Monday, December 18, 2006

Invaders from Mars?

Theories of panspermia have literally been floating around (ha!) for millennia, from the (very) early musings of the ancient Greek philosopher Anaxagoras to the earnest yet wonky hypothesis of Fred Hoyle to the downright loopy advocacy of "directed panspermia" by Richard Hoagland. While the very base idea at the heart of the theory (and note that I use "theory" in the colloquial sense here, rather than the strict scientific sense), actual falsifiable experiments have been hard to come by--the expense, logistics and timeframes involved are prohibitive for any meaningful direct observation.

But there's been some baby steps forward in recent years. Texas State biologist Robert McLean inadvertently found himself in possession of a panspermia experiment following the tragic destruction of the space shuttle Columbia. McLean's results aren't perfect by any means--salvage work of this nature rarely is--but it was significant enough to warrant publication in Icarus earlier this year.

Now comes a report in Mars Today regarding a new paper published in the journal Astrobiology arguing that terrestrial radiation-resistant bacteria are actually transplanted samples of Martian organisms (the full paper downloadable in PDF format here). The paper is fascinating reading--if you have any interest in astrobiology--and the authors present a compelling case. Consider:
Where in our solar system do the radiation doses indicated in the laboratory “training” cycling experiments occur naturally? The challenge is that high radiation dosages necessary for radioresistance training must accumulate during a bacterial lifetime. At present, if radioresistant bacteria are in the active metabolic state, they can survive any observed radiation level on any object of the solar system (including radiation exposure to the unshielded cosmic rays in space) and repair the resultant DNA damage. The only way bacteria could accumulate sublethal doses of radiation exposure is during dormancy. Natural radioresistance training cycles are, therefore, likely to occur in environments where high levels of ionizing radiation are the norm and bacteria experience long periods of dormancy in cold, dry, or cold and dry conditions followed by periods of population re-growth in favorable warm, wet, or warm and wet conditions. Warm periods could be very short because even a couple of days would be enough to re-grow the bacterial population. Based on our laboratory experiments (Fig. 1), we calculated that a minimum of 100 training cycles would be required to increase the radioresistance of ordinary bacteria to the levels observed in radioresistant bacteria.

Earth’s low background radiation makes such “natural” experiments practically impossible. For example, to accumulate a 10–20 kGy radiation dosage, the duration of each dormancy period would need to be several million years, and the total duration of the training process would take more than several hundred million years.

As discovered during the Mars Odyssey mission, environments shared by present-day Earth and Mars include permafrost regions or polar terrains where ground ice accumulates. Though some bacteria are known to fall into a state of dormancy in permafrost, neither ground ice nor permafrost would be stable for long enough periods of time on Earth for dormant populations to accumulate high radiation dosage. Furthermore, some permafrost biota on Earth do not cease metabolic activity even under 20°C (Rivkina et al., 2000), and such biota cannot accumulate any substantial irradiation dosage while metabolically active.

The long and short of it is, Earth's natural background radiation isn't sufficient to generate the evolutionary pressure necessary to generate this level of radiation resistance. That is one of the lynch pins of the argument in favor of Martian origins of these organisms. If you're like me, however, the Oklo natural nuclear reactors immediately come to mind. Almost two billion years ago, heavy deposits of uranium in Africa underwent a series of natural reactions over the course of hundreds of thousands of years. This process would result in Proterozoic populations of bacteria and eukaryotic cells being repeatedly exposed to high levels of radiation, followed by a "recovery" period. Isaac Asimov himself references a variation on this idea somewhat in Robots and Empire. If, over the course of several hundred thousand years distinct populations of microbes developed high levels of radioactive resistance at Oklo, might the resistance we see today be a vestigial legacy? But the paper's authors have already taken this into account.
For example, if one considers the 1.7-billion year old Oklo uranium deposits near Gabon, the radiation level could not exceed 1 Gy/h  100 rad/h (Nagy et al., 1991), while D. radiodurans bacteria are able to grow continuously even at 60 Gy/h  6,000 rad/h. Furthermore, to be exposed to any significant radiation level in any active zone of “natural” nuclear reactors, microorganisms had to withstand the extreme heat generated from the nuclear fusion reactions (up to 360°C). Therefore even the dosage of 1 Gy/h was most likely unreachable for any living bacteria in Oklo. This example underscores the fact that, on Earth, there is no hypothetical place where terrestrial biota would need to withstand high radiation levels.

Being the enthusiastic lay person that I am, however, I remain unconvinced. I want to believe, but I have suspicions that force me to remain skeptical. The authors are dismissive of the "side effect" phenomenon as a potential explanation for the radiation resistance--a trait that evolutionary pressures select for turns out to have a benefit unrelated to the original purpose of said trait. A good example are insect wings. Intelligent design advocates and creationists argue wings could not have evolved, because what good is half a wing? But it's been well-established that insect wings developed from external gills, and that flight was a happy afterthought.

It's with points like that where I get an itchy feeling in the deep nether regions of my brain--some of the arguments these researchers are using to bolster their position strike me as suspiciously similar to those espoused by the Intelligent Design crowd. The paper discusses how radiation-resistant populations of microbes were easily and rapidly developed in laboratory conditions, and since these conditions never existed on Earth and may, in fact, exist on Mars, this shows that radiation-resistant microbes must originate on Mars (I am grossly simplifying this, mind you). But IDers and creationists use much the same logic when discussing, say, the diversity of canine breeds. Since dog breeds didn't diversify into the various types we have today until humans began selectively breeding them, the argument goes that this "proves" the Hand of God is necessary for any speciation to take place. The whole thing brings to mind the old "watchmaker" argument for design from William Paley:
Since watches are the products of intelligent design, and living things are like watches in having complicated mechanisms which serve a purpose (e.g., having eyeballs to enable sight), living things are probably the products of intelligent design as well.

The scientific authors in Astrobiology--Anatoly K. Pavlov, Vitaly L. Kalinin, Alexei N. Konstatinov, Vladimir N. Shelegedin and Alexander A. Pavlov--obviously aren't IDers, and have done some fascinating work here. But from my admittedly unscientific vantage point, the vast majority of their evidence is indirect and circumstantial. The fact that they were able to create radiation-resistant microbes so easily in the lab suggest to me that all cellular life (or single-celled organisms, at least) boasts this adaptability to some degree, rather than the opposite stance that a Martian origin is necessary. In any event, I suspect molecular biologist and biochemists will have more to say on this issue than I, as it should be a fairly straightforward task to break down the genetic ancestry of these supposed Martian transplants to determine if they vary from expected microbiological parameters enough to either suggest or dismiss extraterrestrial origins.

Don't you just love the unknown?

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