Abiogenesis is the field of study whereby the answers to how the chemistry of life was initiated are explored. Unlike evolution, which is described and understood via a number of falsifiable models which explain the observations made in the natural world, most models of abiogenesis will remain unfalsifiable and hence open to debate. The issue being of course that the conditions under which those first chemical reactions occurred no longer exist and trying to recreate an approximation of these in the lab requires a great deal patience, ingenuity and of course some intelligent guess work! Dr. Michael Russell of the Scottish Environmental Research Centre in Glasgow, on whose work this post is based, summed up this issue very well when he wrote...
"Even if you were to make a reactor in the laboratory, and put hydrogen and carbon dioxide and nitrogen in one end, and out pops something like Escherichia coli at the other end, you still couldn't prove that we and our ancestors arose that way. You'd just have a narrative that made it more plausible."
In this post I want to outline Dr. Russell's hypothesis. He suggests that the first life forms may have evolved within small compartments known as micro-caverns within iron sulphide (AKA Iron Pyrite or fool’s gold) deposits in alkaline thermal vents - deep sea hot springs. Please do not to be confuse these with the giant “black smokers” which billow out hydrothermal fluids at 350 – 400 deg C, these alkaline vents are much smaller and the water expelled through them is only c 100 deg C. When these micro-caverns first developed they would have been composed of a semi-permeable iron nickel sulphide (mackinawite) gel, across which a strong vertical chemistry gradient (a chemocline) could develop, allowing molecules to be concentrated. These were according to Russell's hypothesis the incubators for the first self-replicating molecules and the apparatus required to create life.
The building blocks for these prebiotic molecules and the energy required for the reactions to create them, were provided by the constant flow of hydrothermal water through the vents. This water was rich in hydrogen, cyanide compounds, sulphides and carbon monoxide. The chemocline micro-caverns provided a microenvironment that allowed the migration of synthesized compounds from areas of high concentration to areas of low concentration which facilitated the accumulation, and subsequent catalysation (by the Iron and nickel) of these molecules to create simple monomers (including amino acids). These monomers formed short chain polymers (oligomers) such as peptides, which in turn gave rise to the proteins and the nucleotides required to synthesise RNA.
What is interesting about this idea is there was no need for a lipid barrier or cell membrane until the majority of cellular functions had been developed. This means that according to this model LUCA, the "last universal common ancestor” was not a free living organic cell. The organic constituents making up “LUCA” were confined to the mineral based micro-caverns, which was functionally equivalent to a single cell. The lipid membrane required for free living cells was subsequently genetically encoded by the cellular apparatus.
So what is about this hypothesis that gives it any kind of credibility? First of all it overcomes the question of how an accumulation of molecules could occur, without the chemocine these molecules would have been dispersed as soon as they were synthesised. It also explains the initial oligomerisation and subsequent polymerisation was catalysed. But in addition to these ideas this model also provides a fascinating explanation as to how the energy required to enable cellular chemical synthesis may have evolved.
The ionic gradients that would have developed across the mackinawite gel are similar to the gradients that form across cell membranes. Inside the vent, the water would have been hot, alkaline and hydrogen rich, due to reactions between iron and water in a metamorphic process known as serpentinization. On the other side of the membrane outside the vent the oceanic water would have been cold and acidic. This creates a proton gradient, which could have been utilised to provide a source of energy to facilitate metabolic reactions within the micro-cavern. With a few notable exception modern living cells power much of their chemistry by creating similar gradients across their membranes, but rather than using an inorganic proton gradient they use a variety of proteins to do exactly the same thing. The inorganic proton gradient was initially used to power cell chemistry but life adapted, by changing the chemistry of proton gradient by using proteins, another step in life breaking free from the confines of its mineral incubator deep under the ocean, was established.
Russell’s hypothesis was further developed by assimilating another model developed by Professor William Martin of the University of Dusseldorf, I will discuss this in another post.