Forces of Nature

by Professor Brian Cox

  A human being is primarily made up of a pile of elements, of ingredients that come from the Earth.

  The Earth’s oxygen atmosphere is absolutely necessary for the heat-generating, entropy-exporting reactions that allow us to maintain our complex structure, and this is a crucial insight. We appear to ‘defy’ the second law of thermodynamics, but we do not because we are not isolated systems. We are part of a larger, out-of-equilibrium system. The oxygen atmosphere is unstable and ready to react with pretty much anything, given a little nudge. We exploit this imbalance to create and maintain our highly ordered structure, and as long as we keep breathing we can do so, at the expense of radiating a large amount of heat. We are like little water wheels, exploiting a waterfall to power our internal factories. If the waterfall dries up, the wheel stops and the factory falls to bits.

  The unstable oxygen atmosphere is constantly replenished by photosynthesis, itself a biological marvel that we will investigate in some detail in Chapter Four. Photosynthesis is a remarkable process from a thermodynamic perspective. Plants and algae build complex sugars from carbon dioxide and water, decreasing the local entropy and releasing highly reactive oxygen and heat into the atmosphere in the process. How is this possible? Because of the presence of a waterfall – in this case, the temperature gradient between the surface of the Earth and the Sun. Photosynthesis, which sits at the base of the entire food chain on Earth today, is possible only because there is a great external imbalance; in this case, a glowing source of photons 93 million miles away in space.

  In summary, living is possible from a thermodynamic perspective because the natural environment is grossly out of equilibrium. Living things exist in the imbalances, exploiting them to build and maintain their complex structures as a mill uses a waterwheel to extract useful energy from a cascading waterfall, increasing the entropy of the entire system as it does so. In the context of the origin of life, this observation is highly suggestive. Life probably didn’t begin in a gently stewing pond, because the thermodynamic gradients are too gentle to drive the emergence of complexity. Living things need to be coupled into a steady, powerful gradient from the external environment in order to build and maintain their complex structures.

  Zircon crystals capture a snapshot of the environment in which they are created, locking away a record of the chemical make-up of our planet at any given time.

  The external gradients that most living things exploit today were not available to the first organisms. There was little or no oxygen in the atmosphere because photosynthesis put it there, and photosynthesis, the means by which life exploits the gradient between Sun and Earth, is an incredibly complex biochemical process that surely couldn’t have predated life. The search for the origin of life therefore becomes a search for a gradient; a naturally occurring imbalance generated by Earth’s geology that may have provided the spark of life; a geological cradle with a steady energy source that could drive geochemistry up the thermodynamic hill towards biochemistry.

  We commented earlier in the chapter that Darwin’s theory of evolution by natural selection provides the conceptual framework for the scientific exploration of the origin of life. Recall Darwin’s famous lines, ‘Therefore I should infer from analogy that probably all organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.’ This putative primordial form is a population of living things known as LUCA: the Last Universal Common Ancestor of all life on Earth.

  The unbroken chain of life, stretching back 4 billion years, offers an interesting possibility. If LUCA existed, we might hope that the ensuing 4 billion years of evolution by natural selection has not removed all trace of its original biochemistry. There may be commonalities that all extant organisms share, and if so, it’s likely that LUCA possessed them, too. Furthermore, if evolution, the eternal tinkerer, has not managed to replace such processes in any of the endless forms most beautiful that it has delivered during the last 4 billion years, then we might feel at liberty to conclude that these processes are fundamental and necessary components of all life. As such they would be a smoking gun, connecting living things today across 4 billion years, a third of the history of the entire Universe, to the warm little pond.

  Life probably didn’t begin in a gently stewing pond, because the thermodynamic gradients are too gentle to drive the emergence of complexity.

  The genetic code, DNA, is one such commonality. All living things share it, from bacteria to people. There is also another, rather more surprising, thing that we all share, and that has to do with the way we manage our energy. Given what we’ve said about the central importance of thermodynamics to life, this is an exciting and significant observation. There is a common energy management system, and the suggestion is that this is a relic of the conditions present in the cradle of life on Earth. Living things are like books, frozen moments replete with clues about their evolutionary history. In every bacterium cell, in every blade of grass, in every cell in your body, the story of the evolution of life on Earth is documented, incompletely to be sure, but the narrative is not completely erased. Let’s follow this thread to see where it leads, and explore the way that living things manage their energy.

  The Moth and the Flame

  At first glance, the energy-generation mechanisms employed by living things seem quite straightforward. Let us for the moment focus on animals. We burn food in air to release energy, carbon dioxide and water. The basic chemical reaction is shown below. Glucose reacts with oxygen to form carbon dioxide and water, with the release of energy. This is known as an oxidation reaction. As we discussed in Chapter One, oxygen atoms are rather keen on acquiring electrons, and will do so if they are given the opportunity. The ‘burning’ of sugar can be thought of as sugar molecules transferring electrons to oxygen molecules; the sugar is ‘oxidized’, and the oxygen is ‘reduced’. If you remember anything about school chemistry, you’ll probably remember ‘redox’ reactions, and this is an archetypal example. Redox reactions are all about the transfer of electrons, and so is life.

  The basic chemistry of aerobic respiration. Glucose is oxidized to produce carbon dioxide and water, with the release of energy.

  The oxidation of candle wax, producing carbon dioxide and water, with the release of energy.

  There are rare occasions when the necessity to find a visual texture for a television programme delivers more than wallpaper. This is one such occasion. The story of the origin of life, perhaps inevitably, has a gothic tinge. I’m not sure whether this comes entirely from Frankenstein or whether there is something innately unsettling about the subject that leads inexorably into the shadows. Even Genesis is quite Bauhaus at the beginning: ‘And the Earth was without form, and void; and darkness was upon the face of the deep’, although it turns a bit Hendrix when the lights come on and everything is told to get fruitful and multiply: ‘Behold, I have given you every herb …’.

  The title of this chapter comes from a visual metaphor we used during filming that goes to the heart of one of our central questions. What is the difference between living and inanimate matter? What is the difference between a moth and a flame? The basic chemical reaction that powers a moth is the oxidation of glucose. The chemical reaction that powers a candle flame is an oxidation reaction of precisely the same type, as shown below.

  In both cases, electrons are transferred from a long-chain carbon molecule and onto oxygen, but in the case of respiration, some of the energy released in the chemical reaction is syphoned off and used to live. The process by which this happens is intricate, to say the least. In a living thing, the electron doesn’t just jump straight onto the oxygen, releasing all the energy at once. That would be a flame. Instead, the electron is passed between a series of atoms – usually iron – embedded in proteins that tune their appetite for electrons. There is nothing uniquely biological about iron atoms transferring their electrons to oxygen; it is known as rusting. The clever thing is the way that biological structu
res tune the chemistry by embedding the iron atoms in complex molecular structures, enabling them to control the flow of electrons and harness them to do useful things. This chain of embedded iron atoms, which contains around 15 steps in most organisms, is known as the respiratory chain, and it is used not only in respiration in animals, but also in photosynthesis. In one form or another, it is common to all life, and therefore certainly very ancient. All life uses redox chemistry to extract electrons from something and transport them onto something else via respiratory chains.

  There is another component to the energy-management system of life that is even more intricate, and also universal. All living things store part of the energy delivered down the respiratory chain by the flow of electrons in molecules known as adenosine triphosphate, or ATP. These molecules are the universal batteries of life, transferring stored energy around your body and releasing it as needed. The way ATP molecules are manufactured is, to say the least, odd, complicated and, to be honest, downright weird. One of the great joys of making television documentaries is that I get to learn about science outside my field. I still recall how I felt when I read for the first time about how cells manufacture ATP; it was like learning that the carbon atoms in my body were manufactured in the cores of long-dead stars. It is such a wonderful story that it seems it can’t be true. And, just as the fact that we are all made of starstuff connects us to the great spatial sweep of the Universe, so the story of the manufacture of ATP connects us to the great temporal sweep of the history of life on Earth. It points us back all the way to the warm little pond. Here it is.

  ATP Synthase, an exquisite biological machine, shared, along with DNA, by every living thing on the planet.

  An ATP molecule. The orange balls are phosphorous atoms, red are oxygen, blue are nitrogen, black are carbon and white are hydrogen. Some of the hydrogen atoms are omitted for clarity.

  As the electrons are passed down the respiratory chain they are used to pump protons across membranes. For every pair of electrons that makes its way through, ten protons are pumped. The proton gradients are huge. In the vicinity of the membranes, which are only 6 billionths of a metre thick, the electric field strength is 30 million volts per metre, which is roughly what you’d experience if you got hit by a bolt of lightning. This great reservoir of proton potential is used to power a machine known as ATP Synthase, a nano-factory that mints new ATP molecules out of two ‘empty’ molecular battery components known as ADP and Pi. The protons cascade from their reservoirs down great waterfalls, spinning the waterwheel of the ATP Synthase machine at over 100 revolutions per second. The illustration, left, shows a picture of this exquisite biological machine, shared, along with DNA, by every living thing on the planet.

  ‘Life is nothing but an electron looking for a place to rest’

  – Albert Szent-Györgyi

  The intricate chemistry and structure of the respiratory chain, and in particular the use of the proton waterfalls through ATP Synthase to manufacture ATP, the universal battery of life, is surely telling us something about the deep history of life. Recall that we are searching for clues in the biochemistry of living organisms today that might point to the biochemistry of LUCA, and we have found one; the universal use of proton waterfalls as the energy source for the production of ATP. As we emphasised earlier, thermodynamics is key to understanding how life works, and surely how it began. Living things are the most complex physical structures we know of anywhere in the Universe, and building complexity spontaneously from simple building blocks is a delicate business. Life achieves it, in accord with the unbreakable second law of thermodynamics, by using redox reactions to pump protons around. This must be telling us something about how it got going in the first place? You’d be surprised, after all this, if the answer was no!

  LIVING THINGS ARE THE MOST COMPLEX PHYSICAL STRUCTURES WE KNOW OF ANYWHERE IN THE UNIVERSE, AND BUILDING COMPLEXITY SPONTANEOUSLY FROM SIMPLE BUILDING BLOCKS IS A DELICATE BUSINESS.

  A Very Different Eden

  Everything we’ve discussed in this chapter so far is established science. You will find it all in textbooks. We are now going to bring everything together and present a theory of the origin of life on Earth. This is still science, but science at the cutting edge. Some biologists agree with this theory and some don’t, and this is as it should be when new ideas are in the process of forming and being tested. The theory may turn out to be wrong, and if so, its proponents will be delighted because they have learned something about Nature. It didn’t happen this way. Real scientists are delighted when they find out they are wrong, and to me that is one of the greatest gifts that a scientific education can bring. There are too many people in this world who want to be right, and too few who just want to know.

  Past the bioluminescence, beyond the Sun, the lights of the Alvin submarine brought a world of rock chimneys and tubeworms into view.

  Let’s revisit the logic of the argument. We assume that life began on Earth, and we have evidence that this happened at some point earlier than 3.5 billion years ago. We know that the thermodynamic barrier to complexity is great, and we know that in order to overcome this, life must operate in an out-of-equilibrium system; it exists in a waterfall. Today, the waterfalls are the oxygen atmosphere and the Sun, via photosynthesis, and neither was available to the earliest life. There are other waterfalls hidden inside living things – the proton waterfalls that power the great ATP synthase nano-factories – and these are universal; everything on Earth today, with a very few exceptions, uses protons. This suggests that we are looking at very ancient biochemistry; the biochemistry of LUCA.

  Hydrothermal alkaline vents – one of the few places on Earth where you can see how life could have emerged from a restless young planet.

  Today, living things go to extraordinary lengths to create their internal proton waterfalls, using the complex machinery of the respiratory chains, but what if this is a later addition? What if the original energy source that drove life up the thermodynamic hill from geochemistry to biochemistry was a proton gradient? This leads to the question: Was there a place on Earth 3.5 billion years ago where naturally occurring proton gradients could have been harnessed by the first biological machines, allowing for the foundations of life to spontaneously emerge, all the way up to and including DNA, the prerequisite for evolution by natural selection? The answer is yes. What is more, such places still exist on Earth today, and we can visit them.

  Hydrothermal vents are cracks in the ocean floor where freshwater heated by geothermal energy to over 300 degrees Celsius meets the cold salt water of the sea. I visited a vent system whilst filming Wonders of the Solar System in 2009, 2000 metres down in the Sea of Cortez, just off Mexico’s Baja Peninsula. Past the bioluminescence, beyond the Sun, the lights of the Alvin submarine brought a world of rock chimneys and tubeworms into view. It is an ecosystem founded upon clever bacteria that can drag electrons off volcanic hydrogen sulphide – redox reactions again – leaving residue mats of yellow sulphur across the vent fields. Vents like these are known as black smokers, after the particles they bellow out into the ocean.

  In December 2000, whilst diving on the submerged mountain range known as the Atlantic Massif between Bermuda and the Canary Islands, Alvin discovered a different sort of vent system. There are great towers of calcium carbonate, some 60 metres high, raised by warm waters rich in minerals and reactive gases bubbling up from the deep crust. There is something of the fairytale spires about the place, which is why it was named the ‘Lost City’.

  The chemistry of the Lost City vents is different to that of those I visited in the Sea of Cortez. The waters in the vents are much cooler, around 90 degrees Celsius, because the vents are not volcanic in origin. Chemical reactions between warm water and the rocks of the sea floor saturate the structures with methane and hydrogen gases, rather than the volcanic hydrogen sulphide of the black smokers. The conditions are rather like those in the Urey–Miller experiment, which led to a broth of amino acids – the building
blocks of life. The chemical origin of the vents makes a big difference to the pH level inside their porous rocky chambers; black smokers are acidic, whilst the Lost City’s vents are alkaline. These terms may immediately be suggestive to you; acid means an excess of protons, and alkaline means a deficit of protons.

  The waters in the vents of the Lost City are around 90 degrees Celsius – the vents are not volcanic in origin.

  Four billion years ago, the oceans of our planet were acidic, which means they contained an excess of protons. This acidic seawater would have surrounded the alkaline vent systems like those at the Lost City, delivering a natural gradient of protons though the myriad chambers of the towers. The chambers themselves would have been lined with iron and nickel, present in large quantities in the primordial oceans, which act as catalysts in organic chemical reactions. Conditions were stable, warm, permeated with natural proton gradients and, with the unusual presence of hydrogen gas, highly reactive.

  Could it be that this is what LUCA looked like? Not a cell, not a little thing like a bacterium or archaeon, but a warm rocky chamber in a vent system? The argument is compelling, at least to me. Life’s proton gradients, which are absolutely central to the production of ATP, are a smoking gun. The presence of highly reactive hydrogen gas in the vents is another. As we’ll see in the next chapter, photosynthetic organisms go to extraordinary lengths to stick protons – hydrogen – onto carbon dioxide to make sugars. This is fundamental to life, but it happens spontaneously in the presence of hydrogen. You don’t need the machinery of photosynthesis if you have hydrogen around, and you don’t need the respiratory chain to pump protons across membranes if you have naturally occurring proton gradients coursing through your chambers. Everything, from the reactive precursors of organic molecules, complete with catalysts, to the proton gradients to drive the climb up the entropic gradient, are present and correct.