MedicoPlexus
Notes in Human Biology
1.Development of the concepts for origin of life. Theory of Oparin and Haldane.
The universe began 18 billion years ago with the Big Bang after which our sun formed about 5 billion years ago as massive clouds of hydrogen and other elements compressed into a ball of glowing gases.
Local clusters of gases, dust, and larger particles of matter eventually condensed and solidified into the planets of our solar system, including the planet earth.
The best current estimates suggest that the earth and its moon had aggregated as solid bodies by 4.6 billion years ago.
The compaction of heavy elements, plus radioactive decay, generated immense heat inside the new planet, producing a molten metallic core that today is solid at its center.
The earth was bombarded by millions of high velocity planetesimals (bodies of all seizes from space). Their collective impact caused immense amounts of additional outgassing. The outgassing may also have produced hydrogen cyanide (HCN) and oxidized carbon. The atmosphere that formed dates from about 4.4 billion years ago and at that time consisted of CO, CO2, H2, N2 and H2O vapor, and little or no free oxygen. As eons passed, atmospheric temperatures fell, and water could then exist in liquid form. Hot, torrential rains began falling, and the first oceans appeared.
The earth is not too cold for complex molecular processes to proceed spontaneously and for much of the water to remain liquid; nor is it so hot that complex organic polymers are degraded, and water can exist only as vapor. Only earth has acceptable ranges of temperature, gravity, and other factors for the type of life that originated here.
Life arose spontaneously on the early earth by means of chemical evolution from non-living substances. Each living organism is constructed of the same building blocks. These organic molecules themselves are the logical starting point in the search for life’s origins, or at least its monomers.
Monomers could have formed when gases in the ancient atmosphere were energized by heat, radiation, ultraviolet light, or massive displays of lightning. Harold Urey and Stanley Miller, re-created the hypothesized atmospheric conditions on early earth, filling an upper flask with four kinds of gases and the lower flask with a reservoir of water (miniature ocean). At the end of the experiment, the primal “sea” in the bottom flask had collected significant quantities of aa and simple sugars, as well as tarry residues.
Conclusion: Under conditions of heat, humidity, energy, and raw materials similar to those probably present on the earth billions of years ago, amino acids, sugars, fatty acids, and nucleic acid bases can readily form.
The next step toward life’s origin was the spontaneous linking of monomers into polymers such as proteins and nucleic acids.
Energy from sunshine or the earth’s core or energy from ultraviolet light could have driven polymerization reactions. If an experimenter mixes ATP with aa and various condensing agents, aa adenylates form. Such molecules may then undergo a slow, spontaneous polymerization to form polypeptides.
In the 1969s, biologist Sidney Fox found that heating a mixture of dry amino acids and exposing it to water causes tiny spheres of protein like polymers to form, which he called proteinoid microspheres.
Each individual sphere has an outer layer of water and protein molecules and an aqueous interior that moves about rather like cytoplasmic streaming. These spheres can take up and concentrate other molecules from the surrounding solution, can fuse to form larger spheres, can shrink or swell osmotically, and behave as though they have a selective barrier at their surface, even though no lipid is present.
Aleksandr I. Oparin found that solutions of various polymers derived from contemporary cells, such as proteins plus carbohydrates form polymer-rich droplets – so-called coacervates, which not only are reminiscent of tiny cells, but also, under certain circumstances, will divide into smaller spheres. A third type of aggregate in the laboratory is the liposome, a spherical lipid bilayer that forms easily if phospholipids at the correct concentration range are shaken in an aqueous solution.
While there is a profound difference between even the most complex aggregate of molecules and the living cell, the origins of reproduction, information transfer, and metabolism hint at the next set of steps in the appearance of life.
Significantly, many proteinoids formed in the laboratory have catalytic properties, and this suggests that biochemical pathways could have developed among the early polymers. RNA can also function as an enzymatic catalyst, and this contributes to the current view that RNA was the first informational macromolecule.
Additional studies confirmed that various sequences in tRNAs, mitochondrial RNAs, and nuclear RNAs can splice (cut) themselves out of longer RNA molecules. Furthermore, certain of the excised RNA pieces can function enzymatically as ribozymes to catalyze the construction of new RNA molecules on an RNA template. All this activity can occur without proteins, heat, ATP, or GTP added to drive the reactions.
Modern cells still employ enzymatic activity of RNA to splice out introns from exons and to process precursor molecules into mature tRNAs, rRNAs, and mRNAs.
The key to reproduction and inheritance lies in the ability of informational molecules to replicate.
Perhaps, 4 billion years ago, such processes led to the development of the translation machinery – tRNAs, mRNAs, and ribosomes – to make proteins that are copies of RNA templates.
Although DNA is the modern cell’s “code of life” it is usually regarded as the
“last step” in the origin of informational macromolecules. RNA can serve as a template on which a complementary DNA strand can be assembled.
Biologists think that a process resembling natural selection may have led to the first metabolic pathways.
Microspheres in the primordial waters or on clay surfaces would have depended on an external supply of sugars, lipids, bases, and aa to reproduce and maintain structural integrity.
The important thing is that the chemical evolution of life could well have occurred, as demonstrated by laboratory experiments and geological evidence, in just the sort of sequence we have described, governed by the physical and chemical processes and laws at work throughout the universe, then and now.
The oldest fossil establishes life’s origins sometime between 3.5 billion years ago and the stabilizing of earth’s crust about 3.9 billion years ago.
The first cells were in all probability anaerobic – able to survive in the absence of free oxygen – and heterotrophic – unable to make organic nutrients from simple inorganic precursors. These early heterotrophs consumed the amino acids and polypeptides, nucleotides, sugars, and other carbon-containing compounds that had formed spontaneously on earth or had been delivered in meteorites.
Perhaps as early as 3.8 billion years ago photosynthesis arose: structural proteins and enzymes inherited from earlier cells became capable of trapping energy from sunlight, probably at first to generate ATP by means of photophosphorylation and later to store that energy in carbon compounds, the sugars. Cells with such attributes could have created their own nutrients from inorganic precursors and functioned as the first autotrophs, or “self-feeders”.
The emergence of autotrophs was dated to 3.8 billion year ago by ancient rocks contain chlorophyll, the primary light-absorbing molecules of today’s photo synthesizers.
By 2 billion years ago, some of the atmospheric O2 had been converted by sunlight to ozone and had collected in a high-altitude layer, the ozone layer.
The early autotrophs inadvertently created a potentially lethal condition for themselves, because the oxygen released during photosynthesis is poisonous to anaerobic cells, and it probably disrupted biochemical pathways that originated when the environment lacked free oxygen. Eventually, however, cells capable of living in the presence of oxygen and even of exploiting it metabolically emerged: aerobic cells.
In these aerobic cells, cellular respiration originated, allowing organisms to harvest 18 times more energy than by glycolysis and the additional steps of fermentation alone. This extra energy permits much higher rates of growth and reproduction.
Cellular respiration had another major consequence, as well: the beginning of the carbon cycle. During cellular respiration, carbon compounds are largely oxidized to CO2. Thus, that gas can return to the atmosphere to help replenish the CO2 reservoir used for photosynthesis.
Once aerobic cells appeared and the ozone layer screened much of the ultraviolet light from the sun’s penetrating rays, the variety of organisms increased tremendously in the sea, in fresh water, and on land. These cells were all prokaryotic – lacking a cytoskeleton., a nucleus, and other membrane bound organelles.
Another billion years after eukaryotic cells appear. They had a true nucleus and complex organelles. The oldest fossil eukaryote yet discovered lies embedded in rocks dated at 1.5 billion years ago.
Open resource