Liquid Crystals and the Origin of Life
Prior to the evolution of the first living things the world was made of crystals, liquids and gasses. Life’s beginnings must have involved the transition of matter from these states to one of liquid crystallinity.
Some important events in the origin of life on Earth probably include the concentration of carbon, the formation of complex organic molecules, sorting of those molecules into chiral types, the evolution of sustained loops of chemical interactions, formation of a replicator, a coupling of this chemistry and replication to an energy source, the origination of liquid crystallinity, and possibly a transition from indistinct networks of living material into relatively individual, self-contained entities or cells. This story concerns some of the possible scenarios for the concentration of carbon and, particularly, in the words of Robert Hazen (2001), how life was first “crafted from air, water and rock,” or, implicitly, from gaseous, liquid and crystalline matter.
Carbon concentration may have happened through repeated flooding of pools along the shores of early oceans, presumably by the action of the tides. These pools would have been flooded with water containing low concentrations of soluble carbon compounds, and the water would repeatedly evaporate at low tide leaving the heavier carbon behind, causing it to increase to saturation (De Duve and Miller 1991, Parsons et al. 1998). In this scenario life gets started in three dimensions, in solution.
Alternatively, life may have originated at the interface between water and minerals. Simple carbon molecules would have become attached to mineral surfaces, with moderately confining bonds so that they could interact with each other. Energy could come from the formation of the mineral. Wachtershauser (1994) has described early life as a “surface organism” on pyrite, energized by the formation of the pyrite. He says that carbon would enter the system, then undergo transformations for a time without being immobilized, and then be released in aqueous form. Interestingly, then, it seems carbon would have traveled through the surface organism in a kind of limbo between its more common dispersed and mineral forms, much as it does through current organisms and ecosystems.
One scenario has life evolving within connected chambers that result from the formation of certain common minerals, for example weathered feldspar (Parsons et al. 1998), or iron monosulfide (Martin and Russel 2003), perhaps with some molecular interactions/reactions on the surfaces and others taking place in solution within the chamber. More than a million chambers can exist over a square millimeter of weathered feldspar, making up a surface of 130 square millimeters from the microscopic perspective, and the chambers are the same size as modern bacteria (Parsons et al. 1998). In these cases organic reactants may not be destined to “die by dilution,” a potential problem with Wachtershauser’s model (De Duve and Miller 1991), or by irradiation or hydrolysis (Parsons et al. 1998).
Feldspar chambers are interconnected, providing opportunity for horizontal gene transfer (HGT). Woese (2002) argues that extensive horizontal gene transfer is the only way to explain the evolution of cells, given their complexity. In this scheme the components of early cells are relatively modular, and thus amenable to replacement by the products of transferred genes. The complex cells of today show a spectrum of chemical and physical interconnectivity between their components. Less integrated ones, such as aminoacyl-tRNA transferases, show a higher historical frequency of HGT than more integrated ones, such as ribosomal proteins. HGT would have been very important in collections of early, loosely organized cells. HGT could have driven the complexity of cells up through a point where the systems supporting them became sufficiently idiosyncratic that HGT became less important than traditional Darwinian selection based on individual variation. This point of transition Woese (2002) calls the “Darwinian threshold.” Given the common occurrence in complex dynamical systems of critical points and phase changes, transitions across the Darwinian threshold may have been rather drastic. Before it, species, and cell types, as we know them, would not have existed. The cell types of bacteria, archaea and eukaryotes may be relatively “solidified” ancestors of members of three “evolutionarily fluid” communities.
Original “cells” may have had mineral walls, then, through the formation and phase separations of lipids, membranes could have formed tiny semipermeable windows between the mineral cells and the outside environment (Parsons et al. 1998). Eventually various proto-cellular collections of organic molecules could have escaped from the containers, reforming the windows into protective spheres on the way, becoming much more like the cells we recognize today.
Membranes could have been the first liquid crystalline structures to attribute to living systems, but there is reason to believe that the origin of liquid crystallinity came earlier. Short segments of DNA self assemble into several liquid crystalline phases (Nakata et. al. 2007, University of Colorado 2007). While it had already been known that DNA exhibits liquid crystallinity, recent research shows that the assembling segments can be as short as six bases. They tend to stick together end to end, forming aggregates that behave like longer molecules of DNA, which then form a liquid crystal. But this only happens if base pairs match up. Complementary DNA segments form liquid crystals; non-complementary segments do not, meaning that complementary strands in the early stages of chemical evolution would tend to collect and self-organize into liquid crystalline droplets. Noel Clark, one of the researchers into the physical behavior of short strands of DNA, says:
“In essence, the liquid crystal phase condensation selects the appropriate molecular components, and with the right chemistry would evolve larger molecules tuned to stabilize the liquid crystal phase. If this is correct, the linear polymer shape of DNA itself is a vestige of formation by liquid crystal order.”
Like minerals, phase separations could provide a mechanism for concentration of carbon, as well as selection of certain types of molecules over others. The ones that are selected in this way may often be those that are common in living systems. Selection by phase separation may also be an alternative, or partial, explanation for the chirality of biomolecules. For instance molecules related to cholesterol, because of their chirality, when situated within a temperature gradient, organize into liquid crystals with a natural pulse (Cladis et al. 1991). This is termed “breathing mode,” and Cladis says that it is “sort of like the heartbeat turning on” (Peterson 1995).
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Nakata, Michi, Giuliano Zanchetta, Brandon D. Chapman, Christopher D. Jones, Julie O. Cross, Ronald Pindak, Tommaso Bellini, and Noel A. Clark. “End-to-End Stacking and Liquid Crystal Condensation of 6- to 20- Base Pair DNA Duplexes.” Science 318.5854 (2007): 1276–1279.
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University of Colorado at Boulder. “Tiny DNA Molecules Show Liquid Crystal Phases, Pointing Up New Scenario For First Life On Earth.” Science Daily 23 November 2007. 6 July 2008.
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