Expert: John Locke - 8/25/2007

Question
One theory has always stated that silicon lifeforms could exist just as often as carbon-based lifeforms, since silicon has as many potential combinations with other molecules as carbon. What is your view re this theory?

Also, what is your view re the theory of parallel evolution? This idea suggests that what happens on one planet(ie Earth) would also happen on other planets, with life present - what arethe chances, in your view, of humans developing independently on another planet, in an almost identical way?

Thanks
Geoff

Answer
Thanks for using AllExperts, Geoff.

Silicon, like carbon, can form form bonds to other atoms and displays a number of other properties that make it seem viable as a basis for life. Silicon already has a place in the physiology of a number of earth organisms--sea urchins have silicate-based skeletons, as do diatoms. Silicon is part of the metabolism of embryonic rats and chickens. Some of the connective tissues in humans contain silicon. However, there are some key differences between the two that make silicon largely unsuitable as a biological substrate; whatever usefulness it has in extant organisms is quite limited compared to that of carbon.

What's limiting about silicon? Well, for there to be silicon-based life, there would have to be a silicon cycle, so that silicon could be liberated from dead organisms and used again. Assuming that oxygen also exists in this hypothetical environment, the resulting silicon dioxide (one of the more common compounds of silicon) would pose a serious problem: it is not soluble in water, and it could not be carried around an ecosystem in an aqueous or gaseous state. Nor, for that matter, could it be easily eliminated from cells as a waste product. Though silicon can form single bonds to other atoms, its larger atomic radius makes it less suitable to form double and triple bonds, which are an essential part of biology as we know it. Silicanes (the silicon-based equivalents of alkanes) are not stable in long chains and don't readily form rings, also rather essential to the structure and function of organisms. Taken as a whole, the negatives seem to outweight the positives, and accordingly silicon is not considered as a likely substrate for organic life.

But we should take a moment to remember that the above objections (which is the mainstream scientific view, so far as I am aware) are reasons why silicon couldn't form life as we know it. That certainly leaves open the possibility of silicon-based life that exists in a very different environment, and that has very different characteristics. Such an environment, and even the functions of these new organisms, would be extremely foreign when compared to known biological processes. By far the most honest thing that can be said is this: there is no evidence that silicon-based life exists, and there is substantial evidence against silicon forming life as we know it.

On parallel evolution, I'd have to say that Star Trek got it wrong. The odds are strongly against humans developing in an identical or near-identical fashion (and even higher against those humans learning to speak English, which they always seemed to do on the show). I cannot give an exact probability, mostly because the elements involved don't lend themselves to neat mathematical descriptions. Regardless, why do I think it so unlikely that humans would develop again somewhere else?

There are multiple reasons. Evolution depends upon environment: organisms evolve to be best adapted to the environment in which they live, meaning that these hypothetical humans would have had to evolve on a planet with geography and climate similar to that of earth--and while we are now learning of more and more extrasolar planets, none found so far approximate the conditions on earth. That's not the last of it--human evolution depended heavily upon the melting and reformation of glaciers, the rise and fall of land bridges on earth, and variations in global climate. These things are simply too unpredictable and subject to chance to think that they would happen again with any certainty.

In addition, humans came to dominate on earth because other large organisms (dinosaurs and associated reptiles) died out quickly--contingency played an enormous role in the rise of humans. It is too much, almost, to presume that another such contingency would occur and thus allow humans to evolve on another planet.

Of course, humans could arise without the previous hegemony of reptiles--but there are reasons for distrusting this hypothetical, as well. The body plan that dominates among animals (brain and sensory organs located in the head, four limbs, internal organs located in a central trunk, etc.) should not be assumed to have any special place: other perfectly reasonable adaptations could allow for life in a similar environment to that on earth, but without the same form as human life. Humans, as well, have adapted to an atmosphere with a high concentration of oxygen; oxygen, as you probably know, is a powerful oxidizer, and it is thought responsible for a great environmental change during the early history of the earth. Life on earth survived that change by adapting to a primarily aerobic system of respiration. Had this failed to occur, the outcome would likely have made complex life impossible.

Could highly intelligent lifeforms evolve somewhere else in the universe? Absolutely--as seen here on earth, intelligence is a formidable evolutionary advantage. But we must recognize how unlikely the development of humans would be on another planet; in essence, modern humans owe their existence to a collection of both particular environmental circumstances and contingencies that are unlikely to reoccur, anywhere. Regrettably, I believe that the probability of such humans evolving somewhere else is very low.