Last week, we looked at prehistory's five great extinction events and suggested that extraterrestrial sources may be the binding threads in our planet's violent tapestry of death. We briefly outlined extinction events 1 through 4. This week, we conclude with extinctions 4 and 5. Are there any cosmic smoking guns here?
•Extinction 4-The Triassic-Jurassic extinction event occurred 205 million years ago. At the T-J boundary, nearly 48 percent of all zoological and botanical genera-terrestrial and marine life-went extinct.
Most archosaurs-except the lucky dinosaurs- and most therapsids, which include mammals and their immediate evolutionary ancestors, died out. Even many large amphibians became extinct. The result opened up the environment to the surviving dinosaurs. (Of course, the dinosaurs own turn to face the Darwinian firing squad will come at extinction 5.)
Only a few large amphibians emerged from extinction 4 and managed to survive, barely, into the Cretaceous. For example, the giant, lumbering amphibian known as Koolasuchus-one of the paleo stars of the 1999 BBC-TV mini-series "Walking with Dinosaurs"-became the lone survivor of the now utterly vanished biological order Temnospondyli.
As with earlier extinctions, an extraterrestrial agent may have had a hand in the T-J megadeaths, but we're not 100 percent certain.
Could the giant, 62-mile-wide ring-like Manicouagan Reservoir feature in central Quebec be the smoking gun? Maybe, but it's a big maybe.
The now highly eroded, water-filled impact basin probably was excavated by a rocky asteroid three miles in diameter. The scale of the Manicouagan impact should have produced planetwide fires and dust clouds with impact debris raining down for thousands of square miles. Yet, many researchers now believe the crater was formed 12 million years before extinction 4. Ok, if Manicougan isn't the culprit, then why didn't planetwide extinctions occur at the time of that impact? More geological investigations are needed.
•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.