Hi, All,
1) The ancestor of radiodurans is highly unlikely to have been anything like
e. coli, so extrapolating from the rate of e. coli's acquisition of
radioresistance is meaningless. E. coli has been adapted to highly protected
environments for scores of millions of years; they don't like "roughing it."
Radiodurans' great-grandpappy was probably made of sterner stuff.
2) It would have taken many millions of years to evolve radioresistance at
the earth's present level of radiation (at least for e. coli), but the radiation
levels of the early earth (when radiodurans evolved) were much, much higher,
hence the evolution of radioresistance would have been much faster. They don't
call it the "Hadean" epoch for nuthin'!
3) Such a useful trait only has to evolve ONCE. After that, radiodurans can
and will happily move into and thrive in any environment in which other
organisms cannot survive (no competition). They LOVE nuclear reactors and are
hard as hell to get rid of, as they sunbathe in that lovely blue glow...
4) It is most probable that they evolved in naturally occuring nuclear
reactors, like the Aklo "reactor" in Gabon. Such odd geologic features were much
more common on the early earth when the ratio of U235 to U238 were one part in
five or six instead of the ratio of one part in 140 parts as it is today.
5) Occam's razor suggests that requiring a martian origin for an organism
whose terrestial origin is so easily explained is a silly notion. On the other
hand, if you think god doesn't have a sense of humor, look at a penguin. Maybe
they're from mars.
6) The New Scientist is where you go when your idea makes everybody giggle.
Sometimes they giggle because they're too thick to appreciate your genius, but
sometimes they giggle just because it's really funny...
Sterling K. Webb
-----------------------------------------------------------------------------------
Ron Baalke wrote:
> http://www.newscientist.com/news/news.jsp?id=ns99992844
>
> Tough Earth bug may be from Mars
> Stuart Clark
> New Scientist
> September 25, 2002
>
> A hardy microbe that can withstand huge doses of radiation could have
> evolved this ability on Mars.
>
> That is the conclusion of Russian scientists who say it would take far
> longer than life has existed here for the bug to evolve that ability in
> Earth's clement conditions. They suggest the harsher environment of Mars
> makes it a more likely birthplace.
>
> The hardy bugs could have travelled to Earth on pieces of rock that were
> blasted into space by an impacting asteroid and fell to Earth as meteorites.
>
> Deinococcus radiodurans is renowned for its resistance to radiation - it can
> survive several thousand times the lethal dose for humans. To investigate
> how the trait might have evolved, Anatoli Pavlov and his colleagues from the
> Ioffe Physico-Technical Institute in St Petersburg tried to induce it in E.
> coli.
>
> 99.9 per cent deadly
>
> They blasted the bugs with enough gamma rays to kill 99.9 per cent of them,
> let the survivors recover, and then repeated the process. During the first
> cycle just a hundredth of the lethal human dose was enough to wipe out 99.9
> per cent of the bacteria, but after 44 cycles it took 50 times that initial
> level to kill the same proportion.
>
> However, the researchers calculate that it would take thousands of such
> cycles before the E. coli were as hardy as Deinococcus. And on Earth it
> would take between a million and a hundred million years to accumulate each
> dose, during which time the bugs would have to be dormant.
>
> Since life originated on Earth about 3.8 billion years ago, Pavlov does not
> believe that there has been enough time for this resistance to evolve.
>
> Dormant bugs
>
> On Mars, however, the researchers calculate that dormant bugs could receive
> the necessary dose in just a few hundred thousand years, because radiation
> levels there are much higher.
>
> What is more, they point out that the Red Planet wobbles on its rotation
> axis, producing a regular cycle of climate swings that would drive bacteria
> into dormancy for long enough to accumulate such doses, before higher
> temperatures enabled the survivors to recover and multiply. Pavlov reported
> the results last week at the Second European Workshop on Astrobiology in
> Graz, Austria.
>
> David Morrison of NASA's Astrobiology Institute is sceptical that
> Deinococcus came from Mars, pointing out that its genome looks similar to
> those of other Earthly bacteria. But he admits that there's still no obvious
> explanation for the bug's resistance to radiation.
>
> "It is certainly a mystery how this trait has developed and why it
> persists," he says.
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