1. Studies: Human Hunting Led to Extinctions
For more than a century, scientists have
debated what killed off the big animals in Australia and the
Americas. Two new studies place the blame on squarely on ancient
human hunters equipped with fire, spears and an appetite for meat.
The studies, appearing today in the journal Science, conclude
that after early humans migrated into Australia and the Americas,
the heavyweight animals of these new continents were driven to
extinction within a few thousand years.
In the Americas, 73 percent of the large plant-eaters, along
with the saber-toothed cat, were gone within 1,200 years after
humans migrated to the continents about 13,600 years ago. Wiped out
were animals like mammoths, camels, mastodons, large ground sloths
and the glyptodont, a strange armored creature the size of a small
car and weighing more than 1,400 pounds.
In Australia, researchers precisely dated bone specimens of
elephant-sized marsupials, giant snakes, huge lizards and other
extinct animals. They found that the wildlife disappeared within
10,000 years after humans arrived at the down-under continent.
Longstanding Question
The research contributes powerful new evidence to a century-old
debate among scientists intrigued by the question: What killed off
the big animals in newly settled continents of the world?
2. Some have long blamed humans, but other experts
say it could
have been climate change, disease or a gradual
change in habitat.
The two new studies pin the blame firmly on
humans.
"Human population growth and hunting almost invariably leads to
major mass extinctions," said John Alroy of the University of
California, Santa Barbara, author of the study of the American
extinctions.
"The results show how much havoc our species can cause, without
anyone at the time having the slightest idea of what is going on,
much less any intention of causing harm," Alroy said.
Linda Ayliffe of the University of Utah in Salt Lake City said
precise dating of rocks and fossils from 27 sites in Australia and
West Papua New Guinea clearly show that large animals there
disappeared around 46,000 years ago, or about 10,000 years or so
after the arrival of humans.
The rapid demise during that time of 55 species -- every land
animal, reptile and bird in Australia weighing more than 220 pounds
-- is strong evidence for human involvement in the
extinctions, said Ayliffe.
"It is clear that the downward spiral of these animals was
after the arrival of humans," she said.
Ayliffe said the dating is significant because some researchers
3. have blamed the extinctions on extended droughts that occurred
later. But she noted that the animals had withstood climate changes
previously; so it is unlikely they all would have succumbed to
natural forces. Also, disease is improbable since so many different
species of reptiles, birds and mammals disappeared at about the
same time. Diseases are unlikely to affect all species the same
way.
Largest Known Bird
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