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Of all species that have existed on Earth, 99.9 percent are now extinct. Many of them perished in five cataclysmic events.

According to a recent poll, seven out of ten biologists think we are currently in the throes of a sixth mass extinction. Some say it could wipe out as many as 90 percent of all species living today. Yet other scientists dispute such dire projections.

As our panelists debate the issue, they also consider how one species -- Homo sapiens -- may be triggering a modern mass extinction.

are we in the midst of a mass extinction
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Peter WardPeter Ward is professor of geological sciences at the University of Washington in Seattle, where he is also adjunct professor of zoology and of astronomy. He is the author of nine books on biodiversity and the fossil record, and is the principal investigator for the University of Washington's portion of the NASA Astrobiology Institute.

The current mass extinction has been unfolding for millennia, and unlike the greenhouse effect, global warming, or the hole in the ozone, it is visible without sophisticated imagery or complex computer modeling. It is real, and it is happening to a greater or lesser degree all over the globe; it is most apparent, however, in the tropics. It will not eliminate life from the Earth; no mass extinction does that. But enough species will die that the nature of life on the Earth will be forever changed.

Many scientists dispute whether an extinction is currently taking place at all, or suggest that we are facing the prospect but have not yet begun the experience. Others agree that we are indeed in a period of increased extinction, but that the net result will little change the Earth's flora and fauna. I do not share such a sanguine view. I believe that the current extinction is well under way, having started with the dawn of the Ice Age, about 2.5 million years ago, and since then accelerating in its rate of species destruction. In some ways it is very much like the dinosaur-killing event of 65 million years ago, when a biosphere already stressed by rapid changes in climate and sea level was knocked into mass extinction by the impact of asteroids, striking, according to new evidence, simultaneously in North and Central America. A very similar scenario is currently unfolding. Over 2 million years ago, giant glaciers began to cover large portions of the planet, changing climate and sea level on a global scale in the process. And then, 100,000 years ago, another great asteroid hit Earth, this time in Africa. That asteroid is named Homo sapiens.
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