Wednesday, May 02, 2007

Would extraterrestrial life use DNA?


An interesting article in the April 30th issue of the Boston Globe, "Not so alien after all?", profiles the Search for Extraterrestrial Genomes (SETG) project, lead by Harvard geneticist Gary Ruvkun and MIT geophysicist Maria Zuber. They have assembled a team that includes a number of prominent scientists as co-investigators, including Harvard geneticist George Church and cell biology professor emeritus Walter Gilbert, MIT physicist Claude Canizares, Associate Director of the MIT Kavli Institute for Astrophysics and Space Research (MKI) William F. Mayer, and Michael Finney, biotechnology investor and former Chief Scientific Officer of MJ Research.

The goal of SETG is to search for life on Mars that is ancestrally related to life on Earth. The team is developing a device that uses the polymerase chain reaction (PCR) to search for Martian microbes that share DNA sequences with terrestrial microbes. Billions of years ago there was a heavy exchange of meteors between the bodies in our solar system. If Earth bacteria ended up on one of those meteors, survived the rigors of space, the entry of the meteor into the Martian atmosphere and were able to reproduce, the descendants of those microbes should have retained at least some of the DNA sequences of their Earthly ancestors. It's not as far-fetched as it sounds - the fossilized remains of archaebacteria, which live and thrive under extreme conditions here on Earth, have been discovered in 3.8 billion year old sediments. As to whether microbes could survive the trip, just consider the bacteria that hitched a ride on the Surveyor 3 probe that were able to survive the heat of the launch, the freezing vacuum of space, and three years on the Moon with no water or nutrients before being carried back to Earth by Apollo 12.

As the Globe article explains, Ruvkun not only believes that ancient life could have traveled from Earth to Mars, but speculates that the source of life on Earth was also extraterrestrial.
About 4 billion years ago the planets experienced a period of intense bombardment. Meteors came crashing down to the surface, ejecting more rocks into space, some of which came crashing down onto other planets. Life on Earth appeared very quickly after the bombardment -- too fast, Ruvkun believes, to have evolved on its own.The bombardment, Ruvkun believes, could have brought life to Earth from somewhere else. And if life on earth came from somewhere else, then perhaps it also came to Mars from "the same somewhere else," Ruvkun says. Martian rocks have been found on earth, and an analysis of one of them revealed that portions of its core never experienced superheating as it fell to earth, showing that meteors could be viable shuttles for life.
Finding Martian life with identifiable DNA sequences would be an extremely long shot, even more so than the more generic tests used by the Viking Mars landers, which may or may not have found signs of life. Even if the team succeeds in building a device that can operate reliably under the extreme conditions on the Martian surface, there is no guarantee that NASA will approve it's inclusion on a flight. Ruvkun is optimistic, however, that if the experiment goes ahead there will be something for it to find.
"Never bet against life," he said.
Tags:, , (story via the Knight Science Journalism Tracker)

2 comments:

Anonymous said...

Seems like there should be a peer-reviewed reference on the survival of the bacteria on the moon, if true. I assume Mars-to-Earth movement is easier than the other way around, because Mars gravity is less. I'm not sure I would use limited cargo space for this experiment, as opposed to a more generic search for life.
Ford Denison
This Week in Evolution

Peggy said...

Personally, I think the cargo space should be used for experiments that maximize the likelihood of actually seeing a result. Unless they can make the PCR machine very small and extremely reliable, it probably isn't worth the extra weight and space. It would be very cool if such an experiment found life, though.

As for more info on bacterial survival on the moon the original technical publication is Mitchell, F.J. and WL Ellis "Surveyor 3 Bacterium isolated from lunar retrieved television camera" In its Analysis of Surveyor 3 Mater. and Phot. Returned by Apollo 12 p 239-248 (1972). Not available online, but you can order it.

A good review article that cites peer reviewed publications on the survival of microorganisms in space is Nicholson WL et al. "Resistance of Bacillus Endospores to Extreme Terrestrial and Extraterrestrial Environments", Microbiol Mol Biol Rev 64(3): 548-572 (2000). There are also a number of papers that show that bacterial spores can survive under Martian conditions.