Tunguska Impact Could Explain Vanished Dinosaurs
In 1908, an asteroid or comet slammed into Earth's atmosphere, exploding 1,000 times more powerfully than the bomb dropped on Hiroshima.
The subsequent shock wave flattened trees in nearly 800 square miles of remote Siberian forest. The devastated area formed the shape of a butterfly with a 40-mile wingspan.
Two University of Colorado scientists now believe that the Tunguska impact may help them explain, among other things, why dinosaurs disappeared.
They suspect that extraterrestrial impacts like Tunguska somehow damage the genetic material of organisms that manage to survive the blast. Genetic damage, or mutations, would initially kill off many survivors but might also sow seeds of diversity that could flower later.
"I think it's a wacko idea, really"
admitted geneticist Jeff Mitton, who is probing the genes of Tunguskan pine trees for evidence of impact injury.
"It is a long shot, but if it were to work out it would be remarkable to say these comets do something that's mutagenic"
he said.
David Raup, a professor emeritus of physics and evolutionary biology at the University of Chicago, is skeptical.
"To talk about a big scale evolutionary phenomenon from this is, I think, quite a stretch"
he said.
"But if Jeff Mitton is doing it, you must take this seriously"
Five times in the history of the planet, most of the planet's creatures have disappeared.
"And we know that these extinctions are to some degree coincident with ... impacts"
Mitton said.
An asteroid or comet could kill life in several ways. It might kick up enough dust to shade the planet for months at a time, which would starve plant-like organisms that get their energy from sun.
Some researchers also believe that impacts shake the planet, stirring up volcanoes directly across the globe from the strike. Toxic fumes and dust spewed from volcanoes could also have been deadly to organisms.
Mitton and his colleagues suggest that comets and asteroids could have had a third type of effect, one impossible to find in geologic or fossil records: Maybe the extraterrestrial shocks wreaked havoc with creatures' genes, weakening or killing them.
One night last year, Mitton's colleague at the university, botanist Jane Bock, caught the beginning of a TV documentary on the Tunguskan impact, which was heard for hundreds of miles.
Reports at the time said the sky was so bright that Europeans could read their newspapers at night. That's probably because the impact sent debris so high into the atmosphere that sunlight from the other side of the planet reflected off it.
In a brief segment of the documentary, a Russian geneticist spoke of finding massive genetic mutations in plants growing in the strike zone. Bock was astonished and spoke with Mitton about the finding.
"This woman who has been documenting this for more than 10 years seems to have substantial evidence that this site has many more abnormalities than you would expect"
Mitton said.
"We wondered, then, might this be recorded in the DNA of the trees?"
Mitton said.
So they decided to check it out.
Bock had a Russian botanist friend, Nicolai Vasilyev, who offered to collect material from the site for them. Late last year, Vasilyev sent needles from Tunguskan Scots pine trees and Scots pines elsewhere in Siberia.
Mitton has already found detailed genetic information from European Scots pines. He plans to compare the genetic differences among the three populations. If the Tunguskan pines are particularly odd relative to the other two, then something probably messed with their genes. The most obvious culprit: The 1908 impact.
"I think that if Jeff can hit this, then we go to NSF (the National Science Foundation)"
for funding, Bock said.
Smiling, Mitton responded:
"We might hear about a panel that all died laughing."
In his university laboratory, Mitton carefully opened the brown paper packets of pine needles Vasilyev sent from Siberia.
He sprinkled the dried pine needles inside into a pedestal, poured in liquid nitrogen and ground the frozen needles into dust as white vapors from the nitrogen swirled into the air.
Mitton gestured toward a blue cardboard box packed with tubes, vials and an instruction sheet.
"This is a DNA extraction kit"
Mitton said.
"If you can make chocolate chip cookies, you can do molecular biology"
After he pulls the DNA, or genetic material, from the pine needles, he'll send it on to another university lab which will determine "sequences" for him. A mutation is a change to an organism's genetic sequence, Bock explained.
Though neither Bock nor Mitton can explain exactly how a giant asteroid or comet impact could cause mutations, Bock doesn't consider the idea far-fetched.
"Look, we know that cold can induce mutations, heat can induce mutations"
she said.
"If you feed too much caffeine to mice, that breaks chromosomes"
Mitton added.
Most random mutations are nasty, either weakening or killing an organism. So if impacts somehow create mutations, then that genetic damage could increase the lethality of an asteroid or comet impact, harming many of the organisms that somehow managed to make it through the blast.
There could be another, seemingly paradoxical effect of mutations.
"For those that live through it, they've gotten a pulse of something mutagenic that causes them to be more variable"
Mitton explained.
And variation can help a population through stressful times, such as those following an impact, Mitton and Bock contend.
That's because some tiny fraction of a set of random mutations are likely to actually be good for a creature _ a few genetic changes might let it survive better in cold weather, for example.
So most creatures would die off following an impact, but the highly variable few that survived could fuel a small burst of evolutionary motion afterwards, spawning new species.
This is where both Raup from the University of Chicago and Francisco Ayala, an evolutionary biologist and philosopher at the University of California at Irvine, get skeptical.
"New mutations play a very small role, in my view, in the evolutionary process"
Ayala said.
"I believe most of the variation that counts for evolution does not depend, or depends very little, on mutation rates"
In almost any given population, Ayala contends, there is already extravagant variation, which serves as grist for the evolutionary mill. Increasing mutation rates by even 100 times would add a trivial amount of variety relative to what's already there.
"The importance of the explosions and impacts concern transformation of the environment, massive changes in environmental conditions"
Ayala said.
Raup says he'd be less skeptical about Mitton and Bock's idea if they could explain how comets could be mutagenic.
Mitton and Bock admit that's a problem. For the moment, they're deferring to Colorado School of Mines geologist Keenan Lee, who has studied impacts and visited Tunguska in 1998.
Lee says geologists are still arguing about whether it was a comet or unusual asteroid that slammed into Earth's atmosphere above Siberia in 1908. But whatever it was, it came roughly from the east, probably struck the atmosphere at a low angle and exploded with an energy of roughly 15 megatons.
"When it popped up there 6 or 7 kilometers (3.6-4.2 miles) up, it released a great deal of electromagnetic radiation, probably gamma radiation, too"
Lee said.
"Certainly some was visible because people saw it glow and a lot of it was infrared because it set fire to the forest below it instantaneously"
Lee said he "would certainly think" some of that radiation could harm an organism's genes.
Bock sheepishly called the research project "borderline sci-fi."
But if Bock and Mitton are right, evolutionary biologists are going to have to reassess how extraterrestrial impacts may have affected the evolution of life on Earth.
