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What if life on Earth did not begin on Earth?


http://www.stnews.com

Science & Theology News (06/01/2006)
by William Orem

The hypothesis that life on Earth began elsewhere received support from an unlikely source: the Columbia tragedy

Judeo-Christian tradition has long maintained that the dizzying variety of life forms found on our planet are the result of a special moment of Genesis. From this initial premise, theologians, philosophers and now modern scientists have branched out, arguing for a single act of creation on a young Earth, an ongoing process of molecular evolution begun some four billion years ago with replicating nucleic acids and a multitude of intermediate positions.

What has not been at contention in the majority of these debates has been the premise itself: that life on Earth began on Earth.

This is precisely a point that needs to be considered, say contemporary advocates of a hypothesis known as “panspermia.” In its broadest iteration, panspermia is the proposal that life exists throughout the cosmos. As simple speculation, it has a pedigree dating as far back as 500 B.C. with the Greek philosopher Anaxagoras and makes connections with such luminaries as Giordano Bruno, the ex-Dominican astronomer put to death by the Inquisition partly for suggesting that the sky may be full of populated worlds.

One need not posit highly evolved forms, however, to subscribe to the panspermia hypothesis. Indeed, scientists who take panspermia seriously today are more concerned with a humbler expression of life: bacteria.

“As a group, microorganisms are probably the hardiest of any life forms on Earth,” said Robert McLean, a biologist at Texas State University-San Marcos, who has collaborated with NASA to have microbiology experiments performed on the space shuttle. He said his sense of the fledgling science of astrobiology is that “the vast majority of it is going toward microorganisms.”

McLean is interested in biofilm formation. In particular he wanted to know how planktonic and surface-adhering bacterial populations would interact in microgravity conditions. This information has implications for the future of space travel, in which the filtration of water will be critical. In a paper published in the January edition of Icarus, the International Journal of Solar System Studies, McLean describes how he selected three strains — C. violaceum, P. aeruginoa and E.coli — to ride on the space shuttle Columbia in 2003.

“I think there were 70 to 80 experiments total on the flight,” McLean said. “The Columbia was slanted more toward scientific work. That was its primary responsibility.”

On the morning of Feb. 1, 2003,  McLean received the news that broke the nation’s heart. Columbia had disintegrated on re-entry, its left wing seal damaged by the now-infamous dislodged foam. All hands, with their complement of scientific data, were lost.

Exogenesis hypothesis

Under the general rubric of panspermia lies exogenesis, a more modest, and perhaps more testable, hypothesis. In its simplest form, exogenesis is the proposition that life did not begin on Earth but elsewhere in the stellar region.

One likely candidate would be neighboring Mars, where oceans are now believed to have existed as recently as several million years ago. At some point, basic, self-replicating organisms were then transferred to this planet, explaining the surprisingly small window between when the geological record dates Earth’s formation and when the Precambrian fossil record dates the first appearance of life.

The mechanism by which that transfer of microbes could take place is meteorites. A collision of a large enough meteorite impactor with a planetary surface sends up clouds of debris. Retained by the gravitational field, the ejecta may rain back down or stay aloft to form rings or moons. At escape velocity, however, nonvaporized chunks sail away, bound for empty space. In rare instances, they may eventually come within the gravitational fields of other planets or moons. Perhaps, as exogenesis suggests, such newly born asteroids could be bearing travelers.

A couple of days after the Columbia disaster, McLean got a call.

“One of the people who had an experiment [on Columbia] happened to get on The New York Times Web site and saw a picture of some shuttle debris that looked like our payload,” he said. A long-shot idea struck him. Why not test the apparatus for bacteria?

“There was some liquid that survived in the payload. I pulled that out and tried to culture stuff from that and also flushed various sample cells with sterile medium,” McLean said. All three of the bacterial strains he had sent aloft had been wiped out by Columbia’s re-entry, during which temperatures peaked at more than 175° C. But after a week or so he noticed something peculiar in the incubator.

“In one of the cells that I flushed I eventually found a very slow-growing organism,” he said.

The unexpected survivor was a new bacterial strain known as Microbispora. While not common, it is by no means extraterrestrial. Microbispora is found in the Earth’s soil.

“The scientific weakness of this is we don’t absolutely know where it came from,” McLean said. “But I did do controls when I collected the samples to make sure I wasn’t introducing anything on the site. Those were totally clean. None of the solutions I took down to Florida were contaminated at all, so I was not introducing anything.”

The remaining option, which McLean calls his “best guess,” is that the Microbispora infiltration occurred before liftoff. If so, the Columbia crash inadvertently demonstrated the feasibility of a claim that exogenesis critics and panspermia detractors in general have found implausible. It showed that some bacteria can survive the extreme stresses that would be involved in meteorite entry and impact.

Macroscopic survival

McLean was not the only researcher to be surprised. NASA astrobiologist Catharine Conley works at NASA’s Ames Research Center in Moffett Field, Calif. She had sent up a live population of nematodes with Columbia. Nematodes are a species of worm, each roughly the size of the period at the end of this sentence. Conley was using them to conduct a study on bone density disturbances that occur in microgravity, again with a view toward reducing the risk associated with long-term space assignments.

When Columbia broke apart, Conley’s six canisters of worms were scattered in midair and subjected individually to the re-entry burn and impact force. Five canisters were eventually recovered. All five still had live populations inside.

“Certainly the recovery of vital organisms has completely changed my opinion of what we might expect to find in the solar system,” Conley said. She said her thinking about panspermia has been affected by her finding, and the thesis now seems more compelling.

“There has been life on Earth for three or three-and-a-half billion years,” Conley said. “There have certainly been a large number of big things that have hit the Earth and blasted rocks off the Earth in that three-plus billion years.”

“The more I think about it now, it seems the more likely that there have been so many rocks with live organisms [in them] when they left the Earth floating around the solar system that it would be surprising to me if there weren’t living organisms in places that were hospitable to life,” she said.

Indeed, a massive computer modeling study just completed at the University of British Columbia and presented in March at the Lunar and Planetary Science Conference in Texas has made it apparent just how far-ranging these travels may be. In it, a research team simulated a massive Earth strike on the order of the impactor that is believed to have formed the Chicxulub crater 65 million years ago. Their model tracked the debris paths of millions of ejected fragments and found that potentially life-sustaining sites as far a way as Europa and Titan (moons of Jupiter and Saturn, respectively) were eventually hit.

Even some organisms as large as nematodes would be able to survive such an abrupt ejection into space, Conley said.

“The acceleration forces to get to escape velocity for a meteorite turn out to be not that high,” Conley said. “If it’s a big impactor, it will take a long time to penetrate. It’s more the mass transfer that’s actually going to be forcing the smaller rocks off the planet.” And nematodes routinely survive experimental force conditions of 2,000 Gs or higher. The hard part, she said, is surviving space — the university model predicted a transit time of millions of years for the Jovian moons, well beyond the conceivable survival range of even frozen nematodes — and surviving the new environment when it arrives. In that respect, she said microorganisms are more plausible candidates.

“What you really want is something that can make its own food,” Conley said. “In that case, you’re looking for something like a cyanobacteria that make food from light, or one of these hydrogen sulfide metabolizers that’s able to get food from rocks. Or you want something like a lichen, that can both make food from light and also fix nitrogen.”

Such an organism might be able to survive the long haul between worlds and set up shop in a variety of locations. “If I were looking for life on another planet, I would go looking for a lichen,” Conley said.

Natural carriers

Max Wallis, Honorary Research Fellow at the Cardiff Centre for Astrobiology in Wales, has a different mechanism for long-distance transport in mind. That mechanism is a comet.

“Asteroids are pretty dead,” he said. He added, however, that microorganisms are already known to survive in the extreme conditions of the poles. “Comets have the water and carbon, and it’s very accessible. Certainly as an environment for living organisms they’re very much like the Antarctic.”

In an article published in Nature in 1980, Wallis showed that, despite the popular image of comets as dirty snowballs, some may house liquid cores. Six years later, as a member of the European team that sent the Giotto probe to study Halley’s comet, Wallis recognized that the structured surface and gas jets revealed a more complex comet structure than had previously been assumed. He said he suspects some versions of that complexity could operate like natural traveling containers. And because of the gravitational perturbations of Jupiter, comets in the Kuiper belt are, on occasion, swung out of the solar system entirely.

“I think comets can form an environment in their interior in which elementary life can replicate, survive and travel in a sheltered environment to another stellar system,” Wallis said.

The “bombardment periods” of planetary formation would still play a critical role. In this model, a good-sized impactor of the type that were 100 times more frequent in the young solar system would send a population of microorganisms into space, where some would eventually be collected by passing comets. The collection process would be simply mechanical, akin to the way your hair collects airborne particles as you walk through a room full of smokers.

“It’s basically only the elementary life form — the DNA — but that’s the difficulty in starting life,” Wallis said. “Once you’re seeded with life going by in a comet, it would proceed just the way life has evolved on the Earth.”

Wallis said that the natural tendency of comets to begin outgassing in the proximity of stars may serve to litter the orbital path with organic material that then can rain down on whatever environments are nearby. This type of seeding — not just from planet to planet but from star to star — would be true panspermia.

“There is a growing body of evidence to support the idea that life did not originate on the Earth,” said Chandra Wickramasinghe, director of the Cardiff Centre and a major proponent of the panspermia hypothesis.

“The Earth just happened to be one of innumerable planets that came to be drenched with these cosmic genes, and evolution proceeded to piece the genes together as time progressed, subject, of course, to the criterion that the fittest assemblages are always the most likely to survive,” he said.

At his earthbound lab in Texas, however, McLean urges caution.

“I’d like to believe that it’s true,” he said when considering the possibility of extraterrestrial life of any sort. “But I’m mindful of a saying by Carl Sagan that extraordinary claims require extraordinary proof. What we’re reporting is just a really small piece of the puzzle.”