The ten most disturbing scientific discoveries JUN 17 2010
Number three on this Smithsonian Magazine list is "There have been mass extinctions in the past, and we're probably in one now."
Today, according to many biologists, we're in the midst of a sixth great extinction. Mastodons may have been some of the earliest victims. As humans moved from continent to continent, large animals that had thrived for millions of years began to disappear-mastodons in North America, giant kangaroos in Australia, dwarf elephants in Europe. Whatever the cause of this early wave of extinctions, humans are driving modern extinctions by hunting, destroying habitat, introducing invasive species and inadvertently spreading diseases.