RE: Panspermia

From: Rafal Smigrodzki (rms2g@virginia.edu)
Date: Thu Dec 19 2002 - 09:08:31 MST


Robert wrote:
>
> This is a very interesting area that really should be explored more
> fully. I think Eugene's subsequent messages "dis" the whole
> interstellar spread possibility -- *BUT* one doesn't need "LIFE" to
> survive interstellar transport -- one only needs some of the
> components for "LIFE" to survive. If one can get a ribosome or a
> proteosome or a G-coupled receptor (or at least the blueprint for one
> of these) across the divide one may be a huge step up from an
> evolutionary perspective. Hell, even getting the "code" for a single
> DNA repair enzyme (there are dozens in the current basic code set)
> across the divide probably puts one *way* ahead in the game.
>
> Life doesn't have to survive -- its the *code* that counts.
> To refute panspermia you have to make some *very* complex arguments
> about the stability of DNA (which is more stable than RNA), radiation
> levels, transit times, probable radiation exposures, size of a
> "useful" genomic unit, probability that the destination system uses
> "DNA" as its information carrier, etc.

### Personally I'd find it hard to imagine a snippet of DNA capable of
transferring across interstellar distances and then successfully
incorporating into other life. Remember, the DNA triplet code is largely
arbitrary, an accident of history (aside from the codon structure with the
third relatively non-significant base). The exact complement of aminoacids
is not strictly an immutable law of nature, either.

So, for "partial" panspermia you'd need a remarkable coincidence of locally
produced codes, on many levels. For whole organism panspermia you also need
some pretty unusual circumstances - you need a highly radiation-resistant
organism (can anybody calculate the cumulative dose of cosmic rays over the
millions of years it takes for a meteorite to travel to another stellar
system?), with the ability to use prebiotic organic substances, or with an
autotrophic metabolism. We don't know how long it takes to evolve highly
radiation-resistant spores, I wouldn't be surprised if they occurred only
after the prebiotic sources were exhausted by preceding waves of extremely
simple creatures, which implies a lack of the relevant metabolic adaptations
in radiation-resistant bugs. If so, then the successful panspermic organism
would need to be autotrophic, and this requires pretty complex metabolism.
No Mycoplasma-sized genome would suffice, and with a relatively large genome
the ability to resist radiation goes down.

And then, of course, the problem of the sheer immensity of the lifeless
void, diluting the already uncommon spores that survive being ejected by a
volcanic or other blast. Even with the perfect organism the likelihood of
reaching a suitable planet before radiation destroys the genome will be very
small, unless one assumes spores being intentionally carried out of the
gravity well, in stunningly huge numbers.

I give 10e30 odds against non-sentient interstellar panspermia.

Rafal



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