Here's what they did. They took a bunch of pure sugar phosphates1 and dissolved them in water containing salts and metal ions that were likely present in the primordial oceans. They heated the solution up to 70° C and looked at the degradation products. Low and behold, the sugar phosphates degraded and sometime the products were other intermediates in the glycolytic and pentose phosphate pathway, including pyruvate and glucose.
They conclude that ...
It is therefore apparent that heat exposure is sufficient to convert intermediate metabolites of glycolysis and the pentose phosphate pathway into pyruvate and glucose that constitute thermodynamically stable products also in the modern, enzyme‐catalysed metabolism, and to induce isomerization between pentose phosphate metabolites.Next they looked at the rate of degradation using a concentration of 7.5 μM. They found that there was no significant spontaneous degradation at 40° but at 50° or higher there was a lot of spontaneous degradation to pyruvate.
It is assumed that the pathways that mediate sugar phosphate interconversion, glycolysis, the pentose phosphate pathway, as well as the related Entner-Doudoroff pathway and Calvin cycle are evolutionarily ancient, as they are conserved and fulfil their central metabolic functionality virtually ubiquitously. Known as central, or primary, metabolism, their reaction sequences provide ribose 5-phosphate for the backbone of RNA and DNA, building blocks for the synthesis of co-enzymes, amino acids and lipids and supply the cell with energy in form of ATP and redox equivalent.If it's true that the first metabolic pathways were glycolysis and other catabolic (degradation) reactions then there had to be an abundant supply of glucose in the primodial soup (ocean). If that was true, then the authors think they've identified primitive catalysts (e.g. iron) that catalyzed nonenzymatic pathways resembling the modern biochemical pathways. These might have been precursors to the biological pathways that evolved when life originated.
The authors conclude the paper with ...
In summary, we report that plausible Archean ocean chemical compositions serve as catalysts for a series of sugar phosphate interconversions among pentose phosphate pathway and glycolytic metabolites. The reactions connect important metabolic intermediates including ribose 5-phosphate and erythrose 4-phosphate and eventually feed into the metabolite pools of pyruvate and glucose, the stable products of modern glycolysis and gluconeogenesis. These results indicate that the basic architecture of the modern metabolic network could have originated from chemical and physical constraints that existed in the prebiotic earth’s ocean. These findings suggest that simple inorganic molecules, abundantly present in the Archean ocean, may have served as catalysts in early forms of metabolism and facilitated sugar phosphate interconver sion sequences that resemble glycolysis and the pentose phosphate pathway. These results therefore support the hypothesis that the topology of extant metabolic network could have originated from the structure of a primitive, metabolism-like, prebiotic chemical interconversion network.I think this is nonsense. The most common pathway present in all species is gluconeogenesis—the pathway that makes glucose (and glucose 6-phosphate) from carbon dioxide (CO2). I think that was likely to be the most primitive pathway, a hypothesis that's consistent with the Metabolism First scenario for the origin of life [Changing Ideas About The Origin Of Life] [Was the Origin of Life a Lucky Accident?] [Why Are Cells Powered by Proton Gradients?] [Metabolism First and the Origin of Life].
In order to see what the soupists are up against, let's calculate the amount of glucose 6-phosphate that has to be present in the primordial ocean.
The volume of the oceans = 1.3 × 109 km3 or 1.3 × 1021 litres (National Geophysical Data Center (USA)). (The ancient ocean was probably larger.) The molecular mass of glucose 6-phosphate is 260.136 (Wikipedia). (We'll assume that it's all stereochemically pure D-glucose 6-phosphate.)
If the concentration was 7.5 μM then this represents 2366 billion metric tonnes of glucose or 2366 gigatonnes (Gt). This amount of glucose 6-phospahte degrades to other molecules within 24 hours, according to Keller et al. (2014) so that's the amount that has to be resupplied every day in order to maintain a constant concentration of 7.5 μM. This has to continue for millions of years while life evolved.
That's a lot of sugar. To put it into perspective, the total production of cellulose by all the plants on Earth is 180 Gt per year (Stricklen, 2008). (Cellulose is mostly a bunch of glucose molecules attached to each other.) Something would have to produce ten times this amount per day in order to resupply the glucose in the primordial ocean.
I'm not saying that the soupists are wrong. I'm just saying that they don't think through the implications of what they are proposing.
But I'm also saying that papers like Keller et al. (2014) are not good science for another reason. It's okay to advocate for one side of a controversial issue. That's what science is all about. It's not okay to completely ignore your opponents and present opinions as if they represented the overwhelming consensus in the field.
Here's a list of a few papers that are in my "origin of Life" file.
Lane, N., Allen, J. F. & Martin, W. (2010). How did LUCA make a living? Chemiosmosis in the origin of life. BioEssays 32, 271-280.Nick Lane, Bill Martin, and Michael Russell are well-known contributors to the field. None of them would agree that the glycolytic pathway came before the gluconeogenic pathway. You have to make glucose before you can degrade it. None of them would agree that life arose in a primordial soup of amino acids, sugars, and nucleotides.
Lane, N. & Martin, W. F. (2012). The origin of membrane bioenergetics. Cell 151, 1406-1416.
Martin, W., Baross, J., Kelley, D. & Russell, M. J. (2008). Hydrothermal vents and the origin of life. Nature Reviews Microbiology 6, 805-814.
Martin, W. & Müller, M. (1998). The hydrogen hypothesis for the first eukaryote. Nature 392, 37-41.
Martin, W. & Russell, M. J. (2003). On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 358, 59-85.
Martin, W. & Russell, M. J. (2007). On the origin of biochemistry at an alkaline hydrothermal vent. Philosophical Transactions of the Royal Society B: Biological Sciences 362, 1887-1926.
Martin, W. F. (2012). Hydrogen, metals, bifurcating electrons, and proton gradients: the early evolution of biological energy conservation. FEBS letters 586, 485-493.
Nitschke, W. & Russell, M. J. (2009). Hydrothermal focusing of chemical and chemiosmotic energy, supported by delivery of catalytic Fe, Ni, Mo/W, Co, S and Se, forced life to emerge. Journal of molecular evolution 69, 481-496.
Russell, M., Hall, A. & Martin, W. (2010). Serpentinization as a source of energy at the origin of life. Geobiology 8, 355-371.
Russell, M. J. & Martin, W. (2004). The rocky roots of the acetyl-CoA pathway. Trends in biochemical sciences 29, 358-363.
Schwartzman, D. & Lineweaver, C. (2004). The hyperthermophilic origin of life revisited. Biochemical Society Transactions 32, 168-171.
Sousa, F. L., Thiergart, T., Landan, G., Nelson-Sathi, S., Pereira, I. A., Allen, J. F., Lane, N. & Martin, W. F. (2013). Early bioenergetic evolution. Philosophical Transactions of the Royal Society B: Biological Sciences 368, 20130088.
The Keller et al. paper has 61 references. There's only one single reference to a paper by Lane, Martin, or Russel. It's the Sousa et al. (2013) paper. Here's how they refer to it ...
In modern cells, most metabolic reactions are catalysed by protein-based enzymes. Modern enzymes possess, however, highly specialized and complex structures, and it is unlikely that they evolved before a metabolic system was in place. The major question about the early forms of metabolism is thus the nature about their precursors, the first metabolic catalysts (Lazcano & Miller, 1999; Shapiro, 2000; Anet, 2004; Sousa et al, 2013). RNA could be an option; however, central metabolism lacks examples of RNA-catalysed reactions [and synthetic ribozymes that catalyse aldolase reactions require a bivalent metal for its activity (Fusz et al, 2005)]. Here, we demonstrate that simple inorganic molecules, frequently found in sediments dated to the Archean period, can catalyse reactions analogous to what is observed in modern pathways.There's no mention of the fact that prominent authors disagree strongly with the premises of their argument. That's not good science. The paper should never have been published as it is. Blame the journal and the reviewers for failing to do their jobs but, most of all, blame the authors for misleading readers by ignoring contrary opinions. They should be ashamed of themselves.
1. glucose 6‐phosphate (G6P), fructose 6‐phosphate (F6P), fructose 1,6‐bisphosphate (F16BP), dihydroxyacetone phosphate (DHAP), glyceraldehyde 3‐phosphate (G3P), 3‐phosphoglycerate (3PG), phosphoenolpyruvate (PEP), 6‐phosphogluconate (6PG), ribulose 5‐phosphate (Ru5P), ribose 5‐phosphate (R5P), xylulose 5‐phosphate (X5P) and sedoheptulose 7‐phosphate (S7P)
Keller, M.A., Turchyn, A.V. and Ralser, M. (2014) Non‐enzymatic glycolysis and pentose phosphate pathway‐like reactions in a plausible Archean ocean. Molecular Systems Biology 10:725 [doi: 10.1002/msb.20145228] [text: open access]
Sticklen, M.B. (2008) Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol. Nature Reviews Genetics 9:433-443. [doi: 10.1038/nrg2336]