The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 122

the find was not published until 2009 with a clutch of papers in the journal Science .
These included one on its evolutionary significance by Tim D. White, Berhane Asfaw,
Yonas Beyene, Yohannes Haile-Selassie and colleagues. The relevance of this dis-
covery to the biological and human aspects of climate change is that the precursors
to humans evolved in a world that had not been so cool for scores of millions of
years. Indeed, as we shall see (section 4.6.4), modern humans themselves evolved
into the even cooler Quaternary ice age (2 mya-present) of glacials and interglacials.
Although Milankovitch orbital variations were not triggering the very cold glacials
of the Quaternary, they would still have been taking place in the Pliocene (the Earth
has been wobbling about the Sun since its formation). While, in the Pliocene, Mil-
ankovitch factors were not triggering glacials and interglacials, there would still have
been other climate manifestations such as changes in circulation patterns, and hence
regional rainfall and so forth (these have been discerned in geological strata). Indeed,
as there was a small but significant amount of terrestrial ice on the planet at this
time, these Milankovitch factors would not have been insignificant and changes in,
for example, Pliocene monsoon strength have been discerned 1 . The notion arising
from this is that these Milankovitch-paced climatic changes resulted in local/regional
environmental change, and hence evolutionary pressure, the manifestation of one
aspect of which was the evolutionary line that led to modern humans ( Homo sapiens
sapiens 2 ; hereafter often referred to as just H. sapiens ).
There is debate as to the exact nature of the evolution of human ancestors but
considerable evidence again points to an African origin for hablines ( Homo habilis ).
Indeed, in 2011 a largely US team headed by Brenna M. Henn and Christopher R.
Gignoux presented genetic evidence for a South African origin for modern humans.
(Older than the hablines, earlier sivapithecine remains have been found in Turkey
and India from 14-8 mya; sivapithecines are more closely related to orang-utans
than chimpanzees, gorillas and humans.) The exact relationship (and as mentioned
there is little doubt that at least in part there is one) between climate and human
evolution has yet to be clearly elucidated. This is because terrestrial biogeological
and geological records of East African environmental change are rare, geographically
dispersed and incomplete. Atlantic Ocean and (perhaps more importantly) Indian
Ocean sediment records have been used to reconstruct climatic changes in the region.
In 2005 Martin Trauth and colleagues reconstructed the East African climate 0.5-3
mya from 10 lake-basin records in the area. They identified three possible hominid
periods: 2.7-2.5, 1.9-1.7 and 1.1-0.9 mya. They note that these could well have had
an important impact on the speciation and dispersal of mammals and hominids at that
time.
At about 2.75 mya a sharp change in this global cooling took place, in all likeli-
hood due to the interaction of climate-influencing factors that prompted new ocean-
and/or atmospheric-circulation patterns, and not a single threshold event. Notably,

1

The manifestation of Milankovitch variations in geological strata over the past quarter of a million years
was, in 1998, the subject of an excellent Royal Society discussion meeting, the proceedings of which
appeared in a special edition of the Philosophical Transactions of the Royal Society Series B (Shackleton
et al. 1999).

2

Our species is (supposedly) so wise that they named it twice.