As many an astrochemist will tell you without hesitation, polycyclic aromatic hydrocarbons are important molecules to study, because they’re directly relevant to the origins of life. We tend to repeat this like a mantra, and perhaps we don’t always fully appreciate the ramifications of what we’re saying. Contentious, hotly debated and under researched, the origin of life is a difficult and heavily transdisciplinary subject. It’s also a long standing fascination of mine, and the main reason I went into astrochemistry to begin with — so the “Aromatic World Hypothesis” attracted me like a moth to the proverbial flame. And what a flame it is, too!
The molecular building blocks of modern day life, such as amino acids or nucleobases, are fragile things, easily destroyed by increased temperature or ultraviolet light. By contrast, PAH molecules are stable, abundant and everywhere! As the largest fraction of carbon in the known universe is thought to be tied up in interstellar aromatic molecules (with aliphatics and carbon-rich ices comprising much of the rest), Pascale Ehrenfreund et al propose that the earliest life forms could actually have been made from PAHs! Certainly searches for interstellar biomolecules haven’t been especially fruitful. Taking a hint from life that exists on Earth at the moment, if something is abundant, life will probably figure out how to use it.
The definition of precisely what life is has never exactly been clear cut. Debates in the literature have, if we’re honest, only successfully enlightened us on one thing; we don’t really know what life is. Ehrenfreund et al get past this however, by laying down their own definition of a “minimal life form” early on. Their criteria are simple. A molecular system can be considered ‘alive’ if it turns resources into building blocks, replicates and evolves. A compartmentalised genetic system with a mechanism for harvesting energy from its environment. This certainly seems like a fair definition to me, at least for this context of primitive life.
In this regard, the biggest point this paper makes (and it makes it rather well) is that the components for the earliest Earth life were almost certainly very different compared with those of modern life. In my own opinion, nearly 4 billion years worth of quality time with evolution says that should be rather obvious!
The humbling idea here is that the basic ingredients for life probably came from space, brought down to Earth by the chondritic meteors which pummeled it in its early life. Space is, after all, full of places which can act as molecule factories. Hot molecular cores, for instance (they taste like strawberries, remember?). Reference is also made to the ubiquity of the diffuse interstellar bands, and the probable PAH molecules that cause them. Realising this, something lovely becomes apparent about this hypothesis. It allows you to consider life across astrophysical scales. It certainly bolsters the theories that life is abundant across the cosmos. I’m sure even the most staunchly scientific of minds might find that rather exciting. I certainly do!
The problem is how. How can abiotic molecules form the makings for a protocell, and how would they cooperatively organise together? In fact, that question is the raison d’être of this paper. The formation of PAH assemblies and their integration into a minimal life form. Such a “PAH Protocell” would be composed of three main components:
A non-covalent supramolecular assembly of amphiphilic molecules which contain the protocell’s components and maintain ideal conditions for the necessary chemical reactions. In other words, a big bag of stuff that holds everything together.
Amphiphilic surfactant molecules have a hydrophilic head and a hydrophobic tail. This means they tend to self-organise in water into micelles; little spheres composed of a number of these molecules. They can encapsulate other oily molecules and grow, acting as a container for the other molecules therein. Essentially, this is how soap works. The fatty acid molecules that we use as soap form micelles in water. Any biologist will know that micelles are the precursors to liposomes and lipid bilayers; structures commonly found in life’s cellular machinery.
Surprisingly, charged PAH molecules such as anthracenes are also amphiphilic and thus can also form micelles. This actually made me do a double take. PAHs can form micelles? Wow. Seriously, some days I feel like I know a lot about PAHs, while other days they still find new ways to surprise me! So the first protocells could have been based on micelles contructed from charged PAHs. Score one.
For something to be considered alive, it needs to have an autonomous metabolic system of some kind. Regardless how minimal it’s metabolism may be, if there’s no energy to drive it, no life is possible.
Life on earth today is driven by phototrophs. We know them better as plants. One possibility for an early metabolism could also have been phototrophic. PAH molecules could conceivably act as photosynthetic absorbers. They absorb strongly in the blue to near-ultraviolet frequencies. PAH molecules such as anthracene and quinones can act as energy transducers, absorbing photons and becoming excited, before transferring charge to to drive redox chemistry.
Primitive lipids can be formed this way. Given light and a small amount of oxygen, 2-ethyl anthracene will readily convert dodecane molecules into dodecanone and dodecanol, both of which are amphiphilic. This a good pathway to the formation of such early lipids, using trace amounts of O2 — which might eventually replace their charged PAH predecessors.
It’s a worthwhile point that modifying a compound into a useable building block could’ve been the first ever form of metabolism. PAH molecules can catalyse molecular reactions, effectively generating metabolism. The hypothesis is starting to look promising.
Forming an “informational polymer” to act as a genetic carrier is possibly the most difficult part of creating life. Probably even the rate defining step of the whole procedure. It has been noted in other work, however, that certain minerals promote the ordering of molecules on their surfaces, which could in turn assist the formation of such an informational polymer. Two such minerals are montmorillonite and diamond.
A genetic template is effectively a self enhancing catalyst which promotes its own proliferation. Needless to say, something as complex as DNA didn’t spring up overnight (indeed, some viruses even today don’t use DNA). That said, to qualify as a genetic component, a polymer would have to be able to control the formation and growth of the container, for instance by catalysing lipid formation.
It certainly makes sense that if a polymer formed which could catalyse the formation of amphiphilic molecules, it would soon become encapsulated in the molecules it created — provided of course, that it had sufficient material to continue catalysing.
Once again, PAHs could serve this purpose. In water, neutral PAHs also self organise — exhibiting liquid crystal behaviour due to hydrogen bonding and π-π stacking. While this is still highly theoretical, such a stack of PAHs could form a block copolymer with a specific sequence. It’s notable at this point, that PAHs actually stack with the same separation as DNA nucleobases (which is one of the reasons they’re so carcinogenic!). In work which is apparently only tenuously related to this paper, the “PAH World” model proposes π-π stacked PAH molecules as forming a scaffold for early nucleobases.
PAH molecules could have mediated the first genetic code? Their versatility is seemingly without limit.
The biggest problem here (as in all research into the origins of life) is how exactly these components would integrate together into an organism. It’s not very clear, though it’s a fact that random combinations are difficult due to possible cross-reactions between components. Of course, by probability alone, perhaps random chance is the best way to find a solution — finding three components that fit together by trial and error. It certainly seems to be life’s modus operandi on the road of evolution. Identifying what exactly could make the three components described above would certainly be helpful.
Another interesting implication of all of this is that obviously, there must be different possible transitions to get from non-life to life. As such, alien life could be based on entirely different chemical pathways and organisational principles. If life really began so radically differently, it could thus evolve to be even more radically different. Would we recognise it if we saw it? Though we don’t currently know of any alternatives to a carbon-based biochemistry, the idea certainly can’t be discounted.
The keyword here is evolvable. Doubtless some protocells would form which have no potential to evolve. As evolutionary dead ends, these would eventually be out-competed by more adaptable species and die off.
I think I’ve summarised everything I can as succinctly as possible, which is no easy task given the vastness of the subject at hand. It’s a big paper (more like a micro-thesis to be honest). All the same, concepts, both chemical and astronomical, are laid out and explained clearly, all the while quoting the work of such greats as Henning, Tielens, Allamandola, Frenklach and even Sagan himself. This is arguably the most comprehensive discussion for the transition from the chemical to the biological yet written. Well worth a read, whatever your scientific background. This work should be applauded!
Pascale Ehrenfreund, Steen Rasmussen, James Cleaves, Liaohai Chen (2006). Experimentally Tracing the Key Steps in the Origin of Life: The Aromatic World Astrobiology, 6 (3), 490-520 DOI: 10.1089/ast.2006.6.490