Geoscience Reference
In-Depth Information
return to food security when addressing current and future food-security concerns
in Chapter 7.)
The Holocene also saw human migration across the Pacific Ocean. The nature of
this migration has been hotly debated. However, as new tools for genetic analysis
become available it has been possible to obtain a clearer picture. mtDNA analysis
indicates that the ancestors of the Polynesians originated from somewhere in eastern
Indonesia and then migrated east and south, through and around Australasia approx-
imately 11 000 years ago, before moving on to Polynesia, reaching Fiji 3000 years
ago and then on to Samoa and Tonga before 2500 years ago. There is a genetic
cohesiveness among the Polynesians, from Hawaii in the north to New Zealand in
the south and including Melanesia, the peoples of which Captain Cook thought were
different (Gibbons, 2001).
This wave of migration was subsequent to the earlier colonising 'express' route
down to Australia by 55 000 and 46 000 years. The Polynesians clearly had to be
accustomed to substantial sea voyages even if they did as much island-hopping as
possible. However, the earlier coastal express route would have benefited from the
lower sea levels, and the resulting land bridges, in the latter third of the last glacial,
which would have greatly reduced the need for extensive sea voyages.
The arrival of humans in Australia marked the onset of a megafaunal extinction
event. There has been much debate as to whether this was due to climate change or
the presence of humans (although the coincidental timing makes the latter difficult to
ignore). It is true that there were climate fluctuations during the last glacial. However,
there were similar fluctuations earlier in the glacial and indeed previous glacial and
interglacial climatic cycles. It is therefore unlikely that climate alone was the cause
of this extinction.
Some 60 Australian taxa are known to have become extinct, including all large
browsers, whereas large grazing forms, such as the red ( Macropus rufus ) and great
grey ( Macropus giganteus ) kangaroos, survived. The selective loss of browser-
dependent species is suggestive of environmental change but, because of the afore-
mentioned argument, climate change alone cannot account for this. So what may have
happened?
As discussed in the previous chapter when looking at the late-Miocene expansion
of C 4 grasses, herbivorous animals' food can be determined through
13 C analysis.
C 3 plants (including some grasses) have 13 C values of
26‰ while C 4 plants (which
are mostly grasses) have 13 C values of
12‰; consequently, things like fossil teeth
or eggshells provide a record of the proportion of C 4 and C 3 plants in past herbivore
diets. Research by Gifford Miller and colleagues (2005) looked at the proportion
of 13 C in the eggshells of the now-extinct flightless bird Genyornis newtoni and
the Australian emu Dromaius novaehollandiae to provide a record covering the
past 140 000 years in South Australia's semi-arid zone. They found that a marked
shift in the proportion of 13 C took place 50 000-45 000 years ago. They also tested
the 13 C record against other glacial-interglacial times (in 15 000-year groupings)
to see if climate change was affecting the 13 C record. It was not. The suggestion
therefore is that the human newcomers were changing the environment through fire.
The drought-adapted mosaic of trees, shrubs and grasslands became more like the
modern fire-adapted desert scrub. Animals that could adapt survived; those that could
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