Friday, June 25, 2010

Has Life Originated Elsewhere and will it End?

If the genetic code is universal, it is probably because every organism that has succeeded in living up till now is descended from one single ancestor. But, it is impossible to measure the probability of an event that occurred only once. 
Fran├žois Jacob, The Logic of Life (1973)

The wave of optimism that followed the Miller experiment in 1953 has been replaced today by more subdued attitudes. Solving the mysteries of the emergence of life on Earth is now seen as a very difficult proposition. In the face of this, some have proposed that life may have originated elsewhere in the solar system, or even somewhere else in our galaxy. Proponents of this view include the famous biophysicist Francis Crick of DNA fame (his statement on that question seems to have been tongue-in-cheek) and the equally famous cosmologist Fred Hoyle (not so tongue-in-cheek). 

Echoes of Panspermia

In the nineteenth century, the well-known Swedish chemist Svante Arrhenius hypothesized that a lifeless Earth was seeded with life-forms from outer space. Unlike Rael, however, he did not think that all creatures originated from clever extraterrestrial genetic engineers. Arrhenius’s life-forms were simply bacterial spores that drifted through space, landed by chance on Earth, germinated, and started the process of evolution. Arrhenius called this the concept of panspermia, suggesting that maybe life originated somewhere in our galaxy and then became distributed through it. This was an interesting idea, and it solved the problem of the origin of life on Earth. As expected, that idea was criticized from several angles. Granted, spores are hardened, dehydrated, dormant cells formed by some prokaryotic species. They are more resistant to heat, cold, and radiation than dividing cells. Spores are known to be the hardiest forms of life on Earth and some have been revived after spending 100,000 years in their dormant state. A somewhat controversial report even claims that 25to 40-million-year-old spores have been revived in the laboratory. Spores contain a unique enzyme that can repair ultraviolet (UV)-damaged DNA very efficiently, they are resistant to temperatures up to 150°C, and they can withstand pressures as high as 6000 atmospheres and as low as 10 11 atmosphere. They are also quite resistant to gamma radiation. However, are they hard enough to have resisted conditions prevalent in outer space? Assuming bacterial spores were ejected into space by gigantic volcanic eruptions or asteroidal impacts on a solar or on an extrasolar planet, which is possible, it would have taken them perhaps millions of years, depending on the distance between another solar planet and Earth or between their star and the Sun, to reach our planet. Meanwhile, the spores, even if hidden in the midst of dust grains or chunks of rock, would have been exposed to hard cosmic and ultraviolet rays that pervaded interstellar space. How long could they have resisted? Experiments conducted in the laboratory on Earth and aboard spacecraft, mostly with the spores of the soil bacterium Bacillus subtilis, show that panspermia within the solar system and even interstellar panspermia are valid concepts. As we have seen, laboratory experiments have demonstrated the high tolerance of bacterial spores to injuries inflicted by temperature, radiation, and extremes of pressure. Basically, panspermia is possible only if (a) spores can be ejected from another planet and resist the acceleration and heat produced in the escape process, (b) spores can tolerate space travel for perhaps long periods of time, and (c) spores can survive the reentry process into Earth’s atmosphere.

The escape process must have involved accelerations in excess of the host planet’s gravity. Laboratory experiments have shown that spores can tolerate up to 460,000 × g of acceleration, which is plenty to reach escape velocity from terrestrial types of planets. Then, spores must have been able to survive the hostile environment of space during their travel, during which they would have been exposed to stellar protons, electrons, alpha particles, cosmic rays composed of heavy ions, UV radiation, and X rays. They would also have experienced the effects of a high vacuum and extremes of temperature. Several satellite-based experiments have shed a considerable amount of light on the hardiness of spores in outer space. These experiments were conducted by the National Aeronautics and Space Administration (NASA) on Spacelab, the Long Duration Exposure Facility, and Apollo missions 16 and 17; by the European Space Agency aboard the European Retrievable Carrier; and by the Russian Foton spacecraft. The experiments demonstrated that 30 to 80 percent of B. subtilis spores survived 6 years of exposure to vacuum and radiation in outer space near Earth if they were embedded in crystals of glucose or salt, imitating meteoritic rock. Only a thin layer of this material was needed to completely shield the spores from UV radiation. Experiments conducted outside the Van Allen belts— which deflect charged particles, including most cosmic rays, from Earth— have shown that protection from cosmic rays is more challenging. Even so, it has been calculated that some spores would survive up to 25 million years in deep space if shielded by 2 to 3 meters of meteoritic material. The notion of spores embedded in meteorites naturally raises the question of whether bacteria can live in rock. The answer to that question is yes, as living bacteria have been found in boreholes as deep as 2.5 km inside Earth’s crust.

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