Living cells possess: (1) a boundary membrane separating the cell's contents from its external environment; (2) one or more DNA molecules that carry genetic information for specifying the structure of proteins involved in replication of its own DNA , in metabolism, in growth, and in cell division; (3) a transcriptional system whereby RNAs are synthesized; (4) a translational system for decoding ribonucleotide sequences into amino acid sequences; and (5) a metabolic system that provides usable forms of energy to carry out these essential activities.
The first living system(s) were undoubtedly much simpler than any cells alive today. The transition from nonliving to living was gradual, and no single event led to life in all its modern complexity. Even today biologists cannot agree on a definition of life. The following criteria are usually included in attempts to define life. An aggregate of cells is considered "alive" if it (1) can use chemical energy or radiant energy to drive energy-requiring chemical reactions; (2) can increase its mass by controlled synthesis; and (3) possesses an information coding system and a system for translating the coded information into molecules that maintain the system and allow it to reproduce one or more collections of molecules with similar properties.
The best estimate for the age of the earth is 4.6 billion years. The oldest microfossils superficially resembling bacteria have been dated at about 3.5 billion years ago. Thus, chemical evolution (e.g., abiotic syntheses of amino acids and their polymerization into peptides) during the first 1.0 to 1.5 billion years of earth history probably preceded the appearance of cellular life and its subsequent biological evolution.
The major opinion is that earth's atmosphere was nearly neutral, nonoxidizing, and contained primarily nitrogen, carbon dioxide, hydrogen sulfide, and water. Microfossils resembling modern cyanobacteria ("blue-green algae") have been found in limestone rocks called stromalites dated 3.5 billion years ago. Presumably these ancient photosynthetic bacteria produced oxygen as a by-product of splitting water, just as cyanobacteria do today. Over more than another billion years, oxygen slowly began to accumulate, eventually causing the primitive atmosphere to become oxidizing.
There are two major scientific theories regarding how life came to be on earth. It either evolved on earth from nonliving chemicals, or it evolved elsewhere in the universe and was brought to the earth by comets or meteorites (panspermia theory). The belief that life was created by a supernatural force is impossible to support or refute with factual evidence and hence is outside the realm of science.
Amino acids and other precursors of modern biomacromolecules have been found inside meteorites, so chemical evolution of these molecules might have been (and still may be) widespread in the cosmos. In 1953, Stanley Miller, at the suggestion of his mentor Harold Urey, used a reflux apparatus to simulate early atmospheric conditions in an attempt to reproduce the chemical evolution of biological precursor molecules. He recirculated water vapor and other gases (CH4, NH3, and H2) through a chamber where they were exposed to a continuous high voltage electrical discharge that simulated natural lightning. After a few days, the mixture was analyzed and found to contain at least ten different amino acids, some aldehydes, and hydrogen cyanide. Subsequent experiments by Miller and other researchers using different molecular mixtures and energy sources produced a variety of other building blocks of biological polymers.
Sidney Fox and his colleagues heated amino acids under water-free (anhydric) conditions to temperatures of 160-210°C and found amino acids polymerized into proteinlike chains which he called proteinoids, which are branched rather than linear. When dissolved in water, these proteinoids exhibit several properties of biological proteins including limited enzymatic activity and susceptibility to digestion by proteases.
Protein like peptides can also be synthesized from amino acids on clays. Clays consist of alternating layers of inorganic ions and water molecules. The highly ordered lattice structure of clays strongly attracts organic molecules and promotes chemical reactions between them. Polypeptides have been detected in laboratory simulations of these processes.
When solutions of proteinoids are heated in water and then allowed to cool, small, spherical particles called microspheres are formed. These microspheres are about the same size and shape as spherical bacteria. Some are able to grow (add mass) by accretion of proteinoids and lipids and subsequently proliferate by binary fission or budding.
Lipids can spontaneously organize into double-layered bubbles called liposomes, which are leaky enough to absorb various substances such as proteins from the surrounding medium. Substances trapped with in the liposome find themselves in a hydrophobic environment that might provide more favorable conditions for certain kinds of chemical reactions. Thus, lipid bilayers may have promoted both aggregation and catalysis. Vesicles composed of lipid membranes and protein microspheres, but devoid of RNA or DNA molecules, are hypothesized to have existed in the early stages of life. These entities are called progenotes.
Early atmospheric conditions:
N, CO2, H2S, H2O
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