Darwin meets Plato

Plato knew nothing of Charles Darwin and neither knew anything of cells, organelles or proteins. But if both were given an "elixir of knowledge" what might be the course of a conversation between them?

***

Two men sat at a dark, polished wooden table, one of several in the sparsely occupied, cobbled courtyard. A perimeter of shrubs and trees rendered the courtyard edges indistinct, but provided a comfortable shade from the warm sun

"Your wine," said the waiter, placing two bordeaux glasses on the table then, using an ah so, proceeded to remove the cork from a bottle. Addressing the distinguished looking gentleman with the white beard he explained: "A merlot from California" and turning to the tanned individual draped in a himation, "I believe, sir, the farthest region of your Atlantis". The latter raised an eyebrow and eyed the bottle.

When the waiter left, the bearded gentleman picked up the bottle. "Allow me" as he leaned across and poured a quantity into the bottom of the other's glass". He then served himself. They both savoured their drinks, while looking at each other...

"Ah, Plato... of course I am familiar with much of your work, it being something of a staple for many of us in the nineteenth century."

It is most gratifying to know one is appreciated, but what is the nineteenth century?"

It is how we who live some two and a half thousand years after you refer to our era.

As Plato eyed the other, a slow recognition crossed his face. "Carolos Dar-win, I am pleased to meet you, but you have me at an advantage, as I have only just now been given the merest outline of your work and, as yet, have not had time to digest it properly. But perhaps if we were to converse, then the discussion would clarify matters for me."

"Nothing would give me greater delight sir!" said the other

After savouring another sip; "As I understand it, you hypothesise an evolutionary process from the crudest of lifeforms, over time, to the most complex; the one gradually evolving into the other?" asked Plato.

Yes, in broad terms, that is the size of it." replied Darwin

"An evolution from the relative simplicity of an amoeba to the complexity of a human is no trivial proposition. The growth and repair, form and function of cells and organs, the processes such as digestion, immune response and so on is surely a profoundly elaborate, dare I say impenetrable subject."

"I doubt anyone with an understanding of the study would disagree. Therein lies its wonder and much of its allure"

"Perhaps it would be more worthwhile to deconstruct the subject and deal with certain aspects of your comprehensive hypothesis in a more specific manner?"

"Yes, I think that would be far more productive."

"The biological functions, at a cellular level through to a systemic level are mediated by, amongst other things, a tremendous variety of proteins."

"Yes, such is the wonder of life."

"Even so, in a single cell."

"Of course."

"Shall we then specifically discuss the nature of a protein, within a single celled organism?"

"As you will, it appears as appropriate a topic as any."

"Proteins are composed of amino acids, are they not?"

"Indeed."

"And, of the hundreds of amino acids that exist, there are just 20 or so that are used in the formation of proteins.

"As you say."

So let us consider just one, single protein, in the unimaginably complex system that is the living body.

"Yes, I will consider this with you."

"And let us assume a small hypothetical protein of, say, just ten amino acids. So, not really a protein per se, but, rather, an oligopeptide."

"Indeed, a suitably simple starting point."

"Since there are twenty amino acids available for any individual location on the polypeptide chain, there is a one in twenty chance of the correct amino acid being selected by chance for that location."

"yes."

"So for a protein consisting of ten amino acids, there is a 2010 chance of the correct sequence of amino acids being chosen."

"Yes, that would be the probability."

"That would give a one in ten trillion chance of that small oligopeptide forming correctly, by chance alone."

"That would appear to be the correct computation."

"However, in reality, proteins typically consist of several hundred amino acids and so for a realistic, but still relatively small protein of, say one hundred and fifty amino acids, we could run the same calculation again."

"Go ahead, I will follow you."

Well then, ostensibly, one hundred and fifty sites will give 20150 which for ease of calculation is approximately 10195.

"Agreed."

However, to make this calculation more realistic, we will specify the steps involved rather than generalize as we did in our previous simplistic oligopeptide proposition. So for the first step we now have to consider the ratio of functional sequences to nonfunctional sequences, those excellent biochemists have already studied this phenomenon and it turns out to be 1:1074

"No small number."

"The second is step is to consider that amino acids require peptide bonds to create proteins - no peptide bond, no protein. It transpires there is a 50:50 chance of a peptide bond at each site, so for our one hundred and fifty amino acid protein the chance is 1:2149 which for ease of calculation is approximately 1045

"I'm following..."

"Now for step three. Each amino acid has two optical isomers and only the left isomers are suitable for functional protein formation. So again, for our one hundred and fifty amino acid protein the chance is 1:2150 of the correct isomer occuring, which again for ease of calculation we can approximate to 1045."

""I cannot argue with the numbers, so please continue."

"We can now calculate the probablilty of our one hundred and fifty amino acid protein occuring simply by chance.

1074 x 1045 x 1045 = 10164 "

"A considerable number indeed."

"One must further consider that, in isolation, such a protein has no purpose. It's function is defined by its interaction with other proteins, proteins with a similar probability of existence or perhaps with even less probability of existence as our 150 amino acid protein is very small and the majority of proteins are far bigger.

"Indeed so."

"Possibly several hundred amino acids in length, I gather a protein from a muscle arcomere can comprise some 30000 amino acids.

"You have an intersting perspective on the subject."

"A significant issue arises when considering the first protein, how did it arise? There is something of a chicken and egg situation with protein synthesis as additional proteins are required to mediate at various stages in the process. Having calculated the probability of one minor protein, we have to consider the probabilities of the components of the process arising simultaneously and all being in the correct location simultaneously."

"A most complex scenario indeed."

"The probabilites of all this happening by random chance are such as the entire process is, realistically, impossible."

"But, there is a chance, albeit a very slight one so given enough time don't you think that..."

"How much time would you need? Lets look at the probability of that. There is a 10164 probability of our modest one hundred and fifty amino acid protein forming. By way of giving that some perspective, it may be worth remembering there have only been 1016 seconds since the Big Bang occurred. As I understand, the earth is reckoned to be a mere 4 billion years old, most of that without any life whatsoever."

"And your point?"

"Allowing for one thousand or even one hundred thousand random bonds occuring every second, even one hundred million every second, there has not been enough time for our modest protein to evolve far less all the other proteins which are also required for even the most mundane of physiological functions"

"So how do you propose life began and continues to "evolve" for want of a better expression."

"I have no idea. I can only be sure that random spontenaiety is so unlikely as to not be worth serious consideration. One's time would be better spent pursuing alternative hypotheses and theories."
"Tell me", he continued, "Has a functioning protein ever been observed to form spontaneously without the vast molecular paraphernelia observed to be necessary, either naturally or in the laboratory?"

"Never"

"So no-one has any idea whence came the first protein in this supremely intricate mechanism of protein synthesis?"

"Correct"

"I would posit that if an event is observed and the event can subsequently be repeated, or at any rate reproduced, this is the study of science. If an event is observed but for whatever reasons, repetition is not possible this is the study of history. If an event has never been observed and neither is repetition possible, acceptance of such is faith and we are now in the domain of religeon."

"I must aver, as I wrote in 'Descent', when considering the complexity of the eye I believed myself confronted with the handiwork of The Almighty.

"Such is a tempting but, I'm sure you will agree, too evasive a conclusion - and for an active and fertile mind it lacks ... profundity."

I can see this conversation is only warming up, but regretfully, we have finished the wine."

"Perhaps, another bottle?"

"An excellent idea, waiter!"



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