[ Pobierz całość w formacie PDF ]
.So alien biochemistry will, almost surely, differ in every detail fromour own.Even if the biochemistry were exactly the same, however,Earth s lifeforms cannot possibly be duplicated elsewhere, even onanother ordinary Earth, for evolutionary reasons.Parochialphenotypes will not be repeated; even if they have the samebiochemistry the context will be different.So there will be noextraterrestrial insects or dinosaurs or australopithecines, unless theywere exported from Earth by visiting aliens.However, the initiation oflife on Earth could well be a common, almost identical situation on allaqueous planets: being bombarded with asteroids, keeping the seasboiling and the chemistry in ferment, may make them all equivalent.This may sound like part of the Rare Earth argument, but there s adifference.Our point is that whatever happened here worked , soanything sufficiently similar has a good chance of working too although we d expect the result to be different in detail, even if we re-ran the whole of evolutionary history again on this planet.The RareEarth view boils down to whatever happened here is essentialeverywhere.Logically, this confuses sufficiency with necessity;scientifically, it is not sensible to deduce from a single example that asuccessful trick must be the only trick.In fact, given what we now knowabout alternatives to DNA on Earth, it is completely ridiculous toassume that our specific DNA/protein system is the only way to makelife.*220 NOT AS WE KNOW ITOur expectation that there may be some underlying chemical universalsis restricted to aqueous planets: these are likely to be fairly common, butthey re not the only game in town.On aqueous planets, we expect somesimilarity in early chemistry to occur for two reasons.One is bottom-up , based on internal features of chemistry and physics; the other is top-down , based on contextual influences.The bottom-up view stems from the observation that the array ofamino acids made in Miller-type experiments has a fairly rigid pattern.Some amino acids, like glutamine and arginine, are easy to make,while others, like tryptophane and cysteine, are difficult.These requirelonger times, special starting mixtures and are rare in the final mix.Thesame distinction occurs with sugars, fats, and oils: all the startermolecules that pave the way to life.The reasons why some moleculesare easier to make than others have to do with various physicalproperties, which can in principle be predicted by experts.So we expectthere to be a relatively common easy set of chemical compoundsbuilding up in the seas of most aqueous planets as the planet cools andthe seas condense.Whether these are precipitated from the atmosphere,or are received from meteoroids and comets tails, some amino-acidswill be common and others rare.This provides bottom-up constraintson what can happen later.There are some complexities, to be sure, that build on this bottomlayer.We have seen that some simple peptides (amino acid chains tooshort to count as a protein) have turned out to be moderately effectivecatalysts to help other amino acids join together.Some sugars are veryeasily oxidised, and are destroyed by high concentrations of oxygen; asthey are destroyed they reduce the level of oxygen, allowing their fellowmolecules of sugar to survive more easily.This can lead to a repetitivecycle, like that in the BZ reaction, if there is a reservoir of sugar available.So the standard system of small but interesting molecules complicatesitself on all aqueous planets in not-altogether-predictable ways.The initial conditions on a planet that is going to acquire seas, and life likeours, are immensely complicated but they are not, in the importantaspects, very variable.We don t think that anyone will ever argue convincingly that kind-of-life-X will appear on 15 per cent of them, no life on 70 per cent, kind-of-life-Y on 5 per cent, and so on: this is more detail than any theory221 WHAT DOES A MARTIAN LOOK LIKE?could safely predict.In the same circumstances on other aqueousplanets, much the same rules will apply, leading to much the samecomplexities and complications, simply because a planet is so big andlasts so long, and because the space of the adjacent possible is so muchbigger.Phase space for a planet has more dimensions.The top-down reason why the same complexities could appear indifferent places is more subtle.Perhaps some of the reaction-systems aredynamic attractors.That is, many different starting-points willconverge on to the same behaviour.Even if there is not much likelihoodof any particular system getting started anywhere on such an earlyplanet, once it has got started it is autocatalytic: it encourages theproduction of more systems of the same kind.If so, then that systemcould take over locally wherever and however rarely it appears.Thisis what spirals do in the Belousov Zhabotinskii experiment: they arequite difficult to initiate, and appear only very rarely without some kindof intervention, but once a spiral appears it takes over everything elseuntil the whole dish is a seething mass of spiral chaos.One spiral willgobble up any number of target patterns and turn them into morespirals.This is interesting, because a spiral isn t a thing you can t pickit up and lift it out of the dish.It is a kind of organisation, but it canact like a thing.Life is like that too, only hugely more complicated
[ Pobierz całość w formacie PDF ]