Sunday 25 March 2007

Pic of the day: Photosyntesis is a great idea, but amazingly hard to come up with when you need to do it by blind chance. And especially when you have to chance upon it almost immediately after spontaneously assembling yourself from almost nothing and before you die from lack of energy. After that, the rest of us are basically freeloaders.

The origin of life

The origin of species through natural selection is pretty much accepted as a fact of life. While it may seem unlikely that such a simple process could lead to creatures such as us, it is most certainly the assembly line of evolution, whatever else may happen along the way. It is not just the finks of Galapagos anymore: We have looked at the DNA of many different plants and animals, and they are all related in measurable ways. As if that was not enough, we have a vivid demonstration right in front of our eyes: The multi-resistant bacteria, while not yet new species, are clearly evolving from year to year in response to natural selection. Today they are stalking the very hospital corridors from which their ancestors were first banished, half a century ago.

According to the late Stephen Jay Gould, it was not at all surprising that life gradually developed toward greater complexity: It was the only way to go. After all, you could not get less complex than the simplest single celled organisms. He called this "the left wall" of life. As long as an organism was more complex, it could move either way, and evolution would randomly pull it right or left. If it happened to come further right than anything had been before, the complexity of life was extended. If it just moved around in the middle, no one was likely to notice. If it became too simple, it would die. By far most of the life on Earth is single-celled organisms, after all, especially bacteria. We don't live in the age of humans, or the age of mammals, or the age of vertebrates: We live in the age of bacteria, and this is the only age the planet has had since life came to be.

***

This begs the question: How did life come to be in the first place? More precisely, why is the left wall impossible to climb from our side, but evidently ridiculously easy to penetrate from the other side? As evidence I point you to the earliest fossils clearly bearing signs of life. They are from the late Hadean period, which is actually a bit of a euphemism: The conditions on Earth at the time were more like Gehenna than Hades. You know, the lake that burns with fire and brimstone. In all fairness, there are archaea today (the life formerly known as archaebacteria) that live in boiling springs and other less than hospitable places. I believe they are the ones that don't seem to mind radioactivity so much either. There is single-celled life deep in the crust. But they have had billions of years to get used to it. The first cells arrived almost as soon as the planet was cold enough to have liquid water. It is so remarkable, that several otherwise respected scientists have proposed that life must have arrived from space. Perhaps form Mars, although it too was fresh new at the time. Or from comets. Or seeded by some long forgotten civilization from older stars. Normally you would not talk like that in the scientific community, but when the alternative is an act of God (or "sky fairies" as an online friend of mine likes to say) anything else goes.

Of course, it need not be sky fairies. Brian Swimme (a mathematical cosmologist) believes that life is the world attempting to experience itself. And of course there are a host of people saying that life is a property of the cosmos, it is something that naturally happens. I don't buy the "natural" thing because if it was so natural, other similarly complex things would happen too, like sand castles spontaneously arising on the beach (far less complex than life and thus presumably common). And of course there are those who say that it was just a single stroke of luck. And anyway, if life had not existed, we would not be here to talk about it...

If there is a growing consensus at all, it is that life must have evolved from proto-life. This is because life is far too complex to spontaneously come around: We are talking literally a million times more complex than the simple organic chemicals from which it supposedly arose. Even the individual molecules in the simplest cell are far larger and more complex, and less likely to just happen, than the largest organic molecules found in the wild. And there are a million of them and more, in a simple bacterium. So there is no way life could just come together. On the other hand, life has been remarkably good at evolving. So why not assume that life evolved from proto-life? One stage of this proto-life is supposed to be RNA life. Ribonucleic acid is used in modern cells to transfer genetic information from the chemically similar but more stable DNA. But there are some viruses (retrovirus) that skip DNA at all and use RNA for all their genetic information. Admittedly viruses are far too simple to be called life; but their existence proves that RNA can keep genetic information in a reasonable stable form. And recently some forms of RNA have been found that can actually assemble amino acids (the building blocks of proteins) directly from other RNA strings without the use of proteins. This is great news for the RNA proto-life model. Sort of: The largest stretch of RNA that has been encoded so far was 14 units, and it took a stretch of encoder RNA that was ten times as long to do it. So it could not even have reproduced itself. Still, the search goes on.

Theoretically, there must have been at least one step of totally unknown proto-life between RNA-life and ordinary life, and presumably another step before RNA-life (though some say clay may have facilitated this first step). Another thing we know is that proto-life must have been mutating much faster than life as we know it. First off, because otherwise it would not have had time to become life in the very short time span available. Secondly, because the cells we know have extensive repair systems which simply could not have been implemented early, because they are too complex.

***

The questions simply step on each other's toes here. If proto-life evolved much faster than ordinary life, why didn't it run rings around it and evolve further, leaving life in the dust? Or if evolving so fast is a bad thing (as you'd think when modern cells go to extremes to minimize it) how come the proto-life unerringly evolved in the right direction in the short time span it had? If mutations are bad (and they generally are) how did proto-life survive long enough to become life? It did not know, after all, that being life was a good thing and that it ought to change its fundamental nature. The transition is so profound that we are looking at another Big Bang, really: There is no trace of what was before in what is. It is kind of like if we replace ourselves with super-advanced robots, and these spread across the galaxy (a likely event according to some). How would a robot 4 billion years in the future know that it descended from squishy carbon-based life forms? (The Earth is burned up by the growing sun well before that, if you were about to propose fossils.) No, all we have is an assertion that it must have happened because we are here. There must have been something, but according to all other theories that something would not be able to become life unless it was making a beeline for it.

Things don't get much better once life is born. There is only so much nourishment for it. The sea was even then not a soup of organic chemicals, though there were some. They were scattered, though, so searching them out probably required more energy than they gave. Only near certain energy sources, such as underwater mineral geysers ("black smokers") is there a steady supply of molecules with a higher potential (or bound) energy than that inside the cell. The heat and pressure there cause atoms to bind into molecules that are unstable in the open. By absorbing them and using the energy when they fall apart, life can flourish. But such places are few, and tend to be on the bottom of the oceans, in the rifts where the tectonic plates tear apart.

Life would have been very much a niche product, if it existed at all, if someone had not invented photosynthesis. I use the word "invent" jokingly, of course: These were single cells, if even that, and had no idea that sunlight could be harnessed at all. In fact, they had no ideas at all. They did have a mechanism for transferring energy from chemical breakdown and use it to build their own molecules, presumably. Well, unless today's chemosynthetic cells actually descend from photosynthetic cells rather than the other way around. But they presumably lived deep down in the dark, and there was no reason why a cell which happened to be "born" with chlorophyll in just the right place would seek out the sunlight. But then again, this is the staple of evolution: Our language is in pretty much the same situation. We had the capacity for making speech sounds, at the cost of being able to choke on our food, for a long time before we started to speak. You must have wings already before you can fly, and flight has been "invented" independently in insects (at least once, possibly more) and reptiles, birds and bats. The eye has supposedly been "invented" 40 times. It certainly arose with amazing similarities in squids and vertebrates, whose last common ancestor was barely even a slug, and quite blind.

See, the thing is that life seems to be ambling aimlessly around like a blind drunk as long as things are going OK. But when its very existence is threatened, suddenly it gives the pedal and beelines for the only possible solution. The highly unstable proto-life hurries to become life in a few million years, whereupon it immediately discovers the highly unlikely photosynthesis before its supply of organic molecules runs out. Then it lazes around for a couple billion years doing very little except drift in the sea. Actually, life reminds me disturbingly much of myself. Which should probably not surprise me, since I am made up of it from my toes to my scalp. I wish I had a couple billion years to just laze around too… But by all signs, we'll have to evolve like crazy or die trying. And that's not even counting the man-made black holes that are suppose to start rolling out in CERN's Large Hadron Collider this November. If life really was such an easy thing to make, it may not be a big loss if our planet disappears in a gamma glint a few months from now. But I suspect life is a much more rare and precious thing, quite possibly unique in the universe. If so, we better show it some respect.