From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Thu Dec 19 2002 - 13:04:02 MST
On Thu, 19 Dec 2002, Rafal Smigrodzki, commenting on my comments wrote:
> ### 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.
Cool, an interesting debate with Rafal who is more than well qualified
to discuss this topic. :-)
In general I'd agree, this discussion would be something interesting
to potentially structure for the Long Bets foundation (www.longbets.org).
A couple of responses --
- chemical molecules may survive a long time (I think the paper scerir
pointed out may point to this). In fact *most* geological samples,
certainly asteroidal material, are collections of non-covalently
bonded molecules -- DNA being a covalently bonded molecule *should*
survive to a greater extent than its carrier.
- I don't think one can cite arguments that the triplet code is
completely "arbitrary". It is clear one can construct a different
code (and I've even designed one). It might be arbitrary -- but I'd
probably give at least even odds that we will discover that it isn't
(thus at least one long-bet) :-).
> So, for "partial" panspermia you'd need a remarkable coincidence of locally
> produced codes, on many levels.
Agreed. But we don't have a clue as to how much "biological" material
we are being assaulted with that contains a code that is totally useless
on the Earth! There could be tons of the stuff hitting us every day.
> 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?),
But the point of my message *wasn't* that you get "whole organism
panspermia". It was that you *happen* to get some useful information
transfer. You have to balance the cumulative dose of cosmic rays
with the number of copies of G-protein coupled receptor DNA. That
may not be a small number and so surviving the radiation may indeed
be feasible. As I said -- its a complex topic -- so complex that
I'm not enthusiastic about wrestling with it.
> We don't know how long it takes to evolve highly radiation-resistant spores,
Agreed. But we know that there are high-radiation environments out there.
And so for organisms to develop and survive in those environments they
have to develop the defenses. Radiation resistance actually has an
negative effect from an evolutionary standpoint because it slows
down the rate at which mutations propagate through generations
(and could subsequently drive the development of another trait
such as intelligence). So at least in some environments one *should*
see the development of high levels of radiotolerance.
> 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.
Here we are getting into some very complex issues regarding the
evolution of life itself, the code(s) it might be based on,
the environment and the metabolic resources available in it.
This is *way* beyond the question of panspermia.
> 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.
I'll grant the fact that smaller genomes are unlikely to contain
a full complement of DNA preservation or repair genes. But the
Deinococcus genome is only 3.26 megabases compared with the
Mycobacterium tuberculosis genome which is 4.41 megabases
(http://www.aeiveos.com/~bradbury/genomes/) both of which
are smaller than the E. coli genome at 4.64 megabases.
I disagree entirely that a "successful panspermic organism" has to
be "autotrophic". The point I was trying to make was that one
doesn't have to throw organisms across interstellar distances
one only needs to throw information across interstellar distances.
Now, on the receiving end, one does need to have something like
a virus that can pick up a small fragment of DNA that happens
to have something like an ATP receptor site that can be utilized
creatively. I believe there are viruses and/or bacteria that do
pick up DNA, incorporate it into their genomes and gradually may
evolve such that it performs a "useful" function. The question
revolves around whether the "nature" of the universe is to drive
biological systems into "niches" that happen to be able to transfer
information. I'm able to ask that question but don't have a clue
as to how to go about providing an answer.
> 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.
Rafal, I think you are thinking too much along the lines of it has
to be "spore" transfer [which may be what many panspermic proponents
think]. It doesn't have to be that way -- it could be molecular
transfer.
The question isn't "How many complete complex life forms can
I transmit across interstellar space?" The question is "How many
organized pre-evolved "bits" of information can I transmit across
interstellar space that is (a) meaningful; and (b) short-cuts
the development process, where-ever it ends up?"
Very different from the "classic" panspermia hypothesis -- but
one that may be more useful.
> 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.
Supernovas or planetary collisions would seem to be likely sources
of carrying very large numbers of "bits" out of the gravity well.
Obviously in both cases you have to deal with the the genomic
destruction caused by the radiation/heat -- but you have to
complement that with the number of genomes. I've done some
work with these numbers and I would not want to make a judgement
one way or the other -- as I've said it is a very complex technical
argument.
> I give 10e30 odds against non-sentient interstellar panspermia.
10e30 -- that is a *really* big number -- I'd probably take that bet
(against Rafal) -- even though I'm generally opposed to the traditional
panspermia hypothesis.
Robert
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