Louis Pasteur’s pasteurisation experiment illustrates the fact that the spoilage of liquid was caused by particles in the air rather than the air itself. These experiments were important pieces of evidence supporting the Germ Theory of Disease.
Pasteur’s powerful dismissal of spontaneous generation may have had an effect, because the search for the origin of life on Earth seemed to become unfashionable for half a century. It may be too strong to claim that two powerful and brilliant essays by well-respected scientists, published within a few years of each other in the 1920s, re-introduced the quest for the origin of life to respectable scientific circles, but they are certainly symbolic of a resurgent interest. Both are entitled ‘The Origin of Life’. The first was written in 1922 by the Russian biochemist Alexander Oparin, but wasn’t translated into English until 1967. The second was written by the maverick, self-experimenting biologist J B S Haldane and published in the Rationalist Annual in 1929. It’s always difficult to choose adjectives to describe Haldane; perhaps it’s best to say ‘brilliant’ and leave it at that. My favourite quote of his concerns a perforated ear-drum, which he inflicted upon himself in a decompression chamber whilst trying to investigate the effects of varying oxygen levels on the human body: ‘The drum generally heals up; and if a hole remains in it, although one is somewhat deaf, one can blow tobacco smoke out of the ear in question, which is a social accomplishment.’
Neither scientist was aware of the other’s work, but they reached similar conclusions in their eloquently argued essays. Both begin by stating the obvious question raised by Pasteur’s assertion that life can arise only from life. Oparin writes:
‘Pasteur’s experiments showed beyond doubt that the spontaneous generation of microbes in organic infusion does occur. All living organisms develop from germs, that is to say, they owe their origins to other living things. But how did the first living things arise? How did life originate on Earth?’
The idea that life could have its origin beyond Earth is raised by both authors, and set aside. It may be correct, as we have already discussed, but it’s not a useful working hypothesis because, as Oparin notes, it ‘is only the answer to the problem of the origin of earthly life and not in any way to that of the origin of life in general’.
Oparin then turns to the difference between biology and chemistry:
‘Do we have any logical right to accept the fundamental difference between the living and the dead? Are there any facts in the world around us which convince us that life has existed for ever and that it has so little in common with dead matter that it could never, under any circumstances, have been formed or derived from it?’
His answer is an unequivocal no.
‘The specific peculiarity of living organisms is only that in them there have been collected and integrated an extremely complicated combination of a large number of properties and characteristics which are present in isolation in various dead, inorganic bodies. Life is not characterised by any special properties but by a definite, specific combination of these properties.’
Haldane is more succinct:
‘The link between living and dead matter is therefore somewhere between a cell and an atom.’
This is very important. If we are to understand life as a physical phenomenon, we must put aside the extraneous complication introduced by our human experience of living. We are not asking questions about consciousness, or the origin of feelings, or morality, or good or evil, or the other infinite complexities generated by life. We should focus only on the difference between an atom and a single cell, and under what circumstances atoms can self-assemble into structures that we would recognise as being alive.
Haldane’s and Oparin’s essays are lessons in how to think carefully about a difficult problem, and it is remarkable how closely their speculations foreshadow current ideas on the origin of life, especially given the limited understanding of biochemistry available to them. The details of reparation and photosynthesis were sketchy at best, and the discovery of DNA was a scientific lifetime away. Both essays suggest the most probable location for the origin of life as a ‘primeval’ or ‘primitive’ ocean where, in Oparin’s words, ‘individual components of organic substances floating in the water met and combined with one another’ until, switching to Haldane, it ‘reached the consistency of hot dilute soup’.
The idea of a ‘prebiotic soup’, Darwin’s warm little pond, supporting the gradual development of ever more complex organic chemistry, energised by ultraviolet light and a reactive atmosphere, is perhaps the most common picture of the origin of life in popular culture today. This is in part due to a famous experiment carried out in 1953 by Nobel Prize-winning chemist Harold Urey and his PhD student Stanley Miller at the University of Chicago. It’s perhaps not surprising that the Urey–Miller experiment immediately captured the public imagination, comfortably eclipsing Crick and Watson’s discovery of the structure of DNA that same year. Haldane closed his essay by creating a vivid and compelling picture of what they were attempting: ‘The above conclusions are speculative. They will remain so until living creatures have been synthesized in the biochemical laboratory. We are a long way from that goal.’
Louis Pasteur, the father of pasteurisation, and one of the first to expound the theory of ‘all life from life’.
Urey and Miller constructed a model primeval ocean inside a 5-litre sterilised glass flask filled with methane, ammonia and hydrogen to simulate the highly reactive reducing atmosphere that was thought to have existed on the young Earth. A pair of electrodes sent continuous sparks into the flask, mimicking the presence of lightning. The resulting ‘soup’ was then delivered into a cooler flask, the ancient ocean, from which samples could be extracted. The apparatus is shown on here.
A replica of Pasteur’s sealed tube equipment which he used in his experiments to demonstrate that germs are the cause of disease and decay.
After a single day, the primeval ocean in the flask turned an intriguing shade of pink. The experiment ran continuously for just over a week, at which point the ocean in the sterilised flask was tested for signs of organic life. Urey and Miller found amino acids, the building blocks of proteins, the basic components of life. The public response to the experiment was one of great excitement; Miller appeared on the front of Time magazine in 1953, whilst Crick and Watson had to make do with the less glamorous pages of Nature. It’s easy to see why. The Urey–Miller experiment had all the hallmarks of a microbial Frankenstein; the fundamental building blocks of life created from lifeless atoms by a vital spark of electricity. Perhaps if the soup were left for long enough something would crawl out.
Stanley Miller’s apparatus enabled a glass flask containing methane, ammonia and hydrogen to stimulate the reducing atmosphere of Earth and a flask of heated water which created vapour, and a pair of electrodes to mimic the presence of lightning. The ‘primordial soup’ was then delivered into the closed system where it was cooled and condensed into the trap at the bottom.
Watson and Crick with their model of part of a DNA molecule in 1953, the year they published their conclusions on the structure of DNA.
Sixty years on, the Urey–Miller experiment still casts a long shadow over the search for the origin of life. The imagery of the shadow is probably appropriate, because the basic premise of the Urey–Miller experiment is probably wrong. The evidence from the zircons informs us that Earth’s primordial atmosphere was not a reactive chemical cocktail of ammonia, methane and hydrogen. On top of that, the idea that a mixture of amino acids, gently prodded by ultraviolet light and lightning would, over millions of years, coalesce into something as complex as a living cell is highly unlikely. As Nick Lane puts it in his superb book, Life Ascending, if you take a sterilised tin of soup from your shelf and leave it alone for a few million years, perhaps zapping it occasionally with electricity, all that will happen is that the constituent molecules will break apart. It is not very likely that something more complicated than the original constituents will appear. Th
e problem is one of physics, or to be more precise, the branch of physics known as thermodynamics.
Stanley Miller is shown here working in his laboratory at the University of Chicago.
Life, Thermodynamics and Entropy
Even the simplest living cell is an intricate, highly ordered structure. The smallest living things on Earth are bacteria of the genus Mycoplasma. They are only two ten-thousandths of a millimetre across, which still makes them over a billion times the volume of a carbon atom. The simplest known living cells in terms of the number of basic biological building blocks are symbiotic bacteria known as Carsonella ruddii, which contain only 182 different proteins. This isn’t many, given what they have to do, which is to replicate, amongst other things, but they are still extremely complex objects made up of billions of individual interacting atoms.
Planetary nebula at the end of its life around 700 light years from Earth in the constellation of Aquarius.
The problem with the primordial soup hypothesis is that something as complex as a single cell will not emerge by chance in an isolated, gently stewing pond, no matter how long you wait. The physics behind this assertion is encoded into one of the fundamental laws of nature, known as the second law of thermodynamics. It states that things become more disordered as time passes. A broken egg never reassembles. A dead bird decays. I’ve lost count of the number of times it’s been pointed out to me that a song I was involved in producing many years ago called ‘Things Can Only Get Better’ runs counter to the second law of thermodynamics. I accept that this is the case. Things Can Only Get Worse, all things considered.
Ludwig Boltzmann created the equation for entropy, which is now etched on his grave.
The second law of thermodynamics is often stated in the following form: The entropy of an isolated system never decreases. Roughly speaking, entropy can be thought of as a measure of how many ways the component parts of something can be arranged such that it looks the same, and this is a measure of how ordered the thing is. Higher entropy means more disordered, while lower entropy means more ordered. Living things are very highly ordered. The Austrian physicist Ludwig Boltzmann formulated this definition, and the expression for calculating the entropy of a system in this way is written on his grave in Vienna:
S = kB ln W
S is the entropy, W is the number of ways of arranging the components such that they give rise to the same outcome, kB is a constant of proportionality known as Boltzmann’s constant, and the symbol ln stands for natural logarithm. A higher entropy configuration corresponds to lots of ways of arranging things; a lower entropy configuration corresponds to fewer ways of arranging things.
An example might make this clearer. Think about the molecules of air in a room, all whizzing around and bumping into each other. Each molecule moves around the room at random, and could end up anywhere with equal probability, given enough time. It is very unlikely that all the molecules will end up in one corner by chance, leaving the rest of the room as a perfect vacuum. Why is this so? The answer is one of simple statistics. Allow me to introduce two little pieces of jargon, because it makes everything a lot clearer and easier to write about. This is the only excuse for jargon.
Each unique configuration of molecules in the room is known as a microstate of the system. If we want to describe a particular microstate, we need to know the positions and velocities of every single air molecule. We might decide, quite rightly, that this is not something we’re particularly interested in. We’re more interested in things we can observe, like the temperature and air pressure distributions in the room. This more coarsely defined, but more practical characterisation of the state of the room is known as a macrostate.
If each particular configuration of air molecules – each microstate – is equally likely to occur,2 then it follows that the room will be more likely to be in the macrostate that corresponds to the largest number of microstates. Even if we started out with all the molecules in the corner, over time they would end up filling the room. Our system will always head towards the macrostate that consists of the highest number of microstates, which is to say that it will always increase its entropy. The W in Boltzmann’s formula is the number of microstates corresponding to a given macrostate.
Boltzmann’s 1898 molecule diagram showing atomic overlap.
This is the content of the second law of thermodynamics, and it’s hard to argue with it, which is why the physicist Sir Arthur Eddington once said,
‘If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations – then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation – well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.’
NATO observers at a Boltzmann event at the Nevada test site in 1957.
Life appears to run counter to the second law of thermodynamics, because living things are highly ordered. They are macrostates that correspond to a very few microstates, and therefore have a very low entropy. ‘Did I request thee, Maker, from my clay to mould me man?’ An 80kg lump of clay may have all the ingredients necessary to build a human being (it doesn’t, but this is a metaphor), but most random reconfigurations of the ingredients will result in differently configured but indistinguishable lumps of clay. We’d be surprised if we got lucky and arrived by chance in the very particular configuration of ingredients that can sit up and start considering the origin of life. A human seems to be a gross violation of statistical common sense, a physicist doubly so, although I’ve been called worse. A bacterium is not much better. That’s also been said. We’ve taken a little literary latitude here to make a point, however. As we have already noted, we shouldn’t get confused by trying to explain how an organism as complex as a human being emerged from some sort of primordial clay ‘in one go’, because evolution by natural selection does most of the work. Natural selection is a non-random process and one that can drive increases in the complexity of living things quite astoundingly quickly. Having said that, evolution by natural selection has to get going in the first place, and this certainly requires some form of genetic code that can pass information down the generations, as well as all the associated proteins and machinery needed for the copying and replication of genes. We do seem to have a problem.
One of the first scientists to think carefully about this apparent paradox and to offer a solution was Erwin Schrödinger, who is best known for his foundational work in quantum theory. In 1943, Schrödinger gave a series of lectures at Trinity College, Dublin, in which he posed the question: ‘How can the events in space and time, which take place within the spatial boundary of a living organism, be accounted for by physics and chemistry?’ The answer, as Schrödinger noted, is that the events within the boundary of an organism cannot be understood in isolation, because organisms are not isolated systems. They can be understood only when viewed as intimately and essentially coupled to their external environment. If I am allowed two literary allusions in a single sentence without performing the statistically unlikely feat of transforming into Morrissey, I might counter Milton with John Donne; a maker is not required to mould a man because no man is an island.
If you take the 7 x 1027 atoms that make up the average human – mainly oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus, potassium, sulphur, sodium, chlorine and magnesium – and throw them into a box, the result will be a high-entropy uniform distribution of atoms, just like the air molecules spread uniformly about a room. It will be very difficult to encourage them all to ‘get into the tiny corner’ that corresponds to a human being. You might be able to encourage some of the atomic ingredients to form structures by throwing a match into the box, however; there would be a bang as hydrogen and oxygen bind together to form water, but you’d be rightly surprised if a man emerged.
In the summer months, Alpine ibex (Capra
ibex) clamber up the vertical dam at Lake Cingino in Italy to 2200m altitude to lick the salt that seeps through the stones, so as to get the essential minerals that have been dissolved in water, minerals rich in the calcium that they need to stay strong.
And yet a molecule of water is a lower-entropy arrangement of two hydrogen atoms and an oxygen atom than would be the case if they weren’t bound together, and that appears to run contrary to the second law of thermodynamics. What has happened here? The answer is very important. Whilst the entropy of the system of atoms has been lowered by the chemical reaction, a large amount of heat was released. In the jargon, an exothermic reaction has taken place. It went bang. This heat is absorbed by the surroundings, increasing the entropy of the environment by more than the entropy decrease associated with the formation of the water molecules. The entropy of the entire system increases, in accord with the second law.
Living things work in the same way from a thermodynamic perspective. They can become more ordered as long as they pay their debt by exporting disorder in the form of heat into the Universe. You are exporting disorder now as you read this book. You are hastening the demise of everything that exists, bringing forward by your very existence the arrival of the time known as the heat death, when all stars have died, all black holes have evaporated away and the entirety of creation is a uniform bath of photons incapable of storing a single bit of information about the glorious adolescence of our wonderful Universe. You are doing this by burning food in oxygen from the air. This is an exothermic reaction, generating plenty of heat for export and more than compensating for the temporary low entropy configuration of your wasteful, highly ordered body. I do seem to be turning into Morrissey: What are the odds?
Forces of Nature Page 16
