The Carbon Conundrum

Ben Fernando


All life on Earth is ‘organic’, in that it has a biochemistry based on the element carbon. Carbon forms the molecular backbone of every biologically important complex molecule, from the DNA nucleotides which encode our genetic information, to the ATP which transports energy within our cells.

Carbon’s ubiquity in all of life’s wonderfully varied forms implies its use by biology is ancient in origin. Three or so billion years ago, the first organisms to form on our planet must have incorporated carbon-based building blocks (from whatever molecular soup they existed in) into their biochemistry. All their evolutionary descendants, including every living thing that we see around us today, has retained the same fundamental, organic molecular chemistry.

But why carbon? There are many other elements on the periodic table. Does carbon hold some special advantage, meaning that all other life either never evolved or could not compete, or is it just a matter of chemical chance? This is surely one of the most fundamental questions in molecular bioscience. The implications are profound, not just for understanding how life came to evolve on Earth, but for speculating about what life might look like elsewhere in the universe, if it does exist. If we one day find extraterrestrial creatures, be they simple bacteria or Star Trek’s Klingons, will they ‘look’ like us biochemically?

This isn’t just a philosophical question. Searches for life elsewhere in our Solar System, especially on Mars, have often concentrated on searching for the signatures of carbon-based life1. These have been conducted at enormous cost, but despite six decades of searching, no sign of extraterrestrials has ever been found. If we have unfairly limited the parameters of the search, then this might explain why Martian life has so far eluded us. If, on the other hand, carbon is indeed special, then perhaps such searches are being conducted properly and Mars is simply lifeless. Thus, it is worth discussing what, if anything, is special about carbon.

There are 118 elements on the periodic table, of which about 85 are commonly found in nature. Let’s assume that any of them is a potential candidate for the chemical foundation of life, and narrow down the field from there. The first way that we can do this is by considering the ways in which different atoms bond to each other.

Chemical compounds tend to bond in one of three ways: metallic (where a sea of electrons surrounds a lattice of positively charged metal atoms), ionic (where electrons are transferred between metal and non-metal atoms), and covalent (where electrons are shared between different non-metal atoms).

Both metallic and ionic bonding result in the formation of a crystal lattice. Although stable, such structures limit the amount of functional biochemistry that can occur, making it difficult to imagine a complex process such as respiration occurring in a crystal system. Hence, both these types of bonding are not conducive to the evolution of life2.

Figure 1: The periodic table. Each vertical column is known as a ‘group’, and these are numbered from left to right. The semi-metals are coloured blue, and the non-metals orange [sciencenotes.org].

Covalently bonded compounds, though, will readily form complex structures. Thus, we should limit our search to elements which tend to bond in this way, which are the non-metals and semi-metals, found in groups 13 to 18 of the periodic table.

Of these thirty or so elements, we may immediately rule out some. Group 18, the noble gases, are the loners of the atomic world, unwilling to react. Beside them are the halogens, which by contrast can be excluded because they are far too reactive and will only form a single stable bond. This leaves us with about twenty-five elements, in groups 13 to 16.

To narrow these down further, we need to introduce a concept called ‘valency’, which is the number of bonds that a given element will tend to form. Those in groups 13 and 15 will tend to form three bonds per atom, whilst for those in group 16 it’s normally two. Elements in group 14 are ‘tetravalent’, forming four bonds per atom.

Structures like branched chains – which are vital for life – aren’t possible with two bonds per atom in the backbone element. Trivalent atoms offer the potential for increased molecular complexity, but still restrict the variety of possible multiple bonds and rings, which too are biologically crucial3. Therefore, there are sound molecular reasons why early life may have ended up with a group 14 element as its chemical foundation2.

So why carbon? In the galaxy as a whole, heavier elements tend to be less abundant than lighter ones4. So, from a simple view of probability, the reactions which produced the chemical precursor building blocks of life were far more likely to occur involving carbon or silicon than germanium (another group 14 element), simply because the former are vastly more abundant.

This isn’t quite the whole story though – on Earth, silicon is more common than carbon5 – so there must be chemical reasons why carbon-based life is common, but silicon-based life is nowhere to be seen. To explore why this might be the case, let’s consider the reactions of both carbon and silicon with the five other elements which are ubiquitous in all life on Earth today: hydrogen, oxygen, nitrogen, sulphur and phosphorus6.

Hydrogen is the most common element in the Universe4. Carbon will readily react with it to form ‘hydrocarbons’, whilst silicon will form ‘silanes’. Superficially these look similar, however hydrocarbons are reasonably stable and will form branched chains, which as discussed previously are crucial to life. Long silane chains, on the other hand, are far less stable7, and will react vigorously with water or spontaneously disintegrate. This makes it hard to see how silicon equivalents of molecules such as glucose could be biologically useable.

Both sulphur and phosphorus atoms have large diameters, and hence can only bond with complex organic molecules in certain places. Silicon is larger than carbon, so it’s even harder to create silicon-based chains including either of them as they simply do not fit. As both sulphur and phosphorus play crucial roles in the chemistry of life, which no smaller atoms are capable of, this is another serious issue.

That leaves us with nitrogen and oxygen. Silicon’s size again poses an issue here, as it makes forming double or triple bonds very difficult8, whereas nitrogen-carbon multiple bonds occur in several important classes of biomolecules. This means that no silicon analogues of vital functional groups like the pyramidines (from which three of the four DNA nucleotides are derived) can occur.

Perhaps the final nail in the coffin of silicon-based life comes about when we consider its reactions with oxygen. Hydrocarbon chains may be ‘partially oxidised’, where some, but not all, of the hydrogen atoms have been replaced by oxygen. This yields molecules such as amino acids (precursors of proteins), monosaccharides (sugars), and a set of compounds which are of special interest to students – alcohols.

Silicon, on the other hand, is normally completely oxidised to silicon dioxide, forming giant and very stable crystal lattices9. As discussed previously, these are not at all conducive to life. Silicon-based proteins and sugars, therefore, simply cannot exist2.

All this leads to the inescapable conclusion that there are sound biological reasons why carbon-based life evolved on Earth; and we’re not just displaying ‘carbon chauvinism’10,11 when we say this. How widely applicable is our analysis?

Space surveys find high concentrations of complex carbon compounds in the planet-forming disks around other stars12, but no silicon equivalents. The same is true of meteorites, which hold a record of what the Solar System was like at the time our planet formed. Molecules as complex as amino acids have been found13, but only the carbon versions. So carbon compounds definitely have a preponderance on a much wider scale than just the Earth, and far greater potential for biological life than silicon.

Those fans of Star Trek who read this essay hoping to be told that Captain Kirk’s encounters with bizarre silicon creatures were not implausible should not be totally disheartened, however. I will happily admit that that we’ve been rather limited in our definition of what constitutes ‘life’, discussing only molecular biochemistry.

Figure 2: Kirk and Spock (carbon-based) examining a Horta (silicon-based) in an episode of the original series of Star Trek. [Paramount Pictures].

EvolutionSilicon’s preference for forming crystal lattices is its biological undoing – but it makes it far more suited to electronic applications than carbon. We are closer than ever before to creating artificial life, and it may well be that the first breakthrough comes not from a biochemistry lab, but from a computer science one – where the first ‘organism’ is a program embedded within pathways of silicon14, rather than a cell with a molecular backbone of carbon.

References

  1. Klein, H.P., 1979. The Viking mission and the search for life on Mars. Reviews of Geophysics17(7), pp.1655-1662.
  2. Jacob, D.T., 2016. There is no Silicon-based Life in the Solar System. Silicon8(1), pp.175-176.
  3. Tretkoff, E.,2004. Life’s building blocks are found all over galaxy, American Physical Society News (Vol 13, N.7)
  4. Swinburne Centre for Astrophysics and Supercomputing (http://astronomy.swin.edu.au/cosmos/C/Chemical+Composition, accessed 5/3/17)
  5. Service, R., 2016. Researchers take small step toward silicon based life, Science Magazine
  6. Papineau, D., 2010, November. Mineral Vestiges Of The Earliest Habitable Environments On Earth. In 2010 GSA Denver Annual Meeting.
  7. Schulze-Makuch, D. and Irwin, L.N., 2008. Life in the universe: expectations and constraints. Springer Science & Business Media.
  8. West, R., 1987. Chemistry of the Silicon‐Silicon Double Bond. AngewandteChemie International Edition in English26(12), pp.1201-1211.
  9. Dessy, R., 2010 Could silicon be the basis for alien life forms, just as carbon is on Earth?, Scientific American
  10. Cockell, C.S., 1999. Carbon biochemistry and the ultraviolet radiation environments of F, G, and K main sequence stars. Icarus141(2), pp.399-407
  11. Silber, K, July 1999. Is god in the details? Reason Magazine.
  12. Natta, A. et al 2006. Dust in proto-planetary disks: properties and evolution. arXiv preprint astro-ph/0602041.
  13. Engel, M.H. and Macko, S.A., 1997. Isotopic evidence for extraterrestrial non-racemic amino acids in the Murchison meteorite. Nature389(6648), pp.265-268.
  14. Helmreich, S., 1998. Silicon Second Nature: Culturing Artificial Life in a Digital World,. University of California Press, p.3

ben_fernando

“I’m applying techniques from terrestrial seismology to study the structure of the Sun and other stars by analysing their vibrations. I also like tea.”

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