lproven | The Story So Far

A friend of mine,

robthefish, asked:

> can anyone explain what exactly was 'reducing' about the supposed
> 'reducing atmosphere' of early planet Earth
>
> (go on, you know you want to!)

I got a little carried away answering.

Oh, gods. How's your basic chemistry? The one-sentence answer is meaningless unless you know enough chemistry not to have to ask that question.

Reducing is the opposite of oxidizing.

Oxidizing no longer means just breaking molecules up into smaller lumps via attaching them to oxygen atoms, for example burning carbon:

C₂ + O₂ -> CO₂

1 atom of carbon and 1 molecule of oxygen gives 1 molecule of carbon dioxide.

Or "burning" sugar for enery in the mitochondrion. Sugar means glucose. See the Wikipedia article on respiration:

http://en.wikipedia.org/wiki/Cellular_respiration

Now, oxidation used to mean adding oxygen into a molecule; this often implies removing hydrogen. Reduction therefore meant adding hydrogen, or, conversely, removing oxygen.

But these days it's used in a more general sense. Oxygen atoms aren't found on their own, because oxygen is viciously reactive stuff. It has an incomplete electron shell - a "raw", naked oxygen atom has a double net positive charge. It needs 2 extra electrons to complete its shell, but it can't hold on to them on its own, so it bonds very tightly to other atoms with 2 to spare - or 2 with 1 to spare each. A pair of naked oxygen atoms will cling to one another, meaning that they get 12 electrons in their outer shell - each shares 2 of its neighbour's. Better than nothing but not stable - so oxygen is really reactive.

See

http://en.wikipedia.org/wiki/Oxygen

Ozone is O₃ - 3 oxygen atoms sharing. It's even more unstable. Given a nudge, 1 atom falls off, leaving a nice relatively-stable O₂ molecule and a single raw atom - which will instantly find some neighbouring molecule and rip it apart. Ozone is nasty stuff. It's handy up in the stratosphere, blocking ultraviolet radiation, but down here where all the complex molecules are, it's toxic as hell.

When you look at what oxgen does when it bonds with things, it rips apart molecules to liberate free pairs of electrons, which it then bonds with. It "wants" to rip electrons off things.

So you can interpret "oxidization" to mean ripping electrons off things. This is a really useful model; in terms of electron exchange, most simple chemistry can be simplified to things gaining or losing electrons. When you lose 'em, you've been oxidized; when you gain 'em, you've been reduced.

In which case the actual oxygen is irrelevant. It's all about electrons.

So you can, looking at it this way, oxidize things without any actual oxygen, so long as the reaction rips electrons off something. Fluorine (which is really nasty stuff) is even more electronegative than oxygen - the only element, no, substance of any kind, that is.

This being the case, the opposite process to oxidization is reduction - reactions which add electrons to things, and any reactive chemical enough that readily donates electrons is a reducer.

http://en.wikipedia.org/wiki/Oxidize

So for example, the opposite of oxidizing carbon is to reduce it:

C + 2 H₂ -> CH₄

... yielding methane. Similarly you can reduce nitrogen to produce ammonia:

2 N₂ + 3 H₂ -> 2 NH₃

The early atmosphere of Earth comprised no oxygen - it's too reactive, it was all bound up in oxides - but lots of stuff with loads of hydrogen, such as ammonia and methane. The atmosphere of Titan is much like this today and there's a lot of these compounds in the gas giants' atmospheres, too. This kind of atmosphere can attack things and break them down by reducing them - donating hydrogen (because both methane and ammonia are quite unstable and reactive) or, if you look at it that way, simply donating electrons.

However, CH₄ and NH₃ aren't anywhere near as keen to give away electrons as oxygen is to nick them. Both burn like buggery in oxygen, for example, as the oxygen atoms rapidly shred the hydrogen-rich molecules and react with both parts of the fragments, e.g.

CH₄ + 3 O₂ -> CO₂ + 2 H₂O

I can't be bothered to draw the oxidation of ammonia, partly 'cos I can't remember if you get nitrous oxide or nitric oxide and indeed I think it depends on the conditions. The difference between these is left as an exercise for the reader.

So a reducing atmosphere is both good and bad for little proto-living proto-cells. Mainly good.

Why? Well, oxygen's good 'cos using it you can break down all sorts of things, liberating energy as you do so. You already know that oxidizing things liberates energy: burning stuff makes it hot.

But it's bad 'cos it's so reactive. Never mind your food, if you're not really careful handling this oxygen stuff, it oxidizes you. It's dangerous as hell; it's like cooking with an welding torch. Doable and by heck it does the job fast but it has a tendency to incinerate food and chef alike.

So if you're a bunch of long-chain carbon-based molecules experimenting with cooperation and who knows, maybe one day getting together with some fine porous clays and forming a primitive cell, you don't want oxygen around. It very quickly evaporates you into a whiff of water and carbon dioxide and you never get to do anything interesting 'cos suddenly you're so much atmospheric gas. Your protocell evaporates into a puff of smoke, so to speak.

What you want is a nice stable environment where nothing much happens 'cos there's no free oxygen around to burn you all up. It means life is slow, 'cos you can't use oxygen to oxidize your food, you have to do it the slow hard way by using less reactive molecules to oxidize stuff by slowly stripping off individual electrons in a nice slow controlled fashion. It's slow and wasteful - you end up with lots of big complex waste molecules with loads of energy left in them that your basic oxidative tools aren't strong enough to dismember. So for instance if you're a plant and you oxidize glucose without oxygen, you get alcohol.

You break down your C₆H₁₂O₆...

http://en.wikipedia.org/wiki/Glucose

... into ethanol, C₂H₅OH...

http://en.wikipedia.org/wiki/Alcohol

... and some CO₂ and water and stuff. There's loads more energy in that alcohol but your knives aren't sharp enough to cut it.

(Animals, incidentally, do something similar but they get lactic acid as the end result.)

So. Reducing atmosphere. Nice friendly environment for a baby cell to evolve in, 'cos nothing burns it up, but it can't do anything terribly interesting 'cos it's an energy-poor environment. You can break stuff down - eat each other, and eat chemicals formed in the atmosphere by lightning and stuff, or by breaking down simple chemicals in the rocks and so on, but there's no quick way to anywhere. You can't burn anything.

What you need to do, if you want to go places fast, is find a way to trap energy from the environment. What's abundant but not nasty? There's geothermal energy - heat in the Earth's crust - but you cook if you're not careful. There's gamma radiation and so on, but it breaks up your molecules. And there's warmth, infrared radiation, which you can't really trap and harvest but helps chemistry move a bit faster.

And there's light. Loads of it. Streaming out of the sun. Trap that, you're made. Gigajoules of energy falling out of the sky; manna from heaven.

What you need to do is find some way to catch photons and use their energy to build it up, store it. Everyone eats glucose and stuff anyway, so make your own glucose and you're onto a winner. You store your food as food! Brilliant!

And a bunch of what used to be called blue-green algae (but aren't any more on the very reasonable basis that they're not actually algae, so we call 'em cyanobacteria, from "cyan" meaning a sort of bluey-green) found a way of doing this.

Slight snag. Byproduct: er, oxygen. Protocyanobacterium immediately immolates itself. Woof, gone.

So these proto-plants stuck to the old slow ways.

Estimates vary on how soon the bacteria got started - the planet's about 4.5 aeons old, in other words, 4½ thousand million years.

http://en.wikipedia.org/wiki/Age_of_the_Earth

However, some kind of protobacteria probably got going within half a billion years.

http://en.wikipedia.org/wiki/Timeline_of_evolution

Then, suddenly, nothing happened.

For an aeon, a thousand million years, nothing much happened. That's nearly twice as long as there have been animals of any form in existence.

About half way through, some bugs invented photosynthesis. There are safe slow ways of doing it that don't produce oxygen but they're not terribly efficient and don't give you a huge advantage.

Some probably stumbled across the fast way, the oxygen-producing way - and promptly burned themselves to death. The oxygen quickly got used up, oxidizing things - mainly their helpless neighbours - and the status quo resumed.

After an aeon, some things found how to cope with the oxygen and not die. They prospered. At first, the oxygen dissolved in the water, reacting with iron and stuff and not building up. Still a mess, the sea filling with this horrid smelly poisonous gunk, killing bacteria in their trillions. Then as the seas ran out of stuff to react, it started to accumulate and build up in the atmosphere as well.

This is the big shift. Ecodisaster on an epic scale. Almost everything on earth dies, poisoned by this new nasty gas.

A few things live. It takes a while, but they learn to use the oxygen to quickly and efficiently burn stuff for energy. It takes half an aeon to work this out. It is not easy.

Everything today uses glucose and burns it the same way, using the Krebs cycle.

http://en.wikipedia.org/wiki/Krebs_cycle

This is really complex and fiddly. It's unlikely that multiple things evolved it.

From this it is simple to deduce that, probably, everything else on earth died. This one line of bugs (bacteria) - a single cell at one point, in fact - survived and we are all its descendants, every organism on the planet.

Now, things pick up. For 400 megayears (MY), the survivor's sprogs prosper and spread across the planet, diverging and filling up all the ecological niches, learning to eat their cousins alive, or break down their dead bodies, and so on. They specialise. Some in photosynthesising, some in burning glucose, lots in doing a bit of both. We're still talking a planet-sized ball of unicellular slime, though. Most of it green. Giant bogey world.

But after that time, some of the cells have specialised so much that they sort of club together. Some of the big nonspecialists find that they can persuade some small specialised oxygen-burners to move in inside them to digest their food for them, so they don't have to deal with glucose and oxygen and stuff and can just live on straight ATP - the basic fuelstuff of life. This works out really well for both parties, so some of this new crowd do the same trick again: the big jobs persuade little green jobs to join in and get busy with making sugar and getting really leaky, so the outer cell lives on the leakage. The little inner guys really let themselves slip, getting so specialised that they can't live alone - they let the outer cell do most of the work, eating, moving, absorbing & digesting raw materials etc. In the same way, About the only job aside from their main one that the inner guys hang on to it reproducing themselves. Even to this day, they have their own genomes - their own genetic code - and they breed away inside the outer supercell. The burners are called mitochondria and the light-catchers are chloroplasts.

We're not sure yet, but some of these supercells might even have persuaded some kind of information-storage specialist to move in and look after all that business of copying, checking and fixing DNA. The cell nucleus, in other words.

So you now have these compartmentalised cells, with food-making and food-digesting subcells and a central info-storage subcell. These guys are huge but they're really flexible and capable. The 4WD of the unicell world, massive buggers that can go anywhere, do anything. Not very quick, mind, but unstoppable.

What they can't do, though, is fuck. They haven't invented sex yet. So they change r e a l l y s l o w l y and for the best part of another aeon, nothing much happens. Another thousand million years of green slime. Yay.

Then someone invents the idea of like, chopping yourself in half, and like, swapping halves with someone else. Very weird and messy and unhygienic and all rather unpleasant, really. You have to package up your DNA into units, then make pairs of them, so you can trade half with someone else.

But boy, is it fun. And every time you do it, you get several new pairs of genes! Cool!

Evolution picks up its skirts and sprints. From not so much as a gentle stroll as an imperceptibe creep, it gets a rocket up its arse and takes off like nobody's business. In just 200MY, blisteringly quick by the old pace, they invent the idea of getting together in lumps and cooperating. Multicellular life appears. Photosynthesis speeds up, most of the CO₂ in the atmosphere is absorbed - the ammonia and methane is long gone -

... And the whole plot freezes solid as a result. Bugger. Bit overoptimistic there.

When it ~~gradually~~ EDIT: eventually thaws, there's all sorts wriggling about in the mud. Sort of little wormy things, sort of jellyfish things, sponges. Animals. Multicellular ones. And they're all shagging.

In the space of the next 40 MY - an absolute eyeblink, no time at all - every single form of life on the planet today evolves. Every phylum, and a thousand other ones we don't know about, probably. It's an explosion. Suddenly there's life everywhere in the seas, floating, swimming, crawling, walking, digging. Fins, eyes, legs, jaws, backboney-sort-of-things, kidneys, arseholes (big step, those) - you name it, it all appears right now.

From then on, it's a process of winnowing. Most lines go extinct, but some do great, give rise to a thousand variant forms - most of which go extinct, but a handful of which give rise to a thousand variants, and - well, you get the idea. But it all happens inside just over 500MY, and it's still going on.

The story of the world in ninths.

(1st ninth) 4500 megayears ago - Molten ball of rock.

4000 MYA - Cools down, bang, life. Almost instantly. Then nothing else.

3500 MYA - Photosynthesis. Slow safe form. Nothing much else.

3000 MYA - Oxygen-making photosythesis. Everyone dies. Sole survivor does great, ta. Not much else.

2500 MYA - Burning glucose with oxygen invented. No big changes.

2100 MYA - Bacteria get together, invent nuclei, chloroplasts, mitochondria. Doesn't make much difference.

1200 MYA - Sex! Wahey!

1000 - 600MYA - In fairly quick succession, multicellular plants, then multicellular animals

525MYA to date - Everything else you've ever heard of happens.

In the last 0.002% of it, humans evolve. The last 0.0000008% of the history of the world represents the entirety of recorded human history. The Bible says some guy made it all, just for us, in one week, starting October 22nd 4004BC and doesn't mention any of this stuff. It also says that π = 3. It is therefore safe to say it's almost certainly all wrong.