Big Bang Theory
The cosmos, which comprises all the material of reality, may have originated from 10 to 20 billion years ago. Our particular portion of the cosmos is the universe known as the Milky Way. A universe is an island comprising millions of stars, although the term universe is sometimes loosely applied to the entire cosmos. The sun is a medium-sized star lying about two-thirds of the way from the center of the Milky Way. The sun and its planetary satellites make up the solar system. (One alternative theory is that the cosmos always existed much as it does today.) The prevalent view is that the cosmos (life) began with a massive explosion of tightly condensed matter many billions of years ago—the big bang theory. Remnants of that long-ago explosion can be studied with powerful telescopes that can pick up light originating many billions of years ago.
Formation of Solar System
Our solar system probably began as a swirling cloud of gases that eventually condensed into the sun and the planets. The early earth started out as gaseous, but after a while a core of heavy metals, such as nickel and lead, formed. Overlying this core is a relatively thick mantle and a relatively thin crust forming the surface of the earth. One theory holds that the earth was originally cold but heated up under forces of compression in the settling and synthesis of core materials. Radioactivity also produced a great deal of heat. After about 750 million years the earth cooled and the present crust . At this time we live on a relatively cool earth.
The universe we inhabit is not unique and is similar to other kinds of island universes. Nor is the sun a special kind of star. Its location is not unusual, and in size it is medium to small. The planet Earth is larger than Mercury but much smaller than Jupiter or Saturn. In sum, life has arisen under circumstances and in a milieu that fall within a middle range of properties. Conditions on earth, however, are ideal for the development of life as we know it. It is conceivable that such conditions exist on planets of other solar systems we cannot easily observe.
Origin of Life Through the Perspective of Science
Scientific theories about the origin of life on earth require that the earth be billions of years old. There is evidence to support this assumption. One line of evidence comes from observations of other universes and the measurement of the atmospheres of our sister planets. Further evidence is contained in the kinds and proportions of radioactive materials found throughout the universe but especially on the earth.
Lord Rutherford devised a technique, known as a radioactive clock, that gave an age for rocks on the earth’s crust of at least 2 billion years. More recent work with the measurement of two isotopes of lead (‘0<>Pb and 207Pb) yields a minimum value of 3.35 billion years. All these lines of evidence are based on a constant rate of decay of one radioactive element into the next in a radioactive series. Support for the antiquity of the earth is also drawn from the oceans. If the salinity of the oceans today is divided the corrected rate of salt deposition the rivers of the earth, one arrives at a figure suggesting that the oceans are at least several billion years old. Thus, a tentative figure of billions of years is clearly justified.
Creationist Views About Origin of Life
Two separate views exist regarding the origin of life. The creationist view, largely inspired the original narrative in Genesis, maintains that the earth is no more than 10,000 years old, that each species was created separately during a short burst of divine activity some 6000 years ago, and that each species tends to maintain its unique and discrete character through time.
Scientific Views About Origin of Life
Scientific creationism, a recent remodeling of this perspective some conservative geologists and engineers, inspired several unsuccessful battles fundamentalists to alter curricula in U.S. schools to include a creationist alternative in biology classes where evolution is taught.
Origin of Life Through Mechanistic View
An alternative view is that life emerges as a selected point along a continuous spectrum of increasingly complex arrangements of matter. When matter becomes sufficiently complex, we encounter the characteristics associated with life. This is a mechanistic view, but there is room for epiphenomena such as love, conscience, morality, etc., in such advanced forms as humans. Biologists support a natural origin for life.
The Oparin Hypothesis
History of Oparin Hypothesis
The mechanistic view of life suggests that the complex reactions of living things can best be explained the properties of their component parts, and that an orderly progression of cause and effect brought about an emergence of life from aggregations of simple inorganic materials into ever more complex organic macromolecules. A clear and rigorous explanation of how this evolution of life from the abiotic realm of chemistry and physics could have come about was presented A. I. Oparin to his Russian colleagues in 1924. In 1936 his views received worldwide attention.
What is Oparin hypothesis?
The Oparin hypothesis starts with the origin of the earth, about 4.6 billion years ago.
The early atmosphere was almost certainly a reducing one, possibly with large amounts of methane (CH4), steam (H20), ammonia (NH3), and some hydrogen (H2). Such an atmosphere would promote chemical synthesis. As the earth cooled, much of the steam condensed to form the primitive seas. Turbulence in the atmosphere during that cooling period produced violent lightning and thunderstorms. Along with the heat rising from the interior of the earth and the ultraviolet rays of the sun, these bursts of energy produced a variety of simple organic substances in the atmosphere, and these substances soon collected in the early seas.
Major Highlights from Oparin hypothesis
Since (1) no living things were then present to break down these organic materials and (2) the reducing atmosphere promoted an increasing synthesis of energy-rich molecules, the seas incorporated these molecules until the seas took on the characteristics of a hot, dilute soup (a metaphor provided J. B. S. Haldane). The seas were constantly recharged with fresh organic material because a cooling earth produced torrential lightning storms over many thousands of years.
The next stage was extremely crucial to Oparin’s hypothesis. The organic material of the seas, becoming increasingly concentrated, accreted into larger molecules of spatial, or structural, complexity— colloids with special properties of electric charge, adsorptive powers, translational movement, and even the ability to divide after reaching a certain size.
Oparin called these specific colloids of great organizational complexity coacervates. They tended to be shaped into droplets surrounding “cages” of highly ordered water molecules. There thus existed a very clear line of demarcation between the molecules of the coacervate and the surrounding water. The absorptive properties of the droplet caused it to grow, and eventually an actual membrane may have formed at the coacervate-water boundary, increasing the selective permeability of the droplet.
Experimental Work of Oparin
Much of Oparin’s experimental work involved an exploration of the properties of coacervates and their possible role in the evolution of living cells. He believed that at an early stage in the development of living material, amino acids were incorporated into proteins. Since proteins can serve as catalysts, their formation provided the means for an ordering of chemical reactions—the arising of a controlled metabolism. Oparin did not, of course, deal with the reproduction of these complex organizations of organic molecules, because the role of polynucleotides was then unknown. Clearly though, the formation of such information-carrying molecules is crucial to a theory of a gradual evolution of life from simpler abiotic systems.
Stanley Millar Supported the Oparin Hypothesis
Stanley Miller provided experimental support for Oparin’s belief that the conditions and simple inorganic molecules present during the earth’s early history could combine to create the complex organic molecules of living organisms. Miller, a student of the Nobel laureate Harold Urey (University of Chicago), set up a Tesla coil that discharged electric bolts into a closed system containing methane, ammonia, water vapor, and some hydrogen gas.
The results of this energetic stimulation of an atmosphere resembling that of the early earth were spectacular. A variety of organic molecules were generated, including ketones, aldehydes, and acids, but most important of all—amino acids. Since proteins are vital to both the structure and the function of living cells, the creation of amino acids under conditions that were believed to prevail upon the early earth supported the Oparin hypothesis.
Geologist Perspective About Origin of Life
Geologists have more recently revised their estimate of the makeup of the atmosphere during the beginnings of our planet. It is now believed that carbon monoxide, nitrogen, and carbon dioxide were significant constituents of that atmosphere. The Miller-Urey experiments of the 1950s were repeated using the revised atmosphere, and similar yields of organic molecules were achieved. This supported the earlier theories of a primordial transformation of inorganic molecules into the organic building blocks of life.
Sidney Fox’s Experimental Evidence About Origin of Life
Sidney Fox (University of Miami) showed that ultraviolet light can induce the condensation of amino acids to dipeptides and, later, that under conditions of moderate dry heat, amino acids can be polymerized to proteinoids, short polypeptides containing up to 18 amino acids. These proteinoids show a nonrandom arrangement of the amino acids, an advance over random accretions. Particularly exciting was his finding that polyphosphoric acid increases the yields of these polymers, a result suggestive of the present role of ATP in protein synthesis. The proteinoids produced Fox generally assume a specific spherical shape. These tiny spheres (microspheres) show some of the properties of living cells, but they are a long way from a true living structure.
The seminal work of Fox has been extended Cyril Ponnamperuma, a chemist at the Ames Research Center in California. In 1964, he showed that during the thermal polymerization of amino acids, small amounts of guanine form; he thus linked nucleotide synthesis to the synthesis of polypeptides. Later, he reported that adenine and ribose are products of long-term treatment of reducing atmosphere gases with electric current.
Biologist Still Not Convinced!!!!!
Not all biologists believe that the first living forms were produced in primitive oceans. Some theorize that early life began in the hot and extremely thick atmosphere of a long-ago time. The basis for that belief comes from the tendency of polymers to dissociate back into their constituent monomers when water is plentiful and heat and other forms of energy abound. Under such conditions hydrolysis rather than condensation would be encouraged.
Soil Scientist Perspective About Origin of Life
Others have stressed the role of wet soils (J. B. S. Haldane) and clays (Bernal) as stabilizing media for coacervates that were first formed in the turbulent seas. Although clay soils possess a much greater capacity to store water than do the finer sandy soils, the water molecules are electrically attracted to the negatively charged clay soil particles hydrogen bonds. In fact, the electrical attraction of the water molecules to the clay soil particles can be so strong that plants cannot extract water from the soil. There is, then, a difficulty with the view that life could have arisen in stormy seas, where maintaining both structural and functional integrity would have been difficult.
Rejection of Oparin’s Hypothesis
The inevitable question raised formulations such as Oparin’s is why life doesn’t continue to evolve from abiotic sources. The spontaneous generation of life had long ago been disproved Louis Pasteur and, more recently, investigators of microbial systems. Its impossibility is a cornerstone of an evaluation of the distinctiveness of life. But conditions on the earth many billions of years ago were quite different from what they are today. The reducing atmosphere then promoted complexity.
Today’s atmosphere, an oxidizing one, tends to degrade large molecules and complex structures that are not stable. This oxidizing atmosphere promotes a drift toward simplicity rather than complexity. Another significant factor is the presence of living forms now universally distributed in the environment. These organisms gobble up any available energy-rich structures in their never-ending quest for food. The chances of developing complex systems are quite unlikely now that living organisms cover the globe.
Heterotroph to Autotroph
Oparin realized that early living entities dwelt in an environment rich in energy-yielding organic molecules that could be absorbed as food. This ingestion of preformed organic fuels represents a heterotrophic habit. However, in local regions intense competition for vital substances among expanding populations might lead to critical shortages.
Let us designate one such depleted nutrient as A. Under such circumstances, if a mutant appeared that could synthesize A from nutrient B, it would tend to survive while its maladapted competitors would die for want of enough A. As B became depleted, an organism that could synthesize B from C would demonstrate greater survival ability. In this way, organisms would tend to evolve complex enzyme systems enabling them to synthesize their requisite materials from simpler substances—a nutritional habit called autotrophy.
Formation of Oparin- Horowitz Theory
According to N. H. Horowitz, organisms in areas where nutrients were scarce soon evolved long chains of enzyme-catalyzed reaction sequences which afforded them freedom from dependency on the materials of the dilute soup. The evolution of autotrophy as a significant advance in the early evolution of life has been coupled to the earlier contributions of Oparin and is known as the Oparin- Horowitz theory.
Origin of Cells
Complex coacervate droplets maintain their structure within an amorphous (unstructured) liquid medium. Further, an exchange of materials with that environment occurs across the limiting boundary of the coacervate. Although this boundary seems to be made of oriented water molecules and other simple inorganic materials, its properties approach the permeability characteristics of cells and may have been the forerunner of early prokaryotic cells.
A growing complexity of organic materials within the coavervate was dependent on the “foreign policy” of the droplet, as dictated the outer membrane. In turn, the membrane could become increasingly complex as materials brought into the cell were carried to the surface.
Although the evolution of the first cells is crucial to the establishment of a mechanistic hypothesis for the origin of life, a great deal of speculation regarding the transition from prokaryotic to eukaryotic cells also intrigues the imagination of many biologists.