Quote of the Day: The origin of life genome size catch-22

If imitation is the sincerest form of flattery, then Denyse O'Leary won't mind me imitating her Thinkquote of the Day. In compiling my `Evolution Quotes Book' I am finding some great quotes (both old and new) that I would like to share. A Quote of the Day post would be a way for me to do that.

So here is the first one:

"Replication is not perfect. If it were, there would be no variation for selection to act on. But initially the problem would have been too much mutation, and not too little. Most mutations reduce fitness. Selection is therefore needed to maintain a meaningful message. The old game of Chinese whispers demonstrates that, without selection, the result is chaos. How accurate must replication be? Imagine a message-for example, a DNA molecule-that replicates to produce two copies of itself. The two copies replicate to produce four, and so on. During replication, miscopying occurs, and the erroneous copies that result are eliminated by selection. Only perfect copies survive. It is clear that, after each copying, at least one copy on average must be perfect. Otherwise selection cannot maintain the integrity of the message. This places an upper limit on the permissible mutation rate per base copied, or, equivalently, an upper limit on the length of the message, for a given mutation rate. If the genome size, or the mutation rate per symbol, rises above this critical upper limit, the result is an accumulation of mutated messages. This is what Manfred Eigen and Peter Schuster have called the `error threshold'. It is easy to see roughly where this upper limit lies. The requirement is that at least one perfect copy, on average, must be made at each replication. If there are n symbols, this means, approximately, that the probability of an error when replicating a symbol must be not greater than 1/n. In other words, if the genome contains 1000 bases, the mutation rate per base, per replication, must be not greater than 1/1000. The error rate in experiments ... is in the range 1/1000 to 1/10 000. This would permit a genome between 1000 and 10 000 bases. But this involves replication by an enzyme; if there is no enzyme, the error rate is much higher. ... The error rate depends on the medium, the temperature, and so on, but very roughly the wrong base pairs ... once in 20 times. This implies that, before there were specific enzymes, the maximum size of the genome was about 20 bases. At first sight, this is a serious difficulty, and so it was long regarded. It presented a kind of catch-22 of the origin of life. Without a specific enzyme, the genome size is limited to about 20 bases; but with a mere 20 bases one cannot code for an enzyme, let alone the translating machinery needed to convert the base sequence into a specific protein." (Maynard Smith, J. & Szathmáry, E., "The Origins of Life: From the Birth of Life to the Origin of Language," Oxford University Press: New York NY, 1999, pp.34-36)

I don't intend to comment much on these quotes of the day, but to head off the evolutionist "out-of-context quote" brigade, I must mention that Maynard Smith and Szathmáry go on to claim that:

"An escape from this catch-22 of the origin of life came with the discovery that RNA molecules can act not only as templates for copying but also as enzymes" (p.37).

I have another quote by a leading evolutionist to rebut that, but I don't want to start posting multiple quotes in these quotes of the day.

In any case, accepting for the sake of argument Maynard Smith and Szathmáry's point, the significance of this quote is that either: 1) A RNA ribozyme must be able to spontaneously generate above the 20-base error threshold and then make at least one perfect copy of itself every replication (and there is no evidence that it could); or else 2) there is no escape from the naturalistic catch-22:

"Without a specific enzyme, the genome size is limited to about 20 bases; but with a mere 20 bases one cannot code for an enzyme"

and so design is the only remaining alternative!

Stephen E. Jones, BSc (Biol).
`Evolution Quotes Book'