The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 121

S. oppositifolia the species originated in Beringia, part of which was the now sub-
merged land bridge between present-day Siberia and Alaska. It then spread around
the pole to Greenland, Iceland, the northern British Isles, Scandinavia, Siberia and
Canada, and then relied on some 14 refugia when times changed and were harsh
(during glacials). Refugia are ecologically important in climate change terms, as we
shall see later.
It is worth noting now that in terms of polar ecology Beringia was of hemispheric
significance, and we will mention it again later. Beringia, when an exposed land area,
was covered with vegetation even in the coolest of times (including more recently
in the depth of the last glacial some 24 000 years ago, as then the land-bridge area
was substantive due to the sea level being 100 m lower than today). The Pliocene
Beringia vegetation supported populations of large mammals including mammoths,
horses and bison. This Pliocene vegetation was unlike that found in modern Arctic
tundra. Both the Pliocene and modern vegetation communities can sustain a relatively
low population of mammals, but the Pliocene Beringian community was a productive
dry grassland ecosystem that resembled modern sub-Arctic steppe (dry and treeless)
communities. It is likely that Beringia consisted ecologically of a mosaic patchwork
with wet tundra in poorly drained lowlands and marked ecological differences between
the central and its eastern-most regions (Zazula et al., 2003). This added to the habitat
diversity: it was of an appropriate quality for an ecological refugium, enabling it to
provide habitats for a range of species.
For millions of years up to somewhere between 4.8 and 7.4 mya Beringia was
important in terms of both global climate and ecology. Ecologically it formed a
bridge between the North American and East Asian landmasses (enabling animals
to cross between the two) and, in terms of marine ecology and hemispheric climate,
it was a barrier between the Pacific and Arctic oceans (preventing the exchange
of marine life). The tectonic separation of North America and East Asia, between
4.8 and 7.4 mya, split Beringia and allowed water - at times of high sea level - to
flow between the Arctic and the Pacific while hindering terrestrial fauna and flora
travelling between the two continents. This dating of this significant opening was
estimated by the presence of a formerly Arctic and North Atlantic bivalve mollusc,
Ascarte (Marincovich and Gladenkov, 1999). The separation facilitated the high-
latitude re-organisation of ocean circulation including the way heat entered and left
the north polar region. It was yet another factor forcing global climate. As we shall
see, this land bridge continued to form and close as the sea level rose and fell with
the major glacial cycles throughout the Quaternary to the present. One consequence
of this sea-level change was that Beringia's role as a refuge was insecure. Because it
did not flood uniformly in the warm interglacials of the past million years, its highest
hills tended to survive as islands, and do so even today.
Between 5.3 and 2.7 mya the Earth gradually cooled due to the aforesaid combin-
ation of factors of tectonic movement and changes in air and ocean circulation. It
was around this time (4.4 mya) that an early ancestor to humans lived, Ardipithecus
ramidus in Africa. It is an evolutionary connection between later small-brained, small-
canined, upright-walking Australopithecus and the earlier last common ancestor that
we shared with chimpanzees and the bonobo some time before 6 mya. Although fossil
remains of A . ramidus were found in 1992, research providing important details about