C02 + H2O --------- C6H12O6 + O2
Carbon dioxide + Water -------- Glucose (organic molecule) + Oxygen
It doesn’t look like much does it? It's rather unimpressive isn't it? How can such a simple formula hold the key to life? But it does! It makes you feel like a god just knowing it. A god surveying creation before him (or her). But enough of the purple prose. Time for some real science. But before we begin: A plea. I implore you to be patient. The science that follows will be tricky. I have tried my best to strip away any superfluousness, any excess fat, to simplify, to distill, so that what remains is the essence. But to truly understand will require a wee effort on your part. But the rewards; if I may be so bold to say, will be worth it. At the end of it all, if I can make you feel like a genius, if I can make you feel like a crazy lord of time surveying his dominion, than I will have succeeded. Let there be light!
We've already discussed that plants carry out the above reaction by harnessing solar energy during photosynthesis. We mentioned that photosynthesis is a hugely complex process involving numerous enzymes, proteins, electron carriers and an intricate membrane network. To reiterate: the first life on earth could not possibly have photosynthesised. Photosynthesis evolved much later. So, the question remains: How did the first life combine hydrogen with carbon dioxide to build the organic molecules necessary for it to exist?
I hinted that the ocean floor holds the key. Deep down below the surface; projecting from the sea bed like chimneys are hot water vents. These are like fissures or pimples on the earth’s skin where newly exposed rock, from deep within the bowels of the earth, reacts with sea water. It must be understood that water doesn’t just mix and percolate with the rock; it chemically reacts with it. The amount of water bound chemically to the rock in this way is astonishing. It is believed that it is equivalent to the entire volume of water in the oceans. As the sea floor expands from the movements of plate tectonics; these rocks which contain bound water, are ultimately plunged beneath a colliding plate. In what is known in geological parlance as a subduction zone.
Eventually, on the other side of the conveyor belt that powers the movement of the continents these rocks reach the surface of the sea bed. This is where the vents come in. The sea water that was once bound with the rocks is released again through the vents; but not as water (H2O) but as bubbling hydrogen gas.
At the turn of the millennium, scientists aboard the submersible Atlantis stumbled across just one of these vent systems spewing forth hydrogen gas. This vent system was nicknamed the 'lost city' and it had delicate finger like projections made of carbonate that reached up into the inky blackness. These projections gave the lost city a Gothic church look.
The projecting fingers themselves are porous, containing a maze of microscopic compartments like a bee hive. The compartments are 1/12mm in diameter (the same size as an average cell) and are teeming with life. The compartments are the home to colonies of industrious bacteria. The bacteria themselves provide the food source for other larger organisms that thrive in the vents. They are the 'producers' in this underwater food chain but the the key thing is that the entire ecosystem is supported by the reaction of hydrogen gas (H2) with carbon dioxide (CO2). Whereas on the surface this reaction has an initial energy requiring step in which hydrogen first has to be torn from water, but here, the reaction requires no such initial stage. Raw hydrogen bubbling from the ground as a gas is a rare gift on our planet, and life is normally obliged to seek out occult supplies, bound in tight molecular grip to other atoms, as in water or hydrogen sulphide. The reaction occurs painfully slowly, but from the view of thermodynamics it is a free lunch. In fact is is better then a free lunch. You see, not only does the reaction of hydrogen and carbon dioxide occur spontaneously but it also releases energy as a result - energy that can be used to power other reactions. It's like walking into a restaurant, having a meal, and then being paid for it! - the ultimate free lunch. It takes such a free lunch to get life going on a planet. These vents fit the bill as a hatchery for life on earth. They generate organic molecules. They have porous compartments, which can concentrate any organic molecules formed, making the assembly of polymers (long chained molecules) like RNA (Ribonucleic acid - simpler cousin of the more famous Deoxyribonucleic acid DNA). The vents are long lived; the lost city itself has been venting for at least 40,000 years. The walls of the vents contain iron-sulphur minerals which can act as catalysts to speed up and promote the reactions further.
But I hear the whispers of naysayers. I know what you're thinking: such vents are all very well, but how did life progress from such natural reactors to the complex, marvelous tapestry of invention and ingenuity that we see around us? The answer, of course, is unknown but there are clues within all of us. A clue to the past. Yes, within each of us is a 'smoking gun', an ancient remnant of the origin of life itself. All life on earth without exception has within it a set of deeply conserved biochemical reactions, like a living fossil, but not a fossil of bones, nay a fossil of biochemistry. The same reactions that occured at the dawn of life, also occur in the underwater vents and also occur inside your body! Amazing huh? From Bacteria to Penguin. From woolly Mammoth to Chimpanzee we all contain a clue to our primordial past. Don't believe me? I'll show you. We now turn to these for the next step in our wonderful story.
All life on earth shares certain qualities. All living things are composed of cells (with the exception of viruses). All cells in all living things on earth have genes made of DNA; all DNA codes for proteins by means of a universal genetic code. All living things also share the same energy currency, known as ATP. This chemical is like a £10 note, used to 'pay' for all kinds of work that needs to be done to keep life alive. We can infer that all living things inherited these qualities from the first remote common ancestor. All life today also shares a common core of biochemical reactions, at the heart of which is a little cycle of reactions that is the bane of many a biochemistry student: the Krebs cycle! The Krebs cycle occupies hallowed ground in biochemistry - but for me it was something to be rote learned for exams and then quietly forgotten. Yet, within this ancient and dusty cycle of reactions lies the answer to the meaning of life - or, if not that, then at least how it got started!
(The Krebs Cycle - a pain in the ass for all biochemistry students)
I don’t expect you to understand the details. The main things to take are:
- The main purpose of the Krebs cycle is respiration i.e. to generate energy
- At the top you can see a chemical called Pyruvate which is shunted into our cycle
- ATP (the cellular currency of energy) is generated as the wheel turns. The ATP is then used to power life.
- Carbon dioxide and water are produced as waste products of this cycle.
What has any of this got to do with anything you're wondering? Let me explain. At the top of the cycle is a chemical called Pyruvate. Where does Pyruvate come from?
Don't worry about the details (!). All you need to do is look at the top left and read out the name of the first chemical you see...exactly, glucose. And the last chemical at the bottom...pyruvate.
So glucose is absorbed into the body and is converted into Pyruvate via the above series of reactions. Pyruvate in turn is shunted into the Krebs cycle thereby generating energy and releasing carbon dioxide and water as waste products. This is your basic respiration you learnt at school. But wait a minute! Isn’t this the opposite of the reaction we want? Aren’t we trying to combine carbon dioxide (CO2) and water (H20) to make glucose? Aren't these cycle of reactions breaking down glucose to generate C02 and H20. Yes, true but if you look at the cycle again you'll see double arrows between the reactants. This means that the Krebs cycle is reversible. It can run backwards.
Here is a simple version of the cycle to explain my point:
Glucose enters at the top to be broken down into water, carbon dioxide and ATP. If we run it in reverse we can combine water and carbon dioxide, and with the input of some energy, make glucose!
And he said: 'Let there be life, and life sprawled forth from the vents'
Normally the Krebs cycle is used in respiration to break down glucose. But it can also run in reverse to make glucose and other organic molecules. The reversal of this cycle is common in primitive bacteria - especially those found living in hydrothermal vents. It is clearly a primitive way of converting carbon dioxide and water into the building blocks of life. The other amazing thing about the cycle is that it can spin of its own accord. It is thermodynamically stable. In other words it was not invented by living things, it is chemistry that already existed before life emerged. Life simply took the naturally occurring reactions occurring in the ocean vents and made them its own. When life emerged it 'conducted' a score that already existed! Once the Krebs cycle got spinning, side reactions would have been inevitable, giving amino acids and proteins. How much of the core metabolism of life on earth arose spontaneously, and how much is a later product of genes and proteins is an interesting question and quite beyond the scope of this post. But I would hazard a guess that if you were to set up the Krebs cycle in a test tube, give it a spark of energy, and a wollop of inspiration, a whole bunch of proteins and nucleic acids would be synthesised spontaneously. It is a thermodynamic inevitability.
This paints an extraordinary portrait of the first life on earth - our first ancestor. She wasn't some free living, pulsating, moving, throbbing cell but a rocky labyrinth of mineral cells, lined with catalytic walls of iron, sulphur and nickel. The first life was a porous rock that generated complex molecules and energy right up to the formation of proteins and DNA itself. If this doesn’t shake you and evaporate the anaesthetic of familiarity that hovers above you in your daily life, then I don’t know what will. Do you feel like a genius now?
We now turn to DNA and cells. How did they arise from this queer and wonderful ancestor of ours. This rocky labyrinth of mineral cells...
