Cosmologists conclude that the universe as we know it today originated from an explosion that erupted from an extremely dense mass, known as the Big Bang. very soon thereafter, the universe inflated—it expanded at an inconceivably rapid rate. Within the first second after the Big Bang this rapid inflation had ceased, but the universe has continued expanding at a much slower pace ever since. Astronomers have found evidence that galaxies evolved from gravitational effects on swirling gases left over from the Big Bang. The total number of galaxies is estimated to be in the hundreds of billions. Stars formed within galaxies, and in the cores of the stars, helium and hydrogen fused into heavier elements. Additional elements are produced when stars explode. As stars die, many eject the heavy elements, enriching the gas and dust from which new generations of stars (and planets) will be born. Thus, the elements have evolved over the 13 billion years since the first galaxies began to form.
Cosmologists and geologists tell us that between 4 billion and 5 billion years ago the planet Earth formed from the accumulation of matter that was encircling the sun. In earliest times, Earth looked far different from what we see today: it was an inhospitable place scorched by radiation, bombarded by meteorites and comets, and belching noxious chemicals from volcanoes and massive cracks in the planet's crust. Yet it is hypothesized that Earth's atmosphere evolved from those gases emitted, and water might well have been brought to the planet's surface by those comets that were crashing into it.
Meteors and comets bombarded Earth until about 3.8 billion years ago. In such an environment, life could not have survived. After the bombardment ceased, however, primitive replicating structures evolved. Currently, there is not yet a consensus about how these first living things originated, and there are several directions of active research. Before there were living creatures, of course, there had to be organic (i.e., carbon-containing) molecules. Fortunately, such organic chemistry is common throughout space, so the raw material for life was probably abundant. Answering the question of chemical prebiotic evolution involves developing plausible scenarios for the emergence of organic molecules such as sugars, purines, and pyrimidines, as well as the building blocks of life, amino acids.
To explore this question, in the 1950s, scientists began experimenting to determine whether organic compounds could be formed from methane, ammonia, water vapor, and hydrogen—gases that were likely to have been present in Earth's early atmosphere. By introducing electrical sparks to combinations of gases, researchers were able to produce most of the amino acids that occur in proteins—which are the same amino acids found in meteorites—as well as other organic molecules (Miller 1992: 19). Because the actual composition of Earth's early atmosphere is not known, investigators have tried introducing sparks to various combinations of gases other than the original hypothesized blend. These also produce amino acids (Rode 1999: 774). Apparently, organic molecules form spontaneously on Earth and elsewhere, which has led one investigator to conclude, "There appears to be a universal organic chemistry, one that is manifest in interstellar space, occurs in the atmospheres of the major planets of the solar system, and must also have occurred in the reducing atmosphere of the primitive Earth" (Miller 1992: 20).
For life to emerge, some organic molecules had to be formed and then combined into amino acids and proteins, while other organic molecules had to be combined into something that could replicate: a material that could pass information from generation to generation. Modern living things are composed of cells, which consist of a variety of functioning components that are enclosed by a membrane; membranes set cells off from their environments and make them recognizable entities. As a result, origin-of-life research focuses on explaining the origin of proteins, the origin of heredity material, and the formation of membranes.
Origin-of-life researchers joke about their models falling into two camps: heaven and hell. Hell theories point to the present-day existence of some of the simplest known forms of life in severe environments, both hot and cold. Some primitive forms of life live in hot deep-sea vents where sulfur compounds and heat provide the energy to carry on metabolism and reproduction. Could such an environment have been the breeding ground of the first primitive forms of life? Other scientists have discovered primitive bacteria in permanently or nearly permanently frozen environments in the Arctic and the Antarctic. Perhaps deep in ice or deep in the sea, protected from harmful ultraviolet radiation, organic molecules assembled into primitive replicating structures.
The heaven theories note that organic molecules occur spontaneously in dust clouds of space and that amino acids have been found in meteorites. Perhaps these rocky visitors from outer space brought these basic components of life, which combined in Earth's waters to form replicating structures.
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