Problems with the Origin of Biological Membranes in an Early Earth Environment`
Abiogenesis theories require that cellular life on earth must begin with a certain minimum number of components. These components consist of some chemical mechanism to acquire energy to produce work (i.e., some kind of metabolic system), a method to transfer heredity (RNA, DNA, or something similar), and a means to keep these components together (some kind of membrane).
Producing all three components at once is extremely unlikely, so most proponents of abiogenesis hypothesize either metabolism first or replication first scenarios. Both kinds of theories suffer significant flaws. However, as shall be seen, the production of biological membranes under early earth conditions is no trivial task.
Early studies on possible prebiotic membranes began in the late 1950's using aggregated colloidal particles and lipid-like surfactants. Subsequent studies by Oparin examined the possible role of coacervates as membranes. Even though these complexes are unsuitable as possible membrane material because they are inherently unstable, lack the ability to provide a permeability barrier, and lack the ability to encapsulate metabolism, these materials are still prominently featured in modern high school biology textbooks.
The spontaneous formation of bilayer vesicles from phospholipids was first studied in 1965. Although this theory comprises the dominant explanation for the appearance of membranes, it is not without challenging problems. It has been shown that fatty acids will spontaneously form phospholipids in the presence of glycerol and phosphates when heated to dryness. However, Monnard and Deamer point out that it would be extremely unlikely that nature would produce all three chemicals in the same location and then heat them to dryness.
Sources of membrane building blocks
Long-chain hydrocarbons can be formed from carbon monoxide and hydrogen in the presence of certain metals at high temperatures. Deep sea hydrothermal vents have been cited as a potential source of the energy required to synthesize prebiotic molecules, including the building blocks of membranes. Fatty acids and fatty alcohols have been synthesized under these conditions.
These fatty acids will combine with ethylene glycol to form ethylene glycolyl alkanoates and bis-alkanoates, or will combine with glycerol to form monoacylglycerols and diacylglycerols. Others have suggested that the first membranes consisted of highly branched polyprenyl chains, instead of alkyl chains. However, it is unlikely that the starting material would be at sufficient concentrations and it also unlikely that the required phosphorylating agents would have been available on early Earth.
The extraterrestrial origin of membrane components have also bee cited by some investigators. Although lipid-like materials were found in the Murchison meteorite, subsequent studies suggested that those compounds were contaminants, rather than endogenous materials. Even if some membrane building blocks were delivered through extraterrestrial sources, degradation through hydrolysis, photochemical degradation, and pyrolysis would have significantly reduced the amount of such materials.
Even if membrane building blocks were present in sufficient quantities, specific concentrations and other environmental conditions are required for assembly. All possible components other than fatty acids have been eliminated from contention for a lack of plausible synthetic pathways.
The assembly of fatty acids into lipid bilayers is dependent upon the chain length, concentration, pH, and temperature. Short chain fatty acids form vesicles at room temperature when the pH is within half a pH unit of the pKa of the acid. Longer chain fatty acids require higher temperatures (30-70°C). In addition, the concentration of fatty acids must be quite high (130 to 20 mM) for vesicles to form.
The presence of such high concentrations of fatty acids would be unlikely on the primordial earth. Some of the exacting conditions can be moderated by the presence of fatty alcohols with the same hydrocarbon chain length as the fatty acid.
However, the molar ratio must be almost exactly 10:1 (acid:alcohol) in order for any significant effect to be seen. In addition to temperature, pH, and concentration requirements, vesicle formation is highly dependent upon ionic strength and the presence of certain ions. Thus, the presence of sodium chloride at levels found in the oceans of primordial earth causes vesicles to aggregate into sheets, and the presence of Ca+2, Mg+2, Fe+2 at primordial concentrations causes fatty acids to precipitate.
Encapsulation and transport
The mere formation of an enclosed vesicle is not sufficient to guarantee functionality. In order to be useful as a mechanism involved in the naturalistic origin of life, membranes must encapsulate materials necessary to initiate life and be able to transport material in and out of the boundary.
Modern biological membranes contain protein systems that actively and passively allow the exchange of nutrients and wastes. Since these transport systems would not be available on the primordial earth, other systems must have existed to make the process even remotely feasible.
Despite the extremely unlikely appearance of phospholipid membranes under early-earth conditions, most studies that have examined encapsulation have used such membranes. Encapsulation of primitive fatty acid membranes would have to involve repeated rupture and resealing (through agitation) during periods of changing osmotic gradients (increasing and decreasing salt concentrations).
Changing the concentrations of solutes would have the additional problem of likely altering pH, which would disrupt the exacting conditions required for fatty acid membrane assembly. The presence of locations where these exact conditions would exist would be very limited on the primordial earth. In addition, in order for some form of life to be created in this manner, both a primitive replicator and metabolic system must be encapsulated at this time. Of course, such systems would both encapsulate and release materials on each cycle, and it is unclear what kind of equilibrium would be eventually achieved.
Once a stable membrane is formed, some kind of transport system for nutrients/wastes would be required to maintain the metabolism of the proto-cell. Passive transport systems would be the easiest to form, but such systems would automatically achieve equilibrium, making further transport impossible. Obviously, due to their complexity, active transport systems would not be expected to be encoded by a primitive replicator.
Possible sources of energy for proto-cells are heat energy, chemical energy, and light energy. However, none of these forms of energy harvest are compatible with a primitive fatty acid membrane in the presence of known prebiotic chemicals present in the environment. This is because carboxyl head group of a fatty acid membrane mediates proton permeability, eliminating the possibility of generating a proton gradient.
The only way to get around this problem was to use an oleate-arginine system, which slowed the decay of the gradient. However, the unsaturated oleate would not have been present in a prebiotic environment. In addition, the system was inhibited by alkali cations, which would have been present in early earth environments.
The capture of light energy by proto-cells has been hypothesized to occur through encapsulated iron compounds or polycyclic aromatic hydrocarbons (PAH), which absorb light in the near-UV and blue wavelengths. While ferrocyanide and PAH may have been present on early earth, these compounds cannot generate a proton gradient when enclosed within fatty acid membranes.
Growth and Division
Primitive membranes must have the ability to grow and replicate without the aid of biomolecular machinery in order to function in a hypothesized proto-cell. Slow addition of myristoleate micelles to a myristoleic acid/ myristoleate vesicle system results in 90% of the added fatty acid was incorporated into the original vesicles, causing them to grow. Others have used osmotically swollen oleate vesicles to cause growth through vesicle-vesicle fusion.
However, since the presence of these unsaturated fatty acids on early earth is unlikely, the relevance of these studies to the origin of life is questionable. When considered in the context of an RNA world scenario, the requirement for the presence of divalent cations by ribozymes would result in the precipitation of fatty acids, disrupting membranes. Division of membranes may occur when they reach a certain size. The ability of this to occur is dependent upon the size of the membrane and it composition.
However, since the studies were done only with unsaturated fatty acid membranes, it is unclear what relevance there would be to the fission of saturate fatty acid membranes under early earth environments.
Besides the problem that most origin of membranes studies have examined membranes composed of materials that would have never existed on the primordial earth, there is an even more fundamental problem that tends to plague virtually all origin of life research. Once a compound has been declared "prebiotic", researchers immediately begin using the highly purified product at exceptional concentrations. According to Robert Shapiro:
"The observation of a specific organic chemical in any quantity (even as part of a complex mixture) in one of the above sources would justify its classification as "prebiotic," a substance that supposedly had been proved to be present on the early Earth. Once awarded this distinction, the chemical could then be used in pure form, in any quantity, in another prebiotic reaction. The products of such a reaction would also be considered "prebiotic" and employed in the next step in the sequence."
The origin of biological membranes, like the origin of replication and metabolism, is fraught with problems and invokes extremely improbable chemistry. Although some of the building blocks of potential membranes might have been synthesized on early earth, the ones used in modern biological membranes (phospholipids) could not have been.
Therefore, one must hypothesize some kind of primordial membrane that was later discarded in favor of modern membranes. However, even this scenario suffers from insurmountable problems. The extraterrestrial synthesis and delivery of membrane building blocks remain unproved. Although such materials might have been synthesized near hydrothermal vents in the early seas, the assembly of such materials is quite problematic.
Conditions requiring high concentrations, exact pH and temperature, plus the absence of high sodium and small amounts of certain metal ions, prevents the assembly of such components within the earth's early oceans. Conditions that might concentrate fatty acids to sufficient levels to form membranes would also concentrate solutes that disrupt the formation of those membranes. Encapsulation of a proto-cell replicator and metabolic system would be quite problematic, since the conditions that would encourage such activity would likely lead to conditions that would disrupt the primitive membrane completely.
Primitive membranes must be able to transport nutrients and wastes, although passive transport systems would readily reach equilibrium and active transport systems would not be expected to be produced immediately upon encapsulation. Energy acquisition is problematic, since fatty acids membranes cannot generate a proton gradient. Membranes composed of unsaturated fatty acids or phospholipids can generate proton gradients, but would not be expected to have existed in early earth environments.
Virtually all studies that have examined membrane growth and division have used unsaturated fatty acid membranes, which would not have been present on the early earth. Because of this problem, these studies have questionable relevance to the origin of life on earth.
The use of highly purified chemical in extremely high quantities in origin of life studies is questionable at best. Obviously, such experiments are fundamentally flawed, since no such conditions could have ever existed in any early earth environments. No chemists were present 4 billion years ago when life first arose, unless, one considers the ultimate Chemist. As Christian De Duve of Rockefeller University once asked rhetorically, "Did God make RNA?" Maybe he made more than just RNA?
The origin of primitive membranes is one of the more neglected fields in origin of life research. Although fatty acids can be produced under early earth conditions, they are quite resistant to assembly under conditions under which they are generated. In addition, these membranes fail to provide a reasonable mechanism for nutrient/waste transport or any way to produce a proton gradient, so that any "proto-cell" generated would soon die from an inability to use energy.