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HUMANS TO BLAME FOR ICE AGE EXTINCTIONS, STUDY SAYS
By Hillary Mayell
National Geographic News
August 10, 2005

http://news.nationalgeographic.com/news/2005/08/0810_050810_iceage.html

Humans are likely responsible for the extinction of Ice Age megafauna --
large mammals like giant sloths, short-faced bears, mammoths, and
saber-toothed cats -- that occurred in the Americas around 11,000 years ago,
a new study says.

Scientists have long debated whether giant pre-historic mammals disappeared
because of climate change or because humans hunted them to extinction.

The mass extinctions coincided with both the end of the last Ice Age and the
arrival of humans in the Americas around 11,000 years ago. This timing has
made it difficult for scientists to isolate the cause of the species'
disappearance.

But a study comparing the extinction of giant ground sloths in North and
South America with the disappearance of their smaller relatives in West
Indian islands has helped clear up the picture, scientists say.

The researchers say archaeological and fossil evidence strongly suggests
that ancient hunters pushed the animals to extinction.

Giant ground sloths "cruised through" at least 22 major climate cycles as
the continental ice sheets in North America advanced and retreated over the
last two million years, said David Steadman, a paleobiologist at the
University of Florida.

Steadman is a co-author of the new study published in the Proceedings of the
National Academy of Sciences.

"The only thing that's different [at the end of the Ice Age] is the arrival
of people," he said.

Giant Sloth: A Case Study

Until about 11,000 years ago, at least 19 different sloth species lived in
North and South America in a variety of ecosystems. Only a few small,
tree-dwelling sloth species survive today.

Steadman and his colleagues argue that if ecosystem shifts resulting from
climate change caused the sloths' demise, then all extinctions -- on both
islands and the mainland -- should have taken place at the same time, as the
last Ice Age ended between 15,000 to 9,000 years ago.

Radiocarbon dates of bones, dung, and other tissue of extinct sloths place
their last appearance in North America at around 11,000 years ago and at
about 10,500 years ago in South America, Steadman says.

But on the islands of Cuba and Hispaniola -- shared today by modern-day
Haiti and the Dominican Republic -- sloths survived until about 4,400 years
ago.

Their subsequent extinction coincided exactly with the arrival of the first
humans on the islands, Steadman says.

"What [this study] shows us is that there's this great big suggestive
pattern that we find: Wherever human beings first appear all around the
world, these large mammals pretty quickly become extinct," said Gary Haynes,
an archaeologist at the University of Nevada at Reno, who was not involved
in the study.

"[Some] people will say that you have to [establish the cause of extinction]
species by species, and I think they're probably right," Haynes added. "But
the study does create a good model that might make us think that if it
worked for one big animal it's probably what we'll find for other big
animals."

Overkill Vs. Climate Change

Steadman and his colleagues argue that megafauna species on the American
continents, having evolved in an environment without humans, may have been
particularly vulnerable to the sudden appearance of big game hunters.

The 5,000-pound (2,300-kilogram) giant ground sloth is a case in point. In
addition to having no fear of humans, it was the size of a modern-day
elephant, it couldn't hide, and as it name implies, it moved very slowly.

"Walking up to a ground sloth and trying to spear it to death probably
wasn't one of the most macho things they [early hunters] did," Steadman
said. "Any hunter could outrun one."

But other scientists maintain that climate change was the driving force in
Ice Age extinctions. They argue that the retreat of ice sheets from North
America caused a major change in habitat that the giant mammals couldn't
adapt to.

At the peak of the Ice Age around 20,000 years ago, the ice covered much of
North America.

As the sheets melted between 18,000 and 8,000 years ago, warmer temperatures
and shifting rainfall patterns forced plants and animals to move out of old
habitats and into new ones.

Proponents of the climate-change theory add that there's little evidence
that humans hunted anything other than mammoths. Yet species like wild
horses, camels, and saber-toothed cats all went extinct at about the same
time.

"There are no archaeological sites for species other than mammoths, and
perhaps mastodons, where you find a spear sticking out of an animal, and
everyone agrees that there is evidence of human hunting," Nevada's Haynes
said.

"So the lack of kill sites doesn't bother me," he added. "There's a real
lack of a 'smoking gun' implicating either climate change or human hunting,
but that's true for every theory."

Climate change may have been a factor in pushing the animals to extinction,
Steadman says, but it took humans to push them over the edge.

"Animals like the ground sloth, which had a poor ability to regulate body
temperature, should have thrived in a warmer climate," he said.

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