Past climate change - Climate Change: Biological and Human Aspects - page 109

During this time North America was geographically separated from South America
(there was no connecting isthmus). Consequently the smaller early horses could not
migrate south towards the equator. If they had been able to then the modern horses
may have evolved allopatrically (in a population in a different geographic area),
with the more ancient forms surviving today, somewhat in the way the mouse deer
(tragulids) did in the Old World tropics.
Of course, and perhaps of special interest to readers, as noted above, the precursors
to the Hominoidea began diversifying 14-9 mya and this continued with the evolution
of the australopithecines that lived 5-1.2 mya.
From the end of the Miocene, and throughout the Pliocene (2-5 mya), the Earth
cooled. This trend culminated in our current (Quaternary) ice age, consisting of a
series of glacials interspersed by warmer interglacials such as the one we presently
experience. This will be examined next.
3.4 Summary
A key feature of the biosphere's evolution (including its abiotic components of geo-
logy and atmosphere) up to the current (Quaternary) ice age has been the interrelation
of biology and climate. Key evolutionary steps in biological evolution - such as the
developments of photosynthesis, multicellular organisms, vascular plants and new
photosynthetic processes - resulted in changes in both the atmosphere and climate:
the Snowball Earths I and II. A number of these evolutionary steps have had con-
sequences for the re-organisation of elemental cycling, especially carbon, its various
reservoirs and in relations between them. Remember, life on Earth is carbon-based.
So it should not be too great an intellectual leap to appreciate that the evolution of
life will affect the global carbon cycle. So it is perhaps a little surprising that the
notion of evolving life's key evolutionary stages - the development of photosynthesis
and then the cell differentiation of eukaryotes and hence rise of multicelled animals
(metazoans) and plants - caused the Snowball Earths only began to be discussed
seriously at the beginning of the 21st century.
Changes in the carbon cycle, be they major as in Snowball Earths or lesser as in
CIEs of the Phanerozoic eon (past 542 million years), implicitly affected atmospheric
carbon (both carbon dioxide and methane) and therefore changes in the greenhouse
forcing of climate, hence climate itself. Taking all these factors together provided
further evolutionary opportunities that included those that (much later) allowed for
the rise of H . sapiens .
In addition, the biosphere's evolution has been punctuated by a number of extinc-
tion events imposed independently of biological evolution but which have affected
evolution (or ended it). These events have principally been vulcanism-related or aster-
oidal. Of the former some have been associated with (or at least helped to trigger)
carbon releases and both have had global climate effects. However, even asteroidal
biological extinctions, although not directly related to the biosphere's evolution per se,
are necessary components of a cosmology that enables Earth-like planets in general,