First cellular life

Monnard, P.A., and Deamer, D.W. 2002. Membrane self-assembly processes Steps toward the first cellular life. Anat. Rec. 268 196-207. 

Other important historical landmarks include the founding, in 1984, of the Santa Fe Institute, which is one of the leading interdisciplinary centers for complex systems theory research the first conference devoted solely to research in cellular automata (which is a prototypical mathematical model of complex systems), organized by Farmer, Toffoli and Wolfram at MIT in 1984 [farmer84] and the first artificial life conference, organized by Chri.s Langton at Los Alamos National Laboratory, in 1987 [lang89]. 

The central role played by DNA in cellular life guarantees a place of importance for the study of its chemical and physical properties. It did not take long after Watson and Crick described the now iconic double helix structure for a question to arise about the ability of DNA to transport electrical charge. It seemed apparent to the trained eye of the chemist or physicist that the array of neatly stacked aromatic bases might facilitate the movement of an electron (or hole) along the length of the polymer. It is now more than 40 years since the first experimental results were reported, and that question has been answered with certainty. 

D. W. Deamer from the State University of California at Santa Cruz (1998) asks two important questions on the problem of the origin of the first cellular structures these questions must be answered if we want to understand life s beginnings ... 

We must give first an outline of the non-metal pathways which we observe in all cells. We start here because we know nothing about their abiotic chemistry but assume that cellular life arose from it. We shall assume that the basic requirement of all metabolism is the energised and catalysed synthesis of polysaccharides, lipids, proteins and nucleic acids. These are polymers (see Table 4.5), formed from monomers, all of which could have always arisen when energy was applied to the... 

The combination of the material of the first four chapters led us then to give an outline of the thermodynamic systems chemistry common to all cells. We offered little explanation of the origins of cellular life, as such an explanation could well remain beyond our insight. We then concerned ourselves with the way in which the... 

Of course, this is too much of a big picture and to define fife at this level may indeed appear impossible. However, one can scientifically tackle this question by looking at life in its simplest expression, namely microbes and other uiucellular organisms. This is a first, important clarification, which also eliminates (at least for most scientists) the notions of soul or consciousness from the picture. In other words, let us talk only about microbial fife, and try to give a definition of cellular life. 

The autopoietic analysis of life is based on cellular life, the main argument for this being simply that there are no other forms of life on earth. We all know that even the simplest cells are extremely complex, encompassing hundreds of genes and other macromolecules. However, beyond this complexity, the question of what a cell really does, lends itself to a relatively simple answer. Consider Figure 8.1, which schematizes a cell. The first thing one observes is the boundary, a semi-permeable, spherical closed membrane that discriminates the cell from the medium. Here the term semi-permeable means that certain substances (nutrients and other chemicals)... 

It is not yet understood how life began on Earth nearly four billion years ago, but it is certain that at some point very early in evolutionary history life became cellular. All cell membranes today are composed of complex amphiphilic molecules called phospholipids. It was discovered in 1965 that if phospholipids are isolated from cell membranes by extraction with an organic solvent, then exposed to water, they self-assemble into microscopic cell-sized vesicles called liposomes. It is now known that the membranes of the vesicles are composed of bimolecular layers of phospholipid, and the problem is that such complex molecules could not have been available at the time of life s beginning. Phospholipids are the result of a long evolutionary process, and their synthesis requires enzymatically catalyzed reactions that were not available for the first forms of cellular life. 

The conclusion is that membranous vesicles readily form a variety of amphiphilic molecules that would have been available in the early Earth environment, along with hundreds of other organic species. It is likely that during the chemical evolution leading to the first catalytic and replicating molecules, the ancestors of today s proteins and nucleic acids, membranous vesicles were available in the prebiotic environment, and ready to provide a home for the first forms of cellular life. 

The consideration of the minimal cell permits a logical link with the notion of compartments outlined in the previous chapter. Suppose that these 45-50 macromolecules - or their precursors - developed first in solution. In order to start cellular life, compartmentation should have come later on, and one would then have to assume the simultaneous entrapment of all these different genes in the same vesicle. This can indeed be regarded as highly improbable. A more reasonable scenario is one in which the complexity of cellular life evolved from within the same compartment - a situation namely where the 45 (or 206) macromolecules were produced and evolved from a much smaller group of components from the inside of the protocell. How, of course, remains to be seen. 

The field of the origin of life has progressed very much from the time of Stanley Miller s first experiment. However, the main hypothesis, that cellular life derives from inanimate matter, has not been demonstrated yet. It must then be considered still a working hypothesis. Not that we have alternatives within the realms of science, and I have outlined in Chapter 1 why divine creation cannot be considered as an alternative within science. Of course the question of God is not one that is solved in terms of rationality, but in terms of faith, and we are back to zero. 

Current research efforts have progressed to the point where the above processes have been investigated individually, so that the challenge now is to assemble them into an integrated system that exhibits the properties of the living state. This chapter focuses on the self-organizing properties of amphiphilic compounds that produce microscopic compartments necessary for the appearance of the first cellular forms of life. 

Because the lipid components of membranes must be in a fluid state to function as membranes in living cells, it is reasonable to assume that primitive membranes in the first forms of cellular life must also have had this property. Straight-chain hydrocarbons have relatively high melting points due to the ease with which van der Waals interactions can occur along the chains. Any discontinuity in the chains interrupts these interactions and markedly decreases the melting point. As an example, stearic acid contains 18 carbons in its alkane chain and melts at 68 °C, while oleic acid, with a cis-double bond between carbons 9 and 10, has a melting point near 14 °C. If cellular life today requires fluid membranes, it is reasonable to assume that the earliest cell membranes were also composed of amphiphilic molecules in a fluid state. 

Taking this standpoint of constructive biology, we have been working problems listed in Table I both theoretically and experimentally. The first two items in the table are related with the construction of a replicating system with compartment, raised in the questions in Section I. Of course, this problem is essential to consider the origin of a cellular life. However, we do not intend to... 

Cellular life has existed on earth for between 3.8 and 4 billion years [25]. As there was essentially no molecular oxygen in either the atmosphere or oceans for the first two billion years [26, 27], at least the great bulk of life inhabited neutral or reducing environments and appears to have been exclusively prokaryotic. Although photosynthesis is not ruled out during this period, the large-scale photolysis of water does seem unlikely, so that the free energy to drive life had to come from... 

The reductive metabolic core reactions are close enough to bulk physical chemistry to be studied with the statistical mechanics of complete reaction networks of small molecules, yet produce the biomass necessary to support the full complement of compartments, catalysts, prosthetic groups, and genes. The scenario requiring minimal happy accidents is one in which most of the complexity of cellular life developed around this metabolism over the first 0.5-2 Gy. 

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