- The first humans to arrive in Australia destroyed the
pristine landscape, probably by lighting huge fires, the latest research
suggests.
-
- The evidence, published in Science magazine, comes from
ancient eggshells.
-
- These show birds changed their diets drastically when
humans came on the scene, switching from grass to the type of plants that
thrive on scrubland.
-
- The study supports others that have blamed humans for
mass extinctions across the world 10-50,000 years ago.
-
- Many scientists believe the causes are actually more
complex and relate to climate changes during that period, but, according
to Dr Marilyn Fogel, of the Carnegie Institution in Washington, US, chemical
clues gleaned from the eggshells suggest otherwise.
-
- "Humans are the major suspect," she said. "However,
we don't think that over-hunting or new diseases are to blame for the extinctions,
because our research sees the ecological transition at the base of the
food chain.
-
- "Bands of people set large-scale fires for a variety
of reasons including hunting, clearing and signalling other bands.
-
- "Based on the evidence, human-induced change in
the vegetation is the best fit to explain what happened at that critical
juncture."
-
- Carbon clues
-
- Dr Fogel's team, based in the US and Australia, examined
hundreds of fragments of fossilised eggshells found at several sites in
Australia's interior dating back over 140,000 years.
-
- They looked at the indigenous emu and the Genyornis ,
a flightless bird the size of an ostrich that is now extinct.
-
- The type of carbon preserved in eggshells gives a picture
of the food the birds ate.
-
- Before 50,000 years ago, emus pecked at nutritious grasses.
But after humans arrived, about 45,000 years ago, they switched to a diet
of trees and scrubs. Genyornis , however, failed to adapt and died out.
-
- "The opportunistic feeders adapted and the picky
eaters went extinct," said Professor Gifford Miller, of the University
of Colorado at Boulder, US.
-
- "The most parsimonious explanation is these birds
were responding to an unprecedented change in the vegetation over the continent
during that time period."
-
- The data sheds light on the contentious issue of what
led to the extinction of 85% of Australia's large mammals, birds and reptiles,
after about 50,000 years ago, when human settlers arrived by sea from Indonesia.
-
- Climate change theory
-
- Mass extinctions on other continents also coincide with
the arrival of modern humans, suggesting the two events are linked.
-
- In North America, for example, the disappearance of the
likes of mammoths and ground sloths is coincident with the arrival on the
landmass of new stone-spear technologies carried by humans about 12,000
years ago.
-
- But at the time the first settlers reached Australia
there is no evidence of significant fluctuations in climate - they came
later.
-
- However, Clive Trueman of the University of Portsmouth,
UK, disagrees.
-
- He says some large mammals survived long after the sudden
changes in vegetation identified by Dr Fogel's team.
-
- "While there may be a connection between the arrival
of humans and changes in vegetation, as demonstrated by carbon isotopes,
sudden changes cannot be largely responsible for megafaunal extinctions
as the beasts survived for at least 15,000 more years," he told the
BBC News website.
-
- "It is likely that extinctions were not caused by
any single event, but reflect compounding factors such as natural climate
changes associated with the Ice Age fluctuations and, quite possibly, the
arrival of humans."
-
- © BBC MMV
-
- http://news.bbc.co.uk/1/hi/sci/tech/4660691.stm
|
