Evidence

The high temperatures in hydrothermal vents would not allow synthesis of organic compounds, but would instead decompose them

Miller and Bada explain that the hypothesis that life arose at hydrothermal vents is based on a number of misunderstandings concerning the organic chemistry. Specifically, they break the hypothesis up in three steps, and prove one each wrong. The hydrothermal vent origin of life hypothesis steps are as follows: (1) synthesis of amino acids and other essential organic compounds from dissolved methane and other gases, initially at high temperatures, as they pass through a temperature gradient (>350° to -2°C); (2) synthesis of peptides and nucleotides by thermal dehydration and other high temperature reactions; and (3) synthesis of protocells or RNA like molecules at some stage in the thermal gradient, and the conversion of these protolife forms into living organisms as the hydrothermal waters emerge from the vents. From their results, the pyrolytic synthesis of amino acids is not an efficient process at such high temperatures, so the first step fails. Next, the polymerization of amino acids into peptides will not occur under vent conditions as the equilibrium is unfavorable in the presence of liquid water at all temperatures, so step 2 fails. Lastly, organisms made up of RNA-like molecules cannot survive because of the inherent instability of their molecules at high temperatures, so step 3 fails. This suggests that life could not have formed at hydrothermal vents because the results failed each of the three proposed steps involved in the origin of life.

Authors

Miller, S. L., and Bada, J. L.

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Evidence

Extreme thermophiles do not possess mechanisms to protect their macromolecular components against hydrolysis and degradation at high temperatures, like those around hydrothermal vents

Cells of an extreme thermophile were subjected to a temperature of 250°C and a pressure of 260 bar. Bernharadt et al. observed drastic chemical modifications of the amino acids and fast degradation of the polymeric compounds. Thus, indicating that previous predictions that extreme thermophiles could inhabit submarine hydrothermal vents at 250°C is most likely incorrect. These findings support the hypothesis that complex organics do not form at temperatures of 250°C due to degradation.

Authors

Bernhardt, G., Lüdemann, H. D., Jaenicke, R., König, H., and Stetter, K. O

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Evidence

Hydrogen cyanide (HCN) would not have been present in sufficient concentrations to polymerize in the primitive ocean to produce nucleic acid bases and amino acids

Miyakawa et al. were interested in the hydrolytic stabilities of hydrogen cyanide (HCN) and formamide with relevance to the origin of life. They measured hydrolysis rates of HCN and formamide at 30–150°C and pH 0–14. From their results, the steady state concentrations of HCN at 200, 100 and 0°C (at pH 8) were estimated to be 6×10−16 M, 7×10−13 M and 2×10−6 M, respectively. These concentrations are too low for HCN to polymerize to produce nucleic acid bases and amino acids. Furthermore, they estimated that the steady state concentrations of formamide at 200, 100, and 0°C (at pH 8) were 2×10−18 M, 1×10−15 M and 1×10−9 M, respectively. Under these conditions, it is unlikely that formamide could have served a significant role in prebiotic chemistry. As a result, it seems unlikely that HCN could have polymerized in a warm primitive ocean. Instead, it is suggested that eutectic freezing might have been required to have concentrated HCN sufficiently for it to polymerize. If some regions of the primitive Earth were frozen, HCN polymerization might have been important in the formation of complex organics necessary for the origin of life. This suggests that HCN would not have been present in hydrothermal vents because they are too hot to have sufficient concentrations of HCN to form polymers to produce acid bases and amino acids.

Authors

Miyakawa, S., James Cleaves, H., and Miller, S. L.

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Evidence

The pyrolytic synthesis of amino acids is not an efficient process because the yields are extremely low and the products are mostly β-amino acids, not the α-amino acids found in proteins

Lawless and Boynton investigated the thermal synthesis of amino acids and discovered that the predominance of β-amino acids at high temperatures, limited number of amino acids produced, and low yield (0.007%) in their experiment presented problems in the interpretation of chemical evolution. If their results are representative of what may have taken place on the primitive Earth, then they suggest that these small yields of the thermal experiments are discouraging because the formation of a primitive organism in an environment requiring the utilization of β-amino acids, followed by evolution into a modern organism that utilizes α-amino acids, is unlikely. These results suggest that life did not originate at hydrothermal vents and β-amino acids are more likely to be produced near these vents.

Authors

Lawless, J. G., and Boynton, C. D.

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Evidence

The prebiotic synthesis of purine nucleosides is most efficient when it is carried out under dry conditions

Fuller et al. completed the first experiment on successful prebiotic synthesis of purine nucleosides under prebiologically plausible conditions. They found that the purine nucleosides (adenosine, guanosine, inosine, xanthosine) are formed in a dry phase when the corresponding purine bases and D-ribose are heated together in the presence of certain salts and minerals. Additionally, they found that oligomerization of activated nucleotides (such as are formed during phosphorylation) can be promoted in the dry phase. So, it is possible that consecutive stages in prebiological chemical evolution might have occurred in dry phases because when the experiment was also repeated with the reactions in seawater using deoxy-D-ribose in place of D-ribose in the presence of adenine or hypoxanthine, the results indicate the formation of β-deoxyadenosine (>2% yield) and β-deoxyinosine (>5% yield), which are quite low. This suggests that nucleosides can be efficiently phosphorylated in a dry phase, instead of in a high-temperature environment, like hydrothermal vents, because sugars are very unstable at elevated temperatures and high-energy phosphates are hydrolyzed rapidly at 250°C. This supports the hypothesis that complex organics cannot be formed at hydrothermal vents because the synthesis of purine nucleosides are most efficient in dry environments.

Authors

Fuller, W. D., Sanchez, R. A., and Orgel, L. E.

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