Planetary collisions, volcanic eruptions and sudden releases of poisonous gases are the kinds of phenomena typically blamed for the previous mass extinctions on Earth. This time, however, the cause may be humankind.

Rapid population growth, global warming and environmental degradation have ushered in the sixth mass extinction in the Earth's geological history, scientists say.

Though less grave than the mass extinctions caused by planet collision and global volcanic eruption, mankind is "directing" a mass extinction more serious than the ones triggered by natural climate changes, said Shu Degan, a noted paleontologist with the Northwest China University.

About 170 paleontologists from 21 countries expressed their worries about the reducing bio-diversity at the Fourth International Symposium on Cambrian System. The five-day symposium ended Tuesday in Nanjing, capital of east China's Jiangsu Province.

They believe the industrial revolution is thought to have initiated a species disappearing rate alarmingly similar to the five mass extinctions in the Earth's geological history.

The five mass extinctions in geological history were related to drastic changes in nature. The largest extinction occurred about 250 million years ago, eliminating about 95 percent of life. The most well-known extinction, of dinosaurs, occurred approximately 65 million years ago.

About 15,000 species are disappearing, more rapidly than any time in history, according to the 2004 global species survey issued by the World Conservation Union (ICUN). Figures published in Nature, a leading scientific journal, indicate that about 1 million land organisms will disappear in half a century.

"We have limited knowledge about the life composition on the Earth, but habitats are disappearing rapidly and mankind is uprooting life at an unprecedented speed," said Chia-Wei Li, a paleontologist with the Taiwan-based Tsinghua University.

Human activity is causing the present mass extinction to occur more rapidly than those that had been triggered by natural factor. The extinction rate averaged to one species extinct at an hour. The latest rare species suspected to have turned extinct included the Hawaiian thrush, a kind of bird, and picea montigena, a kind of plant.

Though human beings has realized the importance of protecting biological diversity, they still in general pursue their specific interests at the cost of other species, making the extinction of other life inevitable, Li said.

For example, people spray disinfectants when threatened by epidemics such as Dengue or bird flu, drastically accelerating the extinction of some species in a short period of time, Li said.

The rapidly growing population will inevitably have disastrous results, and though humankind may survive, other species will not, said Palmer Allison, director of the Institute for Cambrian Studies in the United States.

The scientists agreed that the present mass extinction may break the biological links between organisms, inevitably affecting humankind, and that people should learn to protect the environment and coexist harmoniously with the other life.

(Xinhau News Agency August 24, 2005)