Mass extinction on Earth ㅡ a mystery that remains unresolved
The sixth mass extinction has begun on Earth, and the main culprit for the extinction is humans, a recent study said.
When a mass extinction happens, 75 percent of species or more disappear. Since about 540,000,000 years ago when animals appeared on Earth, there have been mass extinctions five times.
The last mass extinction occurred 65,000,000 years ago, caused by a comet which crashed in the Yucatan Peninsula, Mexico.
As a result, dinosaurs, which dominated the planet, disappeared.
It is interesting that following the fifth mass extinction in the Stone Age when homo sapiens, the predecessor of modern human beings, began to arise on Earth.
In the Ice Age about 10,000-50,000 years ago, the number of large animals in Asia and Europe decreased by 36 percent, while that of those living in North America dwindled by 72 percent.
Although it was not a mass extinction, mammoths and woolly rhinoceroses disappeared.
Scientists differed over the cause of population change. Some said that mammoth became extinct because of hunting by homo sapiens who scattered around the world.
In a study published in the U.S. PNAS science magazine in 2009, Australian researchers said that kangaroos, measuring 2 meters in height, became extinct as a result of hunting by humans who settled in Australia.
Other researchers, on the other hand, insisted that kangaroos disappeared as they failed to adapt to climate change, saying that global temperatures rose by five to eight degrees in the late Ice Age.
This is because the number of mammoths and wild horses decreased ahead of the arrival of humans in North America, said researchers at the University of Alaska in the U.S.
However, a joint research team, including scientists from Denmark, the U.S., China and Russia, said that the mass extinction in the Ice Age was not caused by only one reason -- hunting by humans or global warming. They published the result of their research in Nature early this month.
Some species became extinct due to one of the two reasons, whereas other species were affected by the two, the multi-national study group said.