If there is a king that rules over all life on Earth it is carbon. All organisms on the planet build each and every part of their cells with molecules whose framework is this atom. We are, as the aliens put it in the first movie of the Star Trek series, ‘units of carbon’. Now, can you build life without it?
As NASA astrochemist Max Bernstein says, there is no element in the entire periodic table with a chemistry more similar to its own than silicon : “It is in the right place in the periodic table, just below carbon. It can form four bonds, like carbon and molecules so similar that it is possible that a whole parallel chemistry can be built. Unfortunately, not all of the mountain is oregano; silicon also has its drawbacks . “We don’t know that the chemistry between hydrogen and silicon is stable like it is with carbon; thus, while the hydrocarbons are stable, the silicon analogs are not. And something similar happens with oxygen: while carbon-oxygen bonds can be made and broken, with silicon they are eternal. This strongly limits the capacity of silicon to be the basis of life, but that does not mean that it cannot play a more important role in biochemistry than it does now, which is practically nil”.
The mystery of silicon
Be that as it may, there is an issue on which there is still no clear answer: the role of silicon in the appearance of life on Earth and why it has completely avoided its use.
If there is something that characterizes living nature, it is the use of practically everything within its reach : metals such as iron, a fundamental component of blood, magnesium in chlorophyll or the rare molybdenum in metabolic processes. Therefore, what is striking is that it has turned its back on silicon, an abundant element that has properties characteristic of both metals and non-metals. Against all odds, this element only appears in some bioinorganic compounds, such as the silicon shells of diatom algae. It has never found a hole in carbon chemistry.
Now, some scientists think that silicon has played an obscure and little-known role and is the one behind one of the most peculiar enigmas in the chemistry of life: the chirality of molecules such as carbohydrates or amino acids.
The strange chirality
Chirality is a geometric property that some molecules have and it comes to say that they cannot be superimposed with their mirror image. The classic example is our hands: it is impossible to superimpose the left with its reflected image in a mirror, which is the right. In the case of the compounds of life, we find that carbohydrates and amino acids present this peculiar property. And here the enigma appears: the first ones are right-handed while the second ones are left-handed. Why is this so if when they occur spontaneously, the same amount of both hands appears? There is no valid answer to this mystery but this is where our silicon has something to say .
One of the most striking hypotheses of how the first self-reproducing molecule could have appeared was proposed by the British chemist Alexander Graham Cairns-Smith in 1985. According to him, the famous primordial soup where the origin of life on our planet was cooked needed a stew in form of clay crystals. The idea is that, before any primitive DNA or RNA, there may have been other systems that stored and copied genetic information . For Cairns-Smith these systems were clay microcrystals, whose basic building unit is silicates. Among their most important properties is that they have great reactivity; In fact, farmers know that it is good to have clay in the fields, as it favors chemical reactions that benefit the plants. Cairns-Smith proposed that the organic molecules lived attached to the clays, which acted as catalysts for their reactions, until one day a “genetic relay” took place: in those primitive molecules the ability to replicate and evolve on their own appeared. If this really happened -and this is something that remains to be demonstrated- the silicon in clays could explain the existence of this chirality of organic molecules : a slight deviation in the position of this atom in the compound was enough to define the preference for a type of orientation in amino acids and carbohydrates.
This is obviously a highly speculative idea, but no more than that of those scientists who believe a silicon life is possible. The truth is that everything indicates that it is, at least, unlikely. We have the proof on our own planet: although silicon is one of the most abundant elements in the earth’s crust (it represents 28% compared to 0.03% for carbon), it is completely absent from the chemistry of life. If life in silicon were possible, only because of the abundance of this element would it have appeared here . There is also no observational evidence to suggest the existence of silicon-based biology, or even just prebiotic silicon products. Nothing in meteorites, comets, the interstellar medium, the atmospheres of the giant planets… silicon oxides have been found in all of them but not substances such as silanes or silicones, which could be the precursors of a silicon biochemistry.
Silicon’s appeal as an alternative to carbon is that, because of its position in the periodic table, much of its basic chemistry is similar. For example, carbon combines with four hydrogen atoms to form methane, CH4, and silicon produces silane, SiH4, silicates are analogs of carbonates, both elements form long chains, or polymers, in which they alternate with oxygen . But the fundamental problem with silicon is its strong affinity for oxygen . When we breathe, carbon is oxidized creating carbon dioxide, which is a gas and is easy to remove from the body. However, with silicon what is produced is silicon dioxide or silica, sand, a solid, clearly a very complicated compound to remove.
Another of the drawbacks of silicon is that it lacks the necessary chemical versatility when it comes to forming the molecules that the metabolism of a living being requires . Yes, it can build long chains, but the capacity of silicon to join atoms such as hydrogen, oxygen, nitrogen, phosphorus, sulfur and metals such as iron, magnesium and zinc is significantly lower than that of carbon. What’s more, when it interacts with other atoms, silicon creates molecules that chemist Norman Pace of the University of Colorado defines as “monotonous compared to the universe of organic macromolecules.” The reason lies in the silicon atom itself, which is much larger than carbon – it has a greater mass and atomic radius – which makes it difficult to form double bonds , something fundamental in a large number of organic molecules, such as ketones. , esters, carboxylic acids… On the other hand, silanes, which would be silicon and hydrogen compounds analogous to carbon alkanes, are very reactive with water and when they form long chains they decompose spontaneously. They are all drawbacks.
Most scientists share what astronomer and extraterrestrial enthusiast Carl Sagan called “ carbon chauvinism ”: life demands this chemical element and no other. Of course, some respond to this by saying that this chauvinism is, in reality, the smoke that blinds our eyes.
Delsemme, A. (1998) Our cosmic origins, Cambridge University Press
Dick, S. J. (1996) The biological universe, Cambridge University Press
Goldsmith, D. y Owen, T. (1993) The search for life in the universe, Addison-Wesley