•Extinction 5-the Cretaceous-Tertiary extinction event, occurred 70 to 65 million years ago and is the most famous extinction-it ushered in the death of the beloved dinosaurs.
"About 17 percent of all families, 50 percent of all genera and 75 percent of species went extinct," according to author Michael Benton. "It ended the reign of dinosaurs and opened the way for mammals and birds to become the dominant land vertebrates. In the seas it reduced the percentage of sessile animals to about 33 percent."
Benton notes that the K-T extinction ("K" is from the German word Kreidezeit or Cretaceous; "T" means Tertiary) was uneven-"Some groups of organisms became extinct, some suffered heavy losses and some appear to have been only minimally affected."
Most space scientists agree that the K-T extinctions were caused by a massive asteroid impact (like the Chicxulub, Mexico, impact), while many geologists believe volcanic activity-such as at the supervolcanic Deccan traps in India-is to blame. Both an impact and supervolcanic event would reduce solar energy falling to Earth and slow down photosynthesis. A few paleontologists even suggest that the extinction was far more gradual due to a drop in sea level or a cooling down of the climate.
But a cosmic agent-an asteroid or comet impact-looks more and more likely at the K-T boundary. In fact, on March 4 of this year, a panel of 41 international scientists agreed that the Chicxulub impact caused this mass extinction.
We'll end with this warning and call-to-action by astrophysicist Neil deGrasse Tyson, director of New York's Hayden Planetarium:
"If humans one day become extinct from a catastrophic collision, there would be no greater tragedy in the history of life in the universe. Not because we lacked the brain power to protect ourselves but because we lacked the foresight. The dominant species that replaces us in post-apocalyptic Earth just might wonder, as they gaze upon our mounted skeletons in their natural history museums, why large headed Homo sapiens fared no better than the proverbially peabrained dinosaurs."
Lou Varricchio, M.Sc., is a former science writer at the NASA Ames Research Center.