Primordial Alphabet Soup
"Thirty-eight years what is arguably the greatest mystery ever puzzled
the scientists - the origin of life - seemed virtually solved by a
single simple experiment." Thus is how the February 1991 issue of
Scientific American begins a review of theories of the origins of
life¹.
The simple experiment, carried out by the University of Chicago
graduate student named Stanley Miller, involved placing a mixture of
methane, ammonia, hydrogen, and water in the sealed flask and zapping
it with electrical sparks. The result was terry goo containing amino
acids, the building blocks of proteins found in living
organisms.
To Miller it seemed but a few inevitable evolutionary steps from this
primordial soup of water and biomolecules to the first living organism.
And from that day, college science students have been thought that
science has explained life's origin. Indeed many students are under the
impression that life itself has been synthesized in a test tube.
Unfortunately, as the article in the Scientific American points out,
scientists are far from understanding life's origin.
First of all, some scientists argued that the condition on the
primordial earth would have been unsuitable for amino acids to form in.
Miller's theory calls for the reducing atmosphere rich in
hydrogen-based gases such as methane and ammonia. But the primordial
atmosphere, some say, consisted mainly of nitrogen and carbon dioxide,
so that the row materials for amino acids and other small biological
molecules would have been missing. In fact, scientists can only guess
about the earth was like billions of years ago, and the guesses they
make can agree or disagree with Miller's theory.
Let's suppose, for the sake of argument that amino acids would have
formed on the primordial earth. And let's suppose they would have piled
up with other biological molecules without being naturally destroyed or
dispersed. We'd then run into another problem: Although the rules for
chemical bonding may allow simple biological molecules to form, these
same rules don't guarantee that the higher forms of organization found
in living organism will arise.
We can illustrate this by simple example. We all know the story of the
monkeys that randomly hit typewriter keys and by chance write
Shakespeare's plays. Monkeys who strikes keys completely at random are
unlikely even to come up with English words, apart from short words
like is or at. But we can improve on the monkeys' performance by
introducing a simple rule.
Here's how the rule works. If the monkey has just typed th, we require
that the next letter be fit for an English word including th. For
example, the next letter might be e, forming the word the, or it might
be r, since thr appears in throw. But the letter couldn't be q or
x, since thq and thx don't come up in English words. By this rule the
monkey always randomly chooses a letter that in English could follow
the last two letters he typed.
Another part of our rule is this: we instruct the monkey that the more
often the letter appears in English after the two he has just typed,
the more he should tend to choose it. For example, e follows th more
often than r does, so after th the monkey is more likely to choose e
than r. (We also let the monkey choose spaces, comas, and periods along
with the twenty-six letters of the alphabet.)
You can think of this rule as an imitation of chemical bonding. An e or
r can bond to th, but q or z can't. allowing the monkey to type
sequences of letters by this rule is like letting molecules form in a
primordial soup by the rules of chemical bonding. I compiled a table of
allowed three-three letter combinations (letter-triples) by running an
essay of mine, on Vedic astronomy, through a computer. Then I
programmed the computer to generate sequences of letters according to
the resulting rule. I call these sequences of letters "sentences", even
though they are generally not punctuated properly. Here is an example:
"To the local thers an ut once scorpith ese, ar an astar. The ma, wers
a godern the sky srittailis othicein volumn of the onsmilky way, thears"
Evolutionists, this seems promising. The computer-monkey is coming up
with many English words, and some even seem to convey a faint glimmer
of meaning. One can imagine that in just a few evolutionary steps the
computer will begin to express profound thoughts - with impeccable
English grammar.
But unfortunately if we read a few pages of this stuff we find no signs
of emerging complex order. We find short English words, often relating
to astronomy, since the letter-bonding rule comes from such words. But
there are no signs of a more complex order needed for the grammatical
expression of thoughts. In the bonding rule, the information for these
complex patterns is simply not there.
Biological chemistry puts before us a similar problem. By the rules of
chemical bonding, atoms of hydrogen, oxygen, carbon and nitrogen will
tend to form amino acids and similar compounds under appropriate
conditions. But those rules are not enough to bring together highly
complex structures found in even simplest living cells.
Of course, our rule for generating letter sequences doesn't take into
account Darwinian evolution by mutation and natural selection. Many
scientists regard this process as essential for the development of
complex order. So it's not surprising, one might say, that our simple
rule cannot produce such order.
But the simple forming of molecules by chemical bonding in a primordial
soup also doesn't involve Darwinian evolution. Darwinian evolution,
calls for a self-reproducing system of molecules. Indeed, one of the
main tasks of origin-of-life theories is to explain how the first
self-reproducing system arose.
In living organism, self-reproduction is a dauntingly complex process
involving proteins, deoxyribonucleic-acid (DNA) and ribonucleic-acid
(RNA). If Darwinian evolution can't take place until such a complex
system is operating, scientists are at a loss to explain how that
complex system has come about.
The only hope has been to suppose that the firs self-reproducing system
was much simpler than the simplest of today's living cells. If somehow
a single molecule could reproduce itself under suitable conditions,
then perhaps it could evolve, develop liaisons with other molecules,
and eventually give rise to the kind of organism that exist today.
One of the most popular scenarios for a self-reproducing molecule has
been the so-called "RNA world." The idea is that the RNA molecule might
be able to catalyze its own replication and so be able to evolve in a
Darwinian manner. It has been shown that RNA molecules can act as
enzymes that act on other RNA molecules. And Manfred Eigen of the Max
Planck Institute has shown that RNA molecules reproducing under the
influence of modern cellular enzymes can undergo a process of Darwinian
evolution.
Bur RNA-world models have problems. One is that RNA would seem unlikely
to form on the prebiotic primordial earth. Another is that RNA cannot
readily make new copies of itself in the laboratory without a great
deal of help from scientists. (For one thing, RNA replication calls for
pure conditions that can be provided in a laboratory but would not be
expected in the nature.)
Still, let's suppose that a self-replicating molecule (which might be
or might not be RNA) did arise on the primordial earth. What might we
expect it to evolve into? To gain some insight into this, I introduced
evolution into the computer-monkey model.
Darwinian evolution rests on the idea of survival of the fittest, or
natural selection. So I defined the fitness of a monkey-generated
"sentence" by looking at how often the letter-triples of that sentence
appear in English. If a sentence has many frequent triples (like the or
ing), it has high fitness. So if replace infrequent or non-existent
triples (like inz) with common ones (like ing), we increase the
sentence's fitness. Essentially, the closer a sentence gets to real
English sentence, the more fit is.
I used survival of the fittest to simulate how evolution might take
place in a population of twenty monkey-generated sentences. For a
sentence to "give birth," I would simple add to the population a copy
of the sentence that might differ by one letter. The copy would be the
offspring, and the differing letter would correspond to a random
mutation.
I divided time to generations. During each generation, the ten fittest
sentences in the population would each give birth to ten offspring. At
the same time, I cruelly killed off the ten sentences of least fitness,
so that the fit sentences multiplied at the expense of the less fir
ones. This was survival of the fittest.
I began with a population of twenty copies of the sentence
"godern the sky srittailis othicein volumn of the onsmilky way,"
generated by the letter-bonding rule. Here is how the fittest sewnetnce
in the population changed at intervals of 200 generations:
"godern the sky srittailis othicein volumn of the onsmilky way,"
"zodur, the sky mriquat isuothyzet, volum, of the oesmilky way,"
"zodur, the sky wriquat isuothyzed, volums, of the oesmilky way."
"zodur, the sky wriquat invothyzed, volums, of the oesmilky way."
"zodur, the lky wriquat unvothyzed, volums, of theboesmilky way."
"zodur, the lky wriquat anvothyzed, volums, of theboesmilky way."
We see that the sentence is indeed evolving. But unfortunately it's not
evolving into anything meaningful. This process of evolution is simply
not able to generate the complex patterns of actual English speech.
My point is this: Assuming that self-replicating molecules could exist
on the primordial earth, where can we expect their evolution to go?
Nowhere meaningful. Such molecules may indeed evolve and grow
molecularly more fit, but there is reason to think they will evolve
into living cells.
Molecular fitness will have something to do with how strongly the
molecule's bonds hold it together and how well the molecule will
catalyze its own replication. This kind of fitness may increase through
Darwinian evolution. But there is no reason to think that anything will
ever emerge from this, other than modified self-replicating molecules
of the same type. There's no reason to suppose that the
self-reproducing molecules will ever give a rise to something
completely different, such as an elaborate system of reproductive
machinery based on DNA, RNA, enzymes, and the famous genetic
code.
My purpose in giving these examples from sequences of letters is not to
claim they prove anything about the origin of life. Rather, I'm simply
illustrating some of the obstacles that theories of life's origin face.
We can talk about these obstacles in purely chemical terms. Such
discussions are necessarily technical.
So, again, here are the two obstacles we discussed:
1.
Natural rules for bonding between atoms may give raise to simple
biological molecules under social circumstances (as in Miller's
experiment), but they cannot give rise to the complex structures needed
to organism to grow and reproduce.
2. If
some hypothetical molecules were able to jump start theiur own
replication, they might evolve by Darwinian natural selection and
random variation. But no one has given any solid reason to suppose they
would evolve into anything more than better self-replicating molecules.
And, of course, it has not been shown that prebiotic molecular
sef-replication could happen.
In the thirty-eight years since Miller's famous experiment, scientists
have come up with many complicated theories about how life might
originated, but they have failed to overcome these and other
fundamental obstacles. Miller himself tends to disapprove of the futile
speculations of the theorists. He argues that what the origin-of-life
field needs is good experiments that actually demonstrate how life got
started. But such experiments are not easy to devise. "I come up with a
dozen ideas a day," Miller says, pausing to reflect, and I usually
discard eth whole dozen²."
References
1. Horgan, J., 1991, "In the beginning…, " Scientific American, February, p. 117.
