the Tertiary period (65.5-1.8 mya) mam-
mals diversified at the end of the Miocene. Back in the Mesozoic era (the age of the
dinosaurs) the largest mammal was the size of a modern house cat. In the Tertiary
this was to change. Here again, equids are illustrative of general mammal diversi-
fication. The early horses (such as Eohippus) were small and well adapted to the
global tropical-like forests of the Tertiary period's early Eocene epoch (55.8-33.9
mya). Their later diversification into increasingly larger forms reflects their being
more suited to a cooler climate (increased size has a lower surface-area-to-volume
ratio, and hence reduced heat loss) and to eating nutritionally poor grasses (which
necessitates gastrointestinal specialisation that takes up room inside the body cavity).
And, as noted, these larger forms were more suited to grassland plains than the smaller
forms which were more suited to moving among the tangle of forests. (Once again,
note the interplay of factors.)