What was the starting-point of this chain of events? There lay the end of this tangled line.'
There is a line that connects us to our ultimate ancestors - some not-yet-alive organisms that inhabited the primitive Earth. No doubt it is a somewhat tangled line: but what sort of a thing is it? What is the nature of the connection within a succession of organisms?
Every organism has in it a store of what is called genetic information. This is a set of instructions about how the rest of the organism, its phenotype, is to be made and maintained. I will refer to an organism's genetic information store as its Library. Man's Library, for example, consists of a set of construction and service manuals that run to the equivalent of about a million book-pages altogether. Simpler organisms, such as bacteria, make do with much less information in their Libraries. But even the thousand pages or so needed for a bacterium is still quite a weighty manual.
The pages in these figurative books are closely printed in a script that uses just four symbols. You can imagine these as, say, the letters a to d filling line after line of page after page with very little apparent rhyme or reason in the order of the symbols. Of course the lack of obvious order allows the possibility that a symbol sequence might be carrying messages of some sort. Although there are suspicions that some Libraries, such as man's, could often do with some crisp editing, there is no doubt that many if not most of the letter sequences do indeed hold messages of some sort. Indeed many such messages have been decoded.
An organism that is big enough to be visible to the naked eye is made up of a large number of compartments, or cells - usually of a variety of sorts with different functions. The materials of our bodies, materials such as skin, bone, blood, nerve, etc., are each made from a few sorts of characteristic cells.
Where is the Library in such a multicellular organism ?
The answer is everywhere. With a few exceptions every cell in a multicellular organism has a complete set of all the books in the Library. As such an organism grows its cells multiply and in the process the complete central Library gets copied again and again.
Indeed the books in the Library can actually be made visible under the microscope just at the moment that cells are dividing and arranging that each of the two new cells will have a complete Library. Just before the cell divides pairs of stubby cord-like structures appear, seemingly clipped together at a point part of the way down their length. Then, when the cell divides, one cord of each pair goes to each of the two new cells. Each of these cords is an instruction manual, one of the enormous books in the Library; and the pairs are two copies of the same manual. Evidently this is all part of a system for an equal share-out.
The human Library has 46 of these cord-like books in it. They are called chromosomes. They are not all of the same size, but an average one has the equivalent of about 20000 pages.
Of course chromosomes are not actually books with pages in them. A somewhat closer analogy for a chromosome would be a closely printed paper tape with the four kinds of letters in one long sequence. If you were actually to type out a tape equivalent to that in a typical human chromosome it would stretch for some 150 kilometres. It would be a long book whichever way you look at it. (Imagine trying to read a book like this on a windy day...)
Although chromosomes may appear as elongated objects the actual message tapes that they contain are much, much longer than the objects seen. The incredibly thin tape in a chromosome is coiled and supercoiled with incredible neatness. It would have to be neat because objects that are as light as these message tapes are continually being violently buffeted about by the molecules around them. (For the tiny components in cells it is always 'a windy day'.)
We humans are ENORMOUS animals. We have several million million cells in us with nearly as many copies, then, of our entire Library currently in print. Each cell equipped with so much information, has a certain autonomy. The cell is a particularly important level in the organisation of large organisms. It is at a level somewhat analogous to the level of an individual in society: a multicellular organism can be thought of as a tightly knit community of cells. We have only a vague idea as to how such a multicellular organisation can be built up and maintained. But at least we can see that the messages that must pass between cells to maintain their collaboration can be fairly simple in principle - messages of the kind ' refer to page so and so and do what it says'.
Fortunately we can forget about the problems of how cells communicate with each other when we are thinking about the origin of life. The important idea here is the autonomous nature of cells. Indeed most organisms on the Earth today are single cells. They can only be seen under the microscope. What we tend to think of as 'life on the Earth' - those organisms that are obvious and visible to us - is a comparatively recent innovation. Although, as already pointed out, there is good evidence for single-cell organisms having been on the Earth 2800 or more million years ago, it seems that it was only about 700 million years ago that well-organised multicellular organisms put in an appearance.
Even among single-cell organisms some are more complex than others. The simplest kinds of free-living forms that we know much about are bacteria. It is natural to wonder whether the very first organisms were not, perhaps, something like our modern bacteria. After all, we can see a general trend in evolution towards more complex creatures. It is sensible to take a hard look at the simplest organisms that we know of. Perhaps then we will be able to define that gap in our understanding that we call 'the problem of the origin of life'.
A favourite creature to talk about is called Escherichia coli. This is not the simplest bacterium, but an amazing amount is known about it. J. D. Watson has estimated that we perhaps know as much as a third of all the chemical reactions that are going on in E. coli - and that is a lot as you will see.
E. coli is a normal inhabitant of the human gut, but is quite capable of living on its own, given suitable nutrients. Only some bacteria are parasites: in this respect they differ from those still simpler 'half-organisms', the viruses, which have to be parasites and so could not have been a first form of life.
'Simple' is a relative term. Even viruses are not that simple, and by any absolute standards, E. coli is not simple at all.
True, E. coli is small by our standards - a rod a thousandth of a millimetre across and about twice as long. It is certainly not ENORMOUS. But it is enormous in its way. It is still vastly bigger than the components out of which it is made.
It is an indication of the sheer complexity of E. coli that its Library runs to a thousand page-equivalents. A better analogy would be a closely typed loop of paper tape: it would be about 10 kilometres long.
If we were to say that we now understand how organisms work, this would not mean that we understand the details of the way in which the masses of information in Libraries unburden themselves into fully working organisms. E. coli's book is only partly read and understood, never mind the books in our Libraries.
No, the more complete understanding is at a more general level. We understand in principle how it is that a machine could reproduce itself in the kind of way that organisms can be seen to. We understand what such a machine has to be like. It turns out, for example, that such a machine, however it is made, whatever it is made of, has to have something like a message tape in it.
Think about it. How can characteristics in parents re-appear in offspring? How could such a thing ever happen - and then go on and on happening for millions of years?
Billy has inherited his father's eyebrows. What does that mean? Father's contribution to the making of Billy was a single sperm - and sperms do not have eyebrows. So what was it that Billy inherited ?
First we have to distinguish between characteristics and determinants of characteristics. Gregor Mendel realised in the 1860s that what must be passed on between generations of people or cats or pea-plants cannot be actual characteristics (tallness, eyebrow shape, flower colour, or whatever) but entities that somehow cause such characteristics to develop as the organism grows from its initial seed'. These entities were to be called genes.
The second crucial distinction to be made is between the inheritance of goods and the inheritance of information. Billy did not inherit his father's eyebrows in the kind of way that he may one day inherit his father's gold watch; but rather as he might inherit, say, the secret of how to make that special toffee that was the backbone of the family's confectionery business. In biology both goods and messages are passed oh from one generation to the next. But it is the messages that are much the most important inheritance: only they can persist over millions and millions of years.
This distinction between goods and information is a case of the ancient distinction between substance and form. While a message may have to be written in some material substance, the message is not to be identified with that substance. The message as such is form. As such it can be reproduced again and again, amplified in principle indefinitely. Through copies of copies of copies... a message may be retained although none of the original material that held it persists.
Forms that can reproduce can be extraordinarily persistent: more stable, in a way, than substances. The complex abstraction that we call Beethoven's Fifth Symphony would not be easily destroyed. You would be sceptical of a newspaper headline that ran 'Fire destroys third movement of Beethoven's Fifth' or 'Opening bars of famed symphony stolen'. Your scepticism would be based on the knowledge that a symphony is not actually a thing; that there are many scores of this symphony in existence (i.e. messages about how to make a performance); and that these scores could easily be reproduced if and when wanted.
This way of persisting, by continually making copies, is certainly part of how organisms succeed. And the reproduction of organisms explains how it is possible for them to have such an extraordinarily complicated way of being. For any other kind of entity it would be hopeless to depend for survival on an intricate interdependence of complex components. Sooner or later something would go wrong and that would be the end of it. So long as a form is uniquely tied to a particular piece of substance, it is vulnerable to accident. (The Mona Lisa really could be destroyed by fire.) But for reproducing beings that caveat does not apply. Reproducing beings can be as complicated as they like. The question is whether their complication tends to improve the effectiveness of their reproduction; that is the only question. How the complexity in organisms arose is another matter; although here too we can begin to see how it might have happened - through natural selection, a process that applies uniquely to reproducing beings.
And we can begin to see how it must be that organisms reproduce. They reproduce through copying the messages that specify them -those very messages that are passed on between generations.
Now it is true that over the shorter term messages are not the only inheritance. There must also be goods, if only the actual books or tapes that hold the messages. Indeed much more than that is needed. The tapes must be read and acted on: a certain amount of automatic equipment will be needed to do this. You can imagine those kinds of machines in automatic factories that carry out instructions fed to them on a magnetic tape. Such machines convert a message into a specific activity. Hence everything can be made by following sufficiently voluminous instructions. Among other things, of course, we have to imagine that these automatic manufacturing machines are able to hammer together brand new automatic manufacturing machines. . .Then at least one of these machines has to be handed on with the messages to the next generation.
One can see, indeed, that when a cell divides more is divided out than just the books of instructions: material over and above the chromosome material is included in each of the two new packages. It is clear that this additional material must contain prefabricated reading and manufacturing equipment.
But the supremacy of the messages remains. Everything in the cell, including all that automatic manufacturing equipment, must be written about somewhere in the Library. If some of these messages happen to have to be read and acted on before a new cell is formed, that is a matter of timing that does not affect the long-term outcome. In the long term, after many generations, all that persist are the messages. Every actual thing, every particular collection of atoms, every particular piece of equipment, every particular water molecule, even every piece of every message tape, will eventually be destroyed or mislaid. Only the messages will survive, the messages themselves: because they are forms, and forms of a particular sort. They can be copies of copies of copies...
So please respect the humble bacterium that is playing this game. It can reproduce, it can evolve. E. colimust have some sort of long-term memory about how to make itself that can outlast its substance. That means that an E. coli must be an automatic factory containing something analogous to control tapes and automatic manufacturing equipment. And that is only part of it. All the equipment must be contained, organised, fed. Pieces for it to work on, energy to drive it, must be provided by the E. coli cell. Apart from the manufacturing machinery that can follow instructions, there has also to be another kind of machinery that instead reprints them - something analogous to a Xerox machine or a tape copier. All these things have to be contrived through the manufacturing machinery duly instructed by appropriate bits of the Library tape.
It may seem hardly surprising that no one has ever actually made a self-reproducing machine, even though Von Neumann laid down the design principles more than 40 years ago. You can imagine a clanking robot moving around a stock-room of raw components (wire, metal plates, blank tapes and so on) choosing the pieces to make another robot like itself. You can show that there is nothing logically impossible about such an idea: that tomorrow morning there could be two clanking robots in the stock-room... (I leave it as a reader's home project to make the detailed engineering drawings.)
There is nothing clanking about E. coli; yet it is such a robot, and it can operate in a stock-room that is furnished with only the simplest raw components. Is it any wonder that E. coli's message tape is so long? (If you remember the paper equivalent would be about 10
2 Messages, messages
kilometres long.) Is it any wonder that no free-living organisms have been discovered with message tapes below '2 kilometres' ?
Is it any wonder that Von Neumann himself, and many others, have found the origin of life to be utterly perplexing ? Consider:
(i) Evolution through natural selection depends on there being a modifiable hereditary memory - forms of that special kind that survive through making copies of copies..., forms which can also be accidentally modified to produce modified effects. It is only effects produced like that that can have a long-term future. There can be no accumulation of appropriate accidents, no kind of progress, without the means to remember.
(ii) Successions of machines that can remember like this, i.e. organisms, seem to be necessarily very complicated. Even man the engineer has never contrived such things. How could Nature have done so before its only engineer, natural selection, had had the means to operate?
We are faced with an if-then-either-or. If life really did arise on the Earth ' through natural causes' then it must be that either there does not, after all, have to be a long-term hereditary memory for evolution, or organisms do not, after all, have to be particularly complex.
As this is a detective story I will not say yet which way the argument will go - whether that ' if will survive and, if it does, whether the 'either' or the 'or' will turn out to be the case.
'My head is in a whirl'. I remarked; the more one thinks of it the more mysterious it grows.'
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