ThePAH world hypothesis is a speculativehypothesis that proposes thatpolycyclic aromatic hydrocarbons (PAHs), known to be abundant in theuniverse,[1][2][3] including in comets,[4] and assumed to be abundant in theprimordial soup of the earlyEarth, played a major role in theorigin of life by mediating the synthesis ofRNA molecules, leading into theRNA world. However, as yet, the hypothesis is untested.[5]
The 1952Miller–Urey experiment demonstrated the synthesis oforganic compounds, such asamino acids,formaldehyde andsugars, from the originalinorganic precursors the researchers presumed to have been present in theprimordial soup (but is no longer considered likely). This experiment inspired many others. In 1961,Joan Oró found that thenucleotide baseadenine could be made fromhydrogen cyanide (HCN) andammonia in a water solution.[6] Experiments conducted later showed that the otherRNA and DNA nucleobases could be obtained through simulated prebiotic chemistry with areducing atmosphere.[7]
TheRNA world hypothesis shows how RNA can become its owncatalyst (aribozyme). In between there are some missing steps such as how the firstRNA molecules could be formed. The PAH world hypothesis was proposed by Simon Nicholas Platts in May 2004 to try to fill in this missing step.[8] A more thoroughly elaborated idea has been published byEhrenfreundet al.[9]
Polycyclic aromatic hydrocarbons are the most common and abundant of the known polyatomic molecules in the visibleuniverse, and are considered a likely constituent of theprimordial sea.[1][2][3] PAHs, along withfullerenes (or "buckyballs"), have been recently detected in nebulae.[10]Buckminsterfullerene (C60) has been identified in theinterstellar medium spaces.[11][12] (Fullerenes are also implicated in the origin of life; according to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth."[13]) PAHs, subjected tointerstellar medium (ISM) conditions, are transformed, throughhydrogenation,oxygenation andhydroxylation, to more complexorganics — "a step along the path towardamino acids andnucleotides, the raw materials ofproteins andDNA, respectively".[14][15] Further, as a result of these transformations, the PAHs lose theirspectroscopic signature which could be one of the reasons "for the lack of PAH detection ininterstellar icegrains, particularly the outer regions of cold, dense clouds or the upper molecular layers ofprotoplanetary disks."[14][15]
In 2013,polycyclic aromatic hydrocarbons were detected in theupper atmosphere ofTitan, the largestmoon of theplanetSaturn.[16]
Low-temperature chemical pathways from simpleorganic compounds to complex PAHs have been demonstrated. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere ofSaturn's moonTitan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it.[17][18]
PAHs are not normally very soluble in sea water, but when subject to ionizing radiation such as solarUV light, the outerhydrogen atoms can be stripped off and replaced with ahydroxyl group, rendering the PAHs far more soluble.
These modified PAHs areamphiphilic, which means that they have parts that are bothhydrophilic andhydrophobic. When in solution, they assemble indiscoticmesogenic (liquid crystal) stacks which, likelipids, tend to organize with their hydrophobic parts protected.
In 2014,NASA announced a database[19] for trackingpolycyclic aromatic hydrocarbons (PAHs) in theuniverse. More than 20% of thecarbon in the universe may be associated with PAHs,[19][20] possiblestarting materials for theformation oflife. PAHs seem to have been formed as early as a couple of billion years after theBig Bang, are abundant in the universe,[1][2][3] and are associated withnew stars andexoplanets.[19]
In the self-ordering PAH stack, the separation between adjacent rings is 0.34 nm. This is the same separation found between adjacentnucleotides ofRNA andDNA. Smaller molecules will naturally attach themselves to the PAH rings. However PAH rings, while forming, tend to swivel around on one another, which will tend to dislodge attached compounds that would collide with those attached to those above and below. Therefore, it encourages preferential attachment of flat molecules such aspyrimidine andpurinenucleobases, the key constituents (and information carriers) of RNA and DNA. These bases are similarly amphiphilic and so also tend to line up in similar stacks.
According to the hypothesis, once the nucleobases are attached (viahydrogen bonds) to the PAH scaffolding, the inter-base distance would select for "linker" molecules of a specific size, such as smallformaldehyde (methanal)oligomers, also taken from the prebiotic "soup", which will bind (viacovalent bonds) to the nucleobases as well as each other to add a flexible structural backbone.[5][8]
A subsequent transient drop in the ambientpH (increase in acidity), for example as a result of avolcanic discharge of acidic gases such assulfur dioxide orcarbon dioxide, would allow the bases to break off from their PAH scaffolding, forming RNA-like molecules (with the formaldehyde backbone instead of the ribose-phosphate backbone used by "modern" RNA, but the same 0.34 nm pitch).[5]
The hypothesis further speculates that once long RNA-like single strands are detached from the PAH stacks, and after ambient pH levels became less acidic, they would tend to fold back on themselves, withcomplementary sequences of nucleobases preferentially seeking out each other and forminghydrogen bonds, creating stable, at least partially double-stranded RNA-like structures, similar toribozymes. The formaldehyde oligomers would eventually be replaced with more stable ribose-phosphate molecules for the backbone material, resulting in a starting milestone for theRNA world hypothesis, which speculates about further evolutionary developments from that point.[5][8][21]
