The origin of cells was the most important step in the evolution of life on Earth. The birth of the cell marked the passage from pre-biotic chemistry to partitioned units resembling modern cells. The final transition to living entities that fulfill all the definitions of modern cells depended on the ability to evolve effectively by natural selection. This transition has been called the Darwinian transition.
If life is viewed from the point of view of replicator molecules, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to keep complex molecules stable in a varying and sometimes aggressive environment; the latter is fundamental for the evolution of biological complexity. If the freely floating molecules that code for enzymes are not enclosed in cells, the enzymes will automatically benefit the neighbouring replicator molecules. The consequences of diffusion in non-partitioned life forms might be viewed as “parasitism by default.” Therefore the selection pressure on replicator molecules will be lower, as the ‘lucky’ molecule that produces the better enzyme has no definitive advantage over its close neighbors. If the molecule is enclosed in a cell membrane, then the enzymes coded will be available only to the replicator molecule itself. That molecule will uniquely benefit from the enzymes it codes for, giving it a better chance to multiply.
Partitioning may have begun from cell-like spheroids formed by proteinoids, which are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could have been the first cellular life forms on Earth.
Another possibility is that the shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles made mostly of water tend to burst quickly, oily bubbles are much more stable, lending more time to the particular bubble to perform these crucial experiments. The phospholipid is a good example of a common oily compound prevalent in the prebiotic seas.
Phospholipids are composed of a hydrophilic head on one end, and a hydrophobic tail on the other. They possess an important characteristic for the construction of cell membranes; they can come together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is not conducive to harbouring water-soluble organic molecules, but a lipid bilayer bubble can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction may have occurred when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the right compounds were released into the medium, the development of the first prokaryotes, eukaryotes, and multi-cellular organisms could be achieved.
The common ancestor of the now existing cellular lineages (eukaryotes, bacteria, and archaea) may have been a community of organisms that readily exchanged components and genes. It would have contained:
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like mitochondria and chloroplasts are remnants of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where at least part of the rest of the cell may have been derived from an ancestral archaean prokaryote cell. This concept is often termed the endosymbiotic theory. There is still debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.
How the current lineages of microbes evolved from this postulated community is currently unsolved but subject to intense research by biologists, stimulated by the great flow of new discoveries in genome science.
Modern evidence suggests that early cellular evolution occurred in a biological realm radically distinct from modern biology. It is thought that in this ancient realm, the current genetic role of DNA was largely filled by RNA, and catalysis also was largely mediated by RNA (that is, by ribozyme counterparts of enzymes). This concept is known as the RNA world hypothesis.
According to this hypothesis, the ancient RNA world transitioned into the modern cellular world via the evolution of protein synthesis, followed by replacement of many cellular ribozyme catalysts by protein-based enzymes. Proteins are much more flexible in catalysis than RNA due to the existence of diverse amino acid side chains with distinct chemical characteristics. The RNA record in existing cells appears to preserve some ‘molecular fossils’ from this RNA world. These RNA fossils include the ribosome itself (in which RNA catalyses peptide-bond formation), the modern ribozyme catalyst RNase P, and tRNAs.
The universal genetic code preserves some evidence for the RNA world. For instance, recent studies of transfer RNAs, the enzymes that charge them with amino acids (the first step in protein synthesis) and the way these components recognise and exploit the genetic code, have been used to suggest that the universal genetic code emerged before the evolution of the modern amino acid activation method for protein synthesis.
The First Cell arose in the previously pre-biotic world with the coming together of several entities that gave a single vesicle the unique chance to carry out three essential and quite different life processes. These were: (a) to copy informational macromolecules, (b) to carry out specific catalytic functions, and (c) to couple energy from the environment into usable chemical forms. These would foster subsequent cellular evolution and metabolism. Each of these three essential processes probably originated and was lost many times prior to The First Cell, but only when these three occurred together was life jump-started and Darwinian evolution of organisms began. (Koch and Silver, 2005)