Francis Crick and Directed Panspermia
Every biologist knows that Francis Crick is the co-discoverer of the structure
of DNA. What is less known, probably, is the fact that Crick was a proponent
of a theory that stands at the border of science, the theory of directed
panspermia.
In 1973, Crick (together with chemist Leslie Orgel) published an article
describing the theory, and in 1981 he dedicated a full book to directed
panspermia, entitled Life itself.
According to Crick, the idea of panspermia – which means “seeds
everywhere” – was proposed by the physicist Arrhenius at the end of the
19th century. Arrhenius suggested that life on Earth originated from space,
that our world was seeded by spores of micro-organisms traveling between
planets.
But because the radiations in space were thought to be too intense for the
spores to survive, Crick and Orgel postulated a variant of the theory in which
spores were transported by an interplanetary spaceship sent by an alien
civilization!
Life itself is a short, very engaging book in which Crick takes the time to
explain some basic features of cosmology, chemistry and biology
throughout the first chapters. Because he knows that directed panspermia
is stretched across the very border of science, Crick adopts a somewhat
skeptical viewpoint, trying to examine and refute the arguments of the
theory. But gradually, once the objections are carefully lifted one by one, he
feels confident to say that directed panspermia is at least possible and,
more importantly, not totally improbable. Thus, at the end of the book he
speculates freely in chapters with amusing titles such as “what would they
[the aliens] have sent?” and “the design of the rocket”!
But let’s quickly sketch the path that leads Crick here. After the first
introductory chapters that I just mentioned, he discusses the mixture of
chemicals required to start life de novo, and he proposes a sequence that
would culminate in the formation of the first replicator molecule, RNA. The
scenario is plausible – actually RNA is still today considered the best
candidate for being the initial replicator – but as Crick notes, p. 87:
“What is so frustrating for our present purpose is that it seems almost
impossible to give any numerical value to the probability of what seems a
rather unlikely sequence of events.”
And p. 88:
“At the present time we can only say that we cannot decide whether the
origin of life on earth was an extremely unlikely event or almost a certainty—
or any possibility in between these two extremes.”
Crick rightly points out that the fact that life exists in the universe (we’re
here!) does not tell us anything about the probability of life’s emergence. It
could be, argues Crick, that life is an extremely rare phenomenon.
From that statement, Crick moves on to the likelihood that other planets in
the universe may be suitable for life. Here he makes a lot of assumptions,
but offers only very little experimental evidence. [That’s not surprising, given
that no exoplanet had been described in 1981! I suppose Crick was
delighted to follow the planet discoveries of Michel Mayor, starting in 1995.]
Thus, after a lot of guesses, Crick comes with an estimation of (p.105) “a
million planets in our galaxy on which we might hope to find oceans of thin
organic soup waiting for life to get going.” Apparently, based on modern
research, this number could be much higher! So Crick is probably on the
safe side with his estimate. What he wants to show here, obviously, is that
life could have begun on another planet than Earth.
His next move is to show that there was ample time since the beginning of
the universe for another highly developed civilization to appear. But what
such a civilization would have sent? interrogates Crick. His guess: a mixture
of bacteria (for example cyanobacteria) and eukaryotes (something like
yeast), which can be packed by billions in a relatively small volume, and
which can survive space travel better than any known organism.
Finally Crick confronts two explanations for life on Earth. In the first one, life
originated on Earth from the primitive soup, without external help. The
second is directed panspermia (DP). How can he decide between them? He
can’t really, though he tries to grant points in different categories. For
instance, Crick sees the uniformity of the genetic code as a support for DP.
Another point granted to DP comes from the fossil record: the oldest fossil
microbes are indeed very old and resemble cyanobacteria. However, Crick
sees the absence of eukaryotes in the ancient fossil record somewhat as an
argument against DP.
The speculative exercise is necessarily limited. There is no way to firmly
decide between DP and the conventional theory, and most of the
estimations and probability are very uncertain. Interestingly, Crick seems
very aware of this fact, and he repeatedly mentions it in his book. For
instance p. 148:
“The first [criticism to directed panspermia], which my wife has voiced
more than once, is that it is not a real theory but merely science fiction. This
is not meant as a compliment, though it might perhaps be taken that
way.”
I already wrote about Crick’s interest in the origin of life in this post, and
directed panspermia represents an interesting complement to his reflections
on the topic.
But despite Crick’s defense of directed panspermia as a valid scientific
theory – or at least as a premature theory that will fully expand in the future
–, it is difficult not to see it as a purely speculative attempt.
An exciting, interesting idea, yes, but without a chance to be proved or
disproved. And this, if we are to follow Sir Popper’s falsification test, puts it
right off science’s lawn.
References:
Crick F. H. C. & Orgel L. E. (1973). Directed panspermia. Icarus 19: 341-346.
Crick F. (1981). Life itself. Simon & Schuster, 192 p.
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