Darwin's Dangerous Idea - Evolution and the Meaning of Life

318 CONTROVERSIES CONTAINED
ence of rational designers—a wildly implausible fantasy, but a possibility
after all.
In our world today, there are organisms we know to be the result of
foresighted, goal-seeking redesign efforts, but that knowledge depends on our
direct knowledge of recent historical events; we've actually watched the
breeders at work. These special events would not be likely to cast any fossily
shadows into the future. To take a simpler variation on our thought
experiment, suppose we were to send "Martian" biologists a laying hen, a
Pekingese dog, a barn swallow, and a cheetah and ask them to determine
which designs bore the mark of intervention by artificial selectors. What
could they rely on? How would they argue? They might note that the hen did
not care "properly" for her eggs; some varieties of hen have had their instinct
for broodiness bred right out of them, and would soon become extinct were it
not for the environment of artificial incubators human beings have provided
for them. They might note that the Pekingese was pathetically ill-equipped to
fend for itself in any demanding environment they could imagine. But the
barn swallow's innate fondness for carpentered nest sites might fool them into
the view that it was some sort of pet, and whatever features of the cheetah
convinced them that it was a creature of the wild might also be found in
greyhounds, and be features we know to have been patiently encouraged by
breeders. Artificial environments are themselves a part of nature, after all, so
it is unlikely that there are any clear signs of artificial selection that can be
read off an organism in the absence of insider information on the actual
history that created the organism.
Prehistoric fiddling by intergalactic visitors with the DNA of earthly species
cannot be ruled out, except on the grounds that it is an entirely gratuitous
fantasy. Nothing we have found (so far) on Earth so much as hints that such a
hypothesis is worth further exploration. And remember—I hasten to add, lest
creationists take heart—even if we were to discover and translate such a
"trademark message" in our spare DNA, or found some other uncontestable
mark of early tampering, this would do nothing to rescind the claim of the
theory of natural selection to explain all design in nature without invocation
of a foresighted Designer-Creator outside the system. If the theory of
evolution by natural selection can account for the existence of the people at
NovaGene who dreamt up DNA branding, it can also account for the
existence of any predecessors who may have left their signatures around for
us to discover.
Now that we have seen this possibility, however unlikely it is, we also see
that, if the skeptics had ever found their Holy Grail, the You-Couldn't-Get-
Here-from-There Organ or Organism, it would not have been conclusive
against Darwinism after all. Darwin himself said that he would have to
abandon his theory if such a phenomenon were discovered (see note 5 of
chapter 2 ), but now we can see that it would always have been log-
A Clutch of Harmless Heresies 319
ically coherent (however lame and ad hoc) for Darwinians to reply that what
they were being shown was telling evidence for the surprising hypothesis of
intergalactic interlopers! The power of the theory of natural selection is not
the power to prove exactly how (pre)history was, but only the power to prove
how it could have been, given what we know about how things are.
Before leaving this curious topic of unwelcome but nonfatal heresies, let's
consider one that is a bit more realistic. Did life on Earth arise just once, or
perhaps many times? Orthodoxy supposes it happened just once, but there is
no skin off its back if in fact life arose twice or ten or a hundred times.
However improbable the initial bootstrapping event may have been, we must
not commit the Gambler's Fallacy of supposing that after it happened once,
the odds rose against its happening again. Still, the question of how many
times life arose independently opens up some interesting prospects. If at least
some of the assignments in the DNA are purely arbitrary, then might there
not have been two different genetic languages coexisting side by side, like
French and English, only entirely unrelated? This has not been discovered—
DNA has clearly coevolved with its parent, RNA—but that does not yet show
that life didn't arise more than once, because we don't (yet) know how wide
the scope for variation in genetic code actually was.
Suppose there were exactly two equally viable and constructible DNA
languages, Mendelese (ours) and Zendelese. If life arose twice, there would
be four equiprobable possibilities: both times Mendelese, both times Zendelese,
Mendelese and then Zendelese, or Zendelese and then Mendelese. If
we ran the tape of life many times, and looked at the times in which life arose
twice, we'd expect that half the time both languages would get created, but in
one quarter of those replays only Mendelese would appear. In those worlds,
the DNA language of all organisms would be the same, even though another
language was just as possible. This shows that the "universality" (at least on
our planet) of the DNA language does not permit a valid inference that all
organisms had arisen from a single progenitor, the ultimate Adam, since, ex
hypothesi in these cases, Adam could have had an entirely independent twin
of sorts, accidentally sharing the same DNA language. Of course, if life arose
many more times—say, a hundred times—under these same conditions, then
the likelihood of only one of the two equiprobable languages' appearing
would plummet to Vanishing. And if in fact there are many more than two
equally usable genetic codes, this would similarly change the implications
about probability. But until we know more about the range of genuine
possibilities and their associated probabilities, we can't get any good leverage
to decide for sure that life arose just once. For the time being, it's the simplest
hypothesis—life only has to have arisen once.