PLEASE NOTE:
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CCNET-ESSAY, 2 February 2000
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PLAUSIBILITY, SIGNIFICANCE & THE PANSPERMIA EPIDEMIC
By Jon Richfield <jonr@iafrica.com>
Panspermia is not intrinsically implausible, but in science it
has
played a minor, inglorious role, not far removed from that of
cold
fusion. The scientific establishment is not especially blind,
hidebound or malicious, but even plain common sense makes its
demands
and a hypothesis must meet certain standards to be taken
seriously,
never mind accepted as a leader among viable alternatives.
For a start, Panspermia as a concept is very poorly defined and
supporting arguments tend to be ill-disciplined to say the least.
They
range from academic musings on academic possibilities, to
passionate
and partisan assertions of the universal inevitability of life,
not
necessarily stopping short of UFO-mongering. At one extreme are
tenuous
speculations that SOME viable living material COULD at SOME TIME
have
been splashed off a planet or have got cobbled together in space
and
SOMEHOW survived indefinite periods of exposure to radiation and
free
radicals and IN PRINCIPLE have arrived intact on a receptor
planet AND
established a viable population. To a biologist, this is not
strictly
impossible, but it is not nearly plausible enough to be exciting.
After
all, in terms of strict logic it also is hard to refute the tooth
fairy
hypothesis, which so far has not cut much of a figure in the
biology
textbooks.
At least the tooth fairy has the advantage that we can check
under our
pillows for the transmutation of discarded hard tissue into
welcome
hard currency. In contrast, how can I distinguish our living
world,
which is as it is because a single spore fell to Earth 4e9BP,
from a
similar living world whose life originated 4e9BP in local clays?
And if
that distinction solves a problem on this planet, how does it
solve the
problem of how life first emerged, wherever that happened? Or do
we
accept not only a steady state universe, but steady state life as
well?
Neither of these is conceptually absurd, but it would take a hard
case
zealot to maintain that either is currently competitive.
At the other end of the scale, zealots tout, more or less as
received
wisdom, the continuous and effective universal dissemination of
viable
spores or viruses from planet to planet throughout the
universe.
So far so fair, but in the heat of debate warriors from both
extremes
skip nimbly from premise to premise. Professional scientists who
would
strip the hide off any student who tried that sort of thing on,
start
out with specific proposals. When these proposals are too
specific,
they encounter fierce resistance from uncongenial specialists who
deploy
no end of unwelcome facts and intrusive logical objections. Under
such
pressure the proponents retreat into hand-waving, only to sally
out
again, even further, when the pressure fails to rout them from
the
fastnesses of vagueness.
All this is Good Stuff. Science would make pretty lame progress
if we
did no speculation, and a bit of bite in the debates adds zest.
After
all, no one forces the gentler spirits among us to participate.
Besides, proposal and criticism of new ideas keep the livelier
among us
alive and hones our ideas, and if it annoys pedestrian fogies to
see us
at play, well, a little sorrow is good for their character and
they can
console themselves with their customary derogatory grumbles.
But a good barney is no substitute for soundness and in science,
argument without soundness soon palls; there is too much real
work that
can be done instead and done more rewardingly.
At the modest end of the scale, almost the only interesting
inference
from Panspermia, if the hypothesis of a unique inoculation is
true, is
the implication that somewhere away from Earth, life has evolved
and
that that life should have certain aspects in common with life on
our
planet. That fact would seem tremendous enough in its own right,
and if
we had the faintest clue as to where to look for the source or
how to
recognise it, it would justify serious investigations and
attempts at a
visit or at least some probes.
When we stop to think about it though, suppose we did discover a
remote
source of inoculation. Work on the genesis of life on Earth is
already
underway and such a discovery need not affect it greatly. Ask
yourself:
either a germ lands in a lifeless, nutritious soup, and it
prospers, or
a similar germ assembles out of that soup in much the way that
the
invader germ might have done on another planet... and it
prospers.
Philosophically it only increases by one remove, the origin of
the
chain of life. *Biologically*, from the perspective of this
planet, I
do not see that it makes any difference at all. Who among us
would
regard such a speculation as grounds for putting other work on
the
back burner to release resources for searching for the parent
planet?
The problem is not only intrinsic implausibility, but lack of
practical
viability. What would you look for, how and why? What makes you
think
that the parent planet is within range of detection, or even that
it
still exists, or that life on that planet ever developed beyond a
few
smears of slime in a dying desert?
To be sure, there are plenty of other lines of research on which
related questions could piggy-back: the study of our solar system
and
of "nearby" stars; the search for extraterrestrial
intelligence; the
investigation of the radiation profiles from our neighbouring
regions of
space; the nature of our own molecular biology; and so on. All of
these
have their own justifications as objects of study on their own
scales
and some of them might point us in directions where we do find a
justification for a study of Panspermia, or at least polyspermia
of some
degree.
But so far, so nothing much.
For the idea that Earth near-miraculously picked up a single or
at most
a few "spores" that ignited our biosphere, let us coin
the term
"oligospermia". As I indicated, I do not regard
oligospermia as being
of much practical importance, nor do I find it plausible (but pay
your
dues and make your own choice!)
Then we have polyspermia. That would be the idea that
interplanetary
infection is rare, but not vanishingly rare. Any planet with a
good
fertile ocean full of abiogenetically generated organic matter
could
expect enough visitors to get a viable biosphere going in a few
hundred
million years. This would imply a far richer supply of sperm
sources
than oligospermia does, but to my mind would have little more
day-to-day
biological significance. For the last two billion years or so,
there
has not been much in the line of biochemicals that you could drop
into
our biosphere in picogram quantities without it getting gobbled
up by
incumbent organisms. Proponents would have to produce something
substantial and falsifiable to lend much interest to polyspermia.
In
science a subject doesn't have to be false to be boring.
Next let's consider Panspermia, which I propose as referring to a
more
ambitious scenario: that the universe is more or less awash with
cosmic
sperms and practically every planet is continually, or at least
frequently (say more than once per century) peppered with viable
organisms. In the more sanguine versions of this theory,
some of those
sperms routinely survive on the planet. Real optimists
regard them as
the driving force behind evolution, continually enriching our
gene
pools! Others take a gloomier view and between copious medicinal
draughts of hot scotch and lemon, they mournfully portray the
visitors
from space as seeds of epidemics of flu and the like. In a
steady-state
(or at least eternal, or at even less than least, a very, very
long-lived) universe, such spores might serve as a steady-state
presence
of life, a sort of diffuse, eternal, cosmic ecology.
In a field of such importance, surely it must be worthwhile to
hunt for
such spores and test their viability in space and on earth?
Why not
solve the dispute at a stroke and then proceed to the more
interesting
implications of the results?
Unfortunately, it is by no means easy to design experiments to
satisfy
everyone. In the face of negative results, panspermic hypotheses
rapidly tail off, first into polyspermia, then into oligospermia,
and
finally into unfalsifiability. You did not find anything?
Obviously
you did not look long enough, skillfully enough and hard
enough. You
did not look in the right places. How do you expect your filter
to pick
up so low a frequency of particles? You looked for the wrong
things
(who says flu germs in space will be in the same form as flu
germs in
eukaryotic host cells?) Your device was not gentle enough; it
destroyed
the specimens. Your tests were not diagnostic enough to recognise
alien
organisms. (Proponents of this sort of argument seem to think
that one
virus is all you need to start an epidemic, no matter what
spoilsport
virologists say.)
And so on. To make things worse, in real life it is no easy
matter to
detect or identify really microscopic life forms or quasi-life
forms
(such as viruses or viroids). Even when one actually has a fair
amount
of material at one's disposal, there is room for argument about
the
nature of, not only fossils, but putative extant soil microbes.
Looking
through the microscope, one does not see all those beautifully
labelled
pictures of cell walls and double helices. So in practice, to
prove a
positive may be a serious challenge, and proof of a negative
becomes
very, very difficult.
But that is no novelty. At best failure to prove a negative
constitutes
confirming instances for either a negative or a positive
hypothesis.
Even to be taken seriously, the failure should fit into a
persuasively
comprehensible conceptual and factual framework.
One example of contorted reasoning is that Panspermia relieves us
of
the intellectual burden of the improbability of getting a
vanishingly
improbable protein (or nucleic acid chain or the like) in the
ridiculously short time and space estimated to have been
available
during the period of abiogenesis on Earth. This is a mouldy old
chestnut, not too appetising even when it was fresh (if it ever
was; I
suspect it to have originated somewhere in a steady-state
panspermic
universe.) The trouble is that it is not even relevant except in
an
effectively eternal, infinite, steady state universe, which ours
is not
currently accepted to be.
Consider: suppose either that life originated here, or elsewhere.
Suppose that it did so by stochastic generation of the necessary
living
structures. But we accept that the stochastic generation of a
unique
set of polymers is far too improbable for us to expect a hit in
our
planetary history. This is inarguable; no competent biologist or
biochemist would deny it for an instant. Unfortunately the same
argument is good for any universe as small as ours seems to be,
even if
we do not limit the scope to one planet. About a googol of
particles in
about 1e10 years is far, far, far too mingy a medium to generate
anything viable with any realistic probability. So on such
assumptions,
to propose panspermia for bluntening Ockham's razor is simply
unreasonable.
There is no evidence that probabilistically independent selection
of
monomers for unique target chains ever would have been a viable
basis
for the genesis of life anywhere. Therefore there is no point to
arguing about 1e-137 probabilities (or whatever ridiculous figure
one
prefers). There are two gaping holes in this idea: we do not know
that
the formation of polymers really is effectively random, and we
have no
hint of a reason to believe that only one (or even one class of)
polymer would be viable. The space of all polymers available to
our
abiogenetic past may be very far from poor in viable
developments. For
all we know the frequency of viable macromolecules in our
ancestral
soup, might be of the order of one in millions or billions,
rather than
one in googols.
In other words, of all the molecules likely to be generated in
our
primeval soup, the frequency of biologically relevant specimens
might
be very high indeed, perhaps hundreds or even millions of orders
of
magnitude greater than a simple-minded stochastic model would
suggest.
Stochastic generation of unique or near-unique target molecules
is
hardly more viable a prospect for panspermists than for
parochialists.
This would not invalidate panspermia, but it certainly would
puncture
claims that panspermia is a sufficient, let alone a necessary,
assumption for the care and feeding of good Ockhamists.
But surely I am being too hopeful? In the light of the by no
means
absolute, but certainly very considerable specificity of extant
biomolecules, where do I get off, criticising the optimism of
panspermists? Granted: there is not a single protein nor a single
nucleic acid to which it is impossible to make a change without
rendering it non-functional. But then, there also are not many
that
offer no opportunity for a single modification that is deadly, or
at
least causes a serious reduction in fitness. Who would bet on our
primeval soup generating a viable mix of near-enough-hits?
The logical problem here is that we cannot assume that the
requirements
for viability in a sterile soup are the same as in the feeding
frenzy
that permeates the soil and ocean today. Any old Heath Robinson
structure could have hobbled on for centuries in a dead, but
fertile,
environment, without fear of interruption. As competition grew
stiffer,
tighter performance tolerances became necessary for survival and
in a
few hundred million years something like the greater specificity
of our
modern molecular structures would have resulted. A few gigayears
of
adaptation later, the original limping abiogenesis would be lost
in the
past, probably together with a job lot of rival systems and
designs that
turned out to be less viable or less lucky. Anyone who doubts
this
might as well ask why Palaeolithic tools are dangerously useless
in a
modern machine shop, in which imprecision of a millimetre or two
would
in many circumstances be disastrous. Ironically, a modern
electric
lathe might not work too impressively in a Palaeolithic machine
shop.
Any of us might fair poorly in the remote Precambrian.
So adaptation was an absolute necessity. A concept of crucial
importance is that the process of adaptation is heuristic as
opposed to
stochastic. Anyone who does not know the implications of that has
a lot
of homework to do; Darwin has passed him by!
Now let's talk monkeys. Lots and lots of monkeys with infinite
stamina
and lots and lots of typewriters; very, very good typewriters
with lots
and lots of consumable supplies. After something like 1e1e7
keystrokes
(sure; call me a liar for the sake of a few googols!) we get a
complete
Shakespeare, right? Maybe. (With MY luck, don't rely on it!) OK,
but
what else do you get on the way? Any half Shakespeares? Any other
full
Shakespeares in different sequences? What? Not even a Pushtu
Limerick?
How about a few Spencers, Schillers, Semmelweisses or
Skytbalies?
(Skytbalie never *actually* existed, but he is the fellow who
*would*
have written the definitive, cogent explanatory treatise on the
nature
of mind, its generation and predictive physics if he *had* in
fact
existed. His work would have made Einstein look like Enid Blyton
for
depth and like a nineteenth-century political-theoretic tract for
clarity and cogency. Unfortunately, if he *had* been real and he
had
gotten round to writing it, the non-existent Skytbalie would have
written his non-existent tract in his non-existent mother tongue
of
Strondskrif, so no one could have read it, since we have no such
language. However, who is in a position to deny that this tract
may
have appeared in the by-products of our monkeys' output, and
maybe there
would also be a fortuitous dictionary... into the other
non-existent
language of Gageluid!)
See? All that Good Stuff may be cramming our monkeys' output and
we are
not even assuming anything heuristic about their products.
Looking
through that mass, we would find the Shakespeares and Schillers
and so
on, while in some other universe these would be sifted out
unrecognised,
while the delighted Strondskriftish would exclaim over the
Skytbalie
tracts. There could be many different systems, mutually
irrelevant,
but intrinsically viable, given a fair chance.
We have no way of knowing what the frequency of meaningful RNA,
DNA or
peptide chains may be within the space of accessible sequences,
combinations and configurations, very likely with strongly
heuristic
associations. Must I necessarily be wrong to guess at frequencies
of
one in millions or billions? Tell me why. (Careful! If
Strondskrif
had indeed been our language (and who are you to say it is
intrinsically
a lesser probability than English) then Skytbalie's works would
have
been far more recognisable to us than Shakespeare's. Trying to
generate
a text in a particular language might be infeasible; generating a
text
in any extant language might be only a few orders of magnitude
easier,
but generating a text in something that might in principle be
viable as
a language, given a suitable semiotic environment might not be
ridiculously difficult at all.)
But would Shakespeare's work really be of any value in such a
gravitationally disastrous mass of paper? Definitely not. We
could
never find it. That is the point of the heuristic generation of
functional polymers in our ancient soil and water. We do not have
to
sort out anything from such beyond-astronomically huge numbers of
molecules; we get perhaps a few trillion tonnes of job-lot
precursor
molecules juggled for a few hundred million years. Any neighbours
that
mutually mean anything constructive to each other get a chance at
a new
generation. Maybe many do, maybe only one ever does. Maybe only
one
ever did in the history of the universe and it happened here.
Maybe it
happened in a few places, but only this one survived. But out of
context, no life ever will.
Most of the molecules that do not fit the emerging templates
simply
never arise. Such a context is a demanding requirement in
this
connection and to get it right first time takes a bit of doing,
in fact
a lot of feedback. One of the greatest problems with any
space-based
abiogenesis is that it is exceedingly difficult to create a
persuasive
scenario for such heuristic feedback. On Earth there was lots of
water
for hydration, support and transport, lots of clay particles for
anchors
and templates, and a good turnover of raw materials for
conversion into
the next generation. In space you would be lucky to get a
peptide, let
alone a virus. And if you did miraculously get a virus, what
meta-miraculous procedure would generate you another virus of the
same
kind in the same region?
If anyone gave me a scroll with all of Shakespeare in it (or even
a
single sonnet, actually) and tried to convince me that he had
found it
in a virgin rock from outer space, I'd want some fairly
persuasive
supporting evidence. All this and in English too?
Elizabethan English?
And on paper? A single glucose molecule from space I just
MIGHT
swallow, but polymerised into shredded cellulose, compacted and
sized
in space? Or splashed off another planet on which cellulose just
happened to be the available substrate? Well, that is the sort of
thing
panspermists are in effect trying to sell us in the form of
diseases
from space. A virus is not just any ball of protein and RNA; it
is a
structure of not only the right proteins in the right
configurations of
the right lock-and-key conformations, but with the necessary
lipid
bilayers, the right enzymes included inside, and the right RNA
chains
inside with them. These need to be assembled in an aqueous
medium, by
the necessary assembly machinery! Anyone who thinks that he can
generate flu viruses just by shaking the ingredients in water
(never
mind in a comet), good luck to him for the first million years'
shaking. And by ingredients I do not mean carbon and
water.
Translate all this into engineering terms: trying to assemble
viruses
at random in space amounts to asking for the proverbial
whirlwind to
assemble a Boeing out of a scrap yard, not in a machine shop with
all
the ready-made components to hand (or to eddy.)
What is more, it is like asking for a Boeing and refusing a
Handley-Page or a Douglas. Of heaven knows how many viruses, we
get,
not just any virus, but an ordered sequence of strains of viruses
of
middle-of-the-road complexity and with a specificity to a modest
subset
of vertebrate hosts. For our miraculous virus from space to have
just
the right neuraminidases and haemagglutinins is asking too, too
much.
We do not get persistent polio or rabies epidemics from space, it
seems,
though both of these viruses are quite infective when they get
into
respiratory tracts or the eye.
What we get is flu.
And the sequences of new strains of flu every few years just
happen to
be logically consistent with Earthside molecular evolution
too?
But wait, sauce for the juice is sauce for the comet. If by my
magical
formula for assembly of living molecules or structures in Earth's
ocean
or clays, I can construct life out of shockingly small samples of
candidate material, by adaptive feedback, then why cannot the
same thing
happen in the far vaster samples of material in space? Who then
needs a
steady state universe?
Fair as far is it goes, but firstly, the idea is then once more
not
very exciting. We can check for life out there, but not for
viruses
raining onto us down here. Secondly, where in space do we get the
heuristic feedback?
No, I repeat, Panspermia has a great deal of development to do
before
it becomes an interesting subject. I may have said this before; I
shall
certainly say it again: certain subjects, such as probability
theory,
are easy in principle, but bristle with traps for the unwary. One
such
subject is evolution, and its treachery extends thence to
permeate the
rest of biology. A neglecter of homework does not make much of a
splash
in biology; more of a dull plop.
And bad science doesn't even make good science fiction unless the
writer is a
genius.
Copyright 2000, Jon Richfield