As stated in the Cell Theory, all cells arise from pre-existing cells. However, when considering the origin of life on Earth, it is not possible for the first cell to have arisen from a pre-existing cell. In this case, a progressive chain of events is responsible for how cells arose.
In order for cells to emerge on Earth, it was necessary for organic molecules to from as they are a fundamental building block of life. One theory to suggest how organic molecules came about is the panspermia theory, originally based on the ideas of Arrhenius in the 1900s (Jeffery and Ross 2007). This theory suggests that organic molecules that could be a basis for the emergence of cells arrived on Earth from the cosmos. There is some evidence to support this theory, including a meteorite found in Murchison, Victoria were scientists discovered that it contained more than 70 amino acids that did not correspond with those on Earth (Jeffery and Ross 2007). However, many scientists refute this theory proposing that such large concentrations of organic molecules needed to support the foundation of cellular life are unlikely to have survived the heating of impact when reaching Earth (Sorrell 1999).
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Another theory is the chemosynthetic theory which was individually hypothesised by two scientists; Alexander Oparin and John Haldane in the 1920s. This idea revived the theory of spontaneous generation as it suggests that organic molecules may have emerged from inorganic substances with the interaction of an energy source (Jeffery and Ross 2007). Haldane paid close attention to ultraviolet light experiments which encouraged the formation of organic compounds from a mixture notably containing water, ammonia and carbon dioxide (Lal 2008). Data such as these led Oparin and Haldane to suggest that the primeval atmosphere did not contain oxygen and furthermore that organic molecules could be formed in this early atmosphere with an energy source. img003.jpg
Figure 1: Urey and Millers experimental set-up (Mader 2008)In 1953, two scientists, Stanley Miller and Harold Urey tested the hypothesis developed by Oparin and Haldane (Campbell et al 2009). As shown in Figure 1, the experiment was set up containing the gases thought to comprise the atmosphere of early Earth; methane, ammonia, hydrogen and water vapour. Also demonstrated in Figure 1, the flask of gases was connected by a tube to another flask containing water in order to represent the ocean of primeval Earth. In the flask of gases, an electrical charge was passed through which is representative of lightning that could have triggered this reaction during the time of early Earth (Lal 2008). The flask was heated and after passing through a condenser, the collected liquid was observed to turn red and cloudy (Jeffery and Ross 2007). After testing, it was found to contain amino acids and other organic molecules. The experiment of Urey and Miller is significant as it supports the hypothesis of Oparin and Haldane and proves that it was possible for organic molecules to arise from inorganic molecules under conditions that mirror that of Early Earth. The emergence of these organic molecules was a fundamental step towards the development of cells.
The development of Macromolecules
Once organic molecules were present on Earth, it was then necessary for macromolecules such as RNA and proteins to emerge in order for the first cell to appear. Within the scientific community, there is a divide in existence as to which macromolecule appeared first and thus became the platform for the emergence of cells. Some scientists argue that proteins emerged first, while others argue that RNA came first and that 4 billion years ago an 'RNA World' existed.
A pioneer of the protein-first hypothesis is Professor Sidney Fox who outlined the theory and supporting evidence in many of his papers. Scientists supporting this theory suggest that amino acids polymerised to form proteinoids, in the medium of shallow pools of water heated by the sun (Mader, 2008). Scientist J.T Trevors of the University of Guelph, Canada acknowledges that in order for these proteinoids to form, a temperature of 65°C may have been required which would have been a common temperature during this period of early Earth. These proteinoids were suggested to have catalytic properties similar to that of enzymes (Mader 2008, Fox 1974). During his experiments to test the formation of proteinoids, Professor Fox placed emphasis on the simplicity of this process that could have easily occurred under the conditions of Early Earth.
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Figure 2: Basic outline of steps in the protein-first theory (Fox 1974)
The proteinoids then form microspheres when they come into contact with water and these microspheres possess many of the properties of modern cells (Mader 2008, Fox 1974). As demonstrated in Figure 2, this protocell then evolved into the contemporary idea of a cell. The figure also demonstrates that genetic information and thus RNA contributed to the formulation of a cell. However, this hypothesis stresses that it was the proteinoids formulated by polymerised amino acids that came before the RNA in order to form the first cell.
Conversely, many scientists support the hypothesis of the 'RNA World' which is considered to have preceeded protein synthesis (Orgel 2004). The discovery by Thomas Cech and Sidney Altman that RNA molecules, labelled ribozymes, could perform enzyme-like catalytic reactions is central to this theory (Campbell et al 2009, Orgel 2004). It is suggested that the structure of a ribosome is composed of a ribozyme, thus excluding the idea that peptides were involved in protein synthesis in the ribosome (Orgel 2004). It is plausible from these arguments, that an 'RNA World' once existed as RNA molecules alone can synthesise themselves, thus performing a stable system of self-replication without the need for proteins (Shapiro 2007).
The Development of the Protocell
After the formation of macromolecules, regardless of whether proteins or RNA emerged first, a protocell containing a structure similar to that of a modern plasma membrane was necessary for the first true cell to arise. Thus it is likely that a lipid-protein membrane developed prior to the formation of plasma membranes exhibited in cells today (Mader 2008). According to Fox in his paper on the Proteinoid Theory, lipid qualities can be found in the hydrocarbon side chains of several amino acids that make up the structure of proteinoids (Fox 1974). It is also possible that a semi-permeable layer may have formed around units known as coacervate droplets, which are formed under very specific conditions (Mader 2008). Eventually, self-replicating structures called liposomes formed from phospholipid molecules in water and it is suggested that protocell membranes may have emerged in a similar way (Trevors 2003, Mader 2008). Another theory on the formation of a primitive membrane considered that the membrane of the protocell formed as a result of the polymerisation of hydrophobic amino acids (Trevors 2003). Under this theory, it is suggested that protocells developed in a hydrophobic medium of hydrocarbons rather than an aquatic medium (Trevors 2003).
Some aspects of the formation of protocell membranes are seen to support of the panspermic theory, mentioned previously in discussing the origin of organic molecules. Lipids extracted from the meteorite found in Murchison, Victoria were shown to form bilayered vesicles when placed in an aqueous environment which supports this theory in the context of the formation of membranes (Griffiths 2007). It is likely that primitive membranes of protocells would have been simpler than modern membranes in terms of their structure and function. It is thought that the structure of such primitive membranes would have allowed for the diffusion of gases and simple amino acids (Trevors 2003). Irrespective of how the first membranes of protocells developed, they were essential in forming true cells as they separated chemical components, including genetic materials from the external environment (Mader 2008).
The First Cell
Once protocells became capable of reproduction, the first true cell emerged as the foundation for biological evolution (Mader 2008). The first true cell is considered to be defined as a membrane-bound structure capable of self-replicating functions (Trevors 2003). In order for such self-replication to occur, protein synthesis must be carried out to produce functional enzymes that foster DNA replication (Mader 2008).
Membranes of cells in this present day are composed of a phospholipid bilayer that provide cells with a clear distinction between the internal and external environment (Trevors 2003). However, contemporary cells also require protein channels that allow for both passive and active transport in a sophisticated manner (Mansy et al 2008). This requires membrane proteins to be embedded in the lipid bilayer of cell membranes, as seen in Figure 3, in order to facilitate transport of molecules according to the needs of the cell (Trevors 2003).
Figure 3: The phospholipid bilayer of modern cells embedded with proteins (Campbell et al 2009)
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Despite disagreements about the ordering of events involving proteins, it is accepted that the formation of cells required RNA in order to provide a platform for replication and that this replication is directed by protein synthesis in cells.
In conclusion, the emergence of the first cells throughout the Earth's history has been a transitional process with many fundamental steps. Primarily, the evolution of small organic molecules laid the foundations for macromolecules to form. These macromolecules provide structures for protocells to form which exist as the primitive basis of modern cells. The biological history of cells is of unequivocal importance in understanding the evolution of life in all forms on Earth.