Quantum Evolution, Johnjoe McFadden. Chapter 4

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Quantum Evolution - Chapter 4 - How did we get here?

‘… But, as I described above, there is evidence for biological carbon-fixing life in rocks 3.85 billion years ago which appears to place the origin of life right back at the tail end of the late bombardment. If the evidence from the Isua rocks is to be believed, then, as close the geological record can tell us, life emerged on Earth as soon as it was possible. The rapid appearance of life on Earth places an important constraint on theories to account for its emergence. We cannot, I believe, rely on extremely unlikely scenarios to get us out of our difficulties. Life’s rapid emergence implies that once conditions on Earth were suitable, life was probable.

Now we come to the crux of the problem. Where did the proto-cell come from and how? The standard scenario (which we will examine more closely below) envisages life spontaneously emerging from a primordial soup of chemicals in the ancient ocean. Somehow, we must account for the spontaneous emergence of a cell with enzymes, ribosomes, RNA and DNA with all the 500 or so genes of the proto-cell. How did this structure emerge from the chemical soup?

I am sure I don’t need to remind you of all those busily typing monkeys to persuade you that an organism with 500 genes, each made up of about 1,000 DNA bases could not ever have arisen entirely by chance. A billion universes each populated by billions of typing monkeys could not type out a single gene of this genome. Hoyle and Wickramasinghe (see below) describe the likelihood of the event as equivalent to the chances that a tornado sweeping through a junkyard might assemble a Boeing 747.

The simplest living cell could not have arisen by chance. Just like the eye, the proto-cell must have evolved from simpler ancestral cells, presumably by a process of natural selection. But this is where the first big problem with the origin of life arises. What were those simpler entities? Darwinian evolution depends on its gradualism. Each small step in the evolutionary ladder must be viable and each must represent a tiny improvement on its progenitor. This is why we do not need to examine the fossil record to find the antecedents of the modern eye. They are all around us. Those simpler eyes are still in use precisely because they are viable structures that do the same job of seeing today that they did when they first evolved millions of years ago. If the proto-cell arose by Darwinian evolution from simpler ancestors then each ancestor must similarly have been viable and each must have represented a small advance that was selected by the process of natural selection. What has happened to these ancestors of the proto-cell? Why don’t we see any of them today? If microbes that lived 3.85 billion years ago could replicate with less than 500 or so genes, why don’t today’s microbes survive with fewer genes?

The problem in a nutshell is that today’s microbes need at least several hundred genes to grow and replicate. We have no reason to believe that the proto-cell, the last common ancestor of all cellular life, used fewer genes. How did life make the leap from the primordial chemical soup to the proto-cell?’

[on complexity theory]

‘…A relative newcomer to the origin of life field is complexity theory. Most people are familiar with its alter ego, chaos theory, in the guise of the butterfly effect, in which a butterfly flapping its wings in an Amazonian rain forest could start a chain of air disturbance that eventually causes a hurricane to strike on the other side of the world. The basis for the effect is the extreme sensitivity of chaotic systems, like the weather, to starting conditions. The other side of the chaos coin is that very complex systems can spontaneously generate order rather than chaos: order for free.

The weather is a complex system generated by the random movements of trillions of molecules of air and water. Even if the positions and velocities of every one of these trillions of molecules were known at one point in time it would be a computationally impossible task to calculate the state of the weather a few moments later. The problem is too complex and the solutions are too chaotic. However, any meteorologist could tell you that, at any particular time, there is likely to be an anticyclone (an area of high-pressure) over the Azores. Although complex systems, like the weather, have a near-infinite number of possible states, they have a tendency to fall into attractors, of simple ordered behaviour. The Azores anticyclone is such an attractor. Despite the near-infinite number of ways that air can travel around the Atlantic, the most stable pattern is a weather system centred over the Azores.

The ability of complex systems to generate order is proposed to be involved in phenomena as diverse as chemistry, meteorology and world economics. The spontaneous order of complex systems has some similarity to life. The Azores anticyclone is an ordered structure that feeds on the winds, just as living cells are ordered structures that feed on chemicals in their environment. Both the anticyclone and living cells are dynamic systems rather than static structures; both are continually renewed by the material flowing in and out of them.

There has been a glut of popular books on complexity in recent years so I refer the reader to any of dozens of them to gain a fuller understanding of this fascinating field. Our interest is the claim made by many complexity researchers that complexity’s order for free generated the first living cells. Stuart Kauffman of the Santa Fe Institute is one of the leading proponents of complexity. His theory of the origin of life starts with primordial soup containing billions of different kinds of molecules. In such a complex system it is quite likely that some molecule, say A, will catalyse the formation of some other molecule B. It may also happen that B will happen to catalyse the formation of C which will go on to catalyse D and so on in a series: A ® >B ® C ® D ® E ® >F ® G etc. However, in such a complex system there is also a possibility that one of the components along the series (say F) will happen to also catalyse the formation of A from the primordial soup, giving catalytic closure of the cycle, A ® ® >C ® D ® >E ® F ® >A ® B ® >C… and so on. The resulting autocatalytic set could continually perpetuate itself by >feeding on the primordial soup to form a kind of anticyclone of interlocking chemical reactions. The sets could even replicate whenever a few drops of the soup containing one autocatalytic set splashes into another pool to start a new cycle. New chemicals invading a cycle would initiate mutations leading to new and more complex sets. Eventually a genetic take-over could have coupled one of these catalytic sets to RNA or DNA. The autocatalytic set would have become enclosed within membranes and the first living cell was born.

The ability of complex systems to spontaneously generate order is impressive. I remember being mesmerised by one example when I tried it out in my laboratory: the Belosov-Zhabotinski chemical reaction. The reaction is very simple; you mix a few chemicals to make a purple solution in a shallow dish and wait a few minutes. First you see tiny blue dots that grow into a series of circles and waves that soon fill the entire plate. It seems almost magical that a featureless dish of inky water spontaneously generates these ordered patterns and waves of oscillating colour.

Many aspects of the natural world almost certainly depend on this self-organisation. It is probably involved in many aspects of biology, particularly ecology and embryology; but is it capable of generating life? Kauffman and other complexity theorists bolster their ideas by performing computer simulations in which they show that autocatalytic cycles do spontaneously emerge from their digital primordial soups. However, the problem with much of complexity theory is that it is too rooted in this kind of digital simulation and takes little regard of wet life. Computer demonstrations of self-organisation can be found on hundreds of web-sites but no one has yet managed to find a complex chemical system that spontaneously generates an autocatalytic set. Yet it ought to be easy. Complex chemical systems are generated every time you bake a cake or boil a saucepan of soup. The gunk that forms the predominant product of most primordial soup experiments is a highly complex chemical system. Yet, no autocatalytic sets have (as far as we know) emerged from any of these complex chemical systems. My guess is that the spontaneous emergence of autocatalytic sets is only feasible in computers, where each set can be isolated from the jumble of reactions going on around them. In real chemical soups, each component gets caught up in a thousand side reactions with gunk that inevitably dilutes and dissipates any emerging autocatalytic sets.

A second and more important objection I have to complexity theory, as a theory to explain the phenomenon of life, is that it is not relevant to the generation of ordered structures inside living cells. As we shall be exploring in Chapter 6, the self-organisation of either the Belosov-Zhabotinski reaction or the Azores anticyclone is generated by the random interaction of billions of molecules. They are phenomena of big numbers of particles and have structure only at the macroscopic scale; at a molecular level there is only chaos and random motion. Yet, as we shall discover in the next chapter, cells have ordered structures all the way down to the level of fundamental particles. The macroscopic structures of living cells are not generated by random incoherent motion but by the directed motion of individual particles. Life is a phenomenon of small numbers and must be described by a different set of rules than complexity theory. It seems to me to be entirely unlikely that a system that generates order through random incoherent motion could have spontaneously given rise to a system – life – that generates order by an entirely different process.’