The theory or theories that seek to explain how biomolecules, subcellular structures, and ultimately living cells came into existence.
Many myths, stories and hypotheses have been proposed. Some are still under investigation, while others remain contested or persist as statements of religious faith.
Also see: DYNAMIC STATE THEORY, ‘LITTLE BAGS’ THEORY OF EVOLUTION, MINERAL THEORY, PHENOTYPE THEORY, PRIMORDIAL SOUP, PROGENOTE THEORY
G Blandino, Theories on the Nature of Life (New York, 1969)
The origin of life on Earth is a scientific problem which is not yet solved. There are many ideas, but few clear facts.
Most experts agree that all life today evolved by common descent from a single primitive lifeform. It is not known how this early life form evolved, but scientists think it was a natural process which happened about 3,900 million years ago. This is in accord with the philosophy of naturalism: only natural causes are admitted.
It is not known if metabolism came first or genetics. The main hypothesis which supports genetics first is the RNA world hypothesis, and the one which supports metabolism first is the protein world hypothesis.
Another big problem is how cells developed. Melvin Calvin, winner of the Nobel Prize in Chemistry, wrote a book on the subject, and so did Alexander Oparin. What links most of the early work on the origin of life is the idea that before life began there must have been a process of chemical change. Another question which has been discussed by J.D. Bernal and others is the origin of the cell membrane. By concentrating the chemicals in one place, the cell membrane performs a vital function.
History of studies into the origin of life
Until the early 19th century many people believed in the regular spontaneous generation of life from non-living matter. This was called spontaneous generation, and was disproved by Louis Pasteur. He showed that without spores no bacteria or viruses grew on sterile material.
In a letter to Joseph Dalton Hooker on 11 February 1871, Charles Darwin proposed a natural process for the origin of life.
He suggested that the original spark of life may have begun in a “warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. A protein compound was then chemically formed ready to undergo still more complex changes”. He went on to explain that “at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed”.
Haldane and Oparin
No real progress was made until 1924 when Alexander Oparin reasoned that atmospheric oxygen prevented the synthesis of the organic molecules. Organic molecules are the necessary building blocks for the evolution of life. In his The Origin of Life, Oparin argued that a “primordial soup” of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine in ever-more complex fashions until they formed droplets. These droplets would “grow” by fusion with other droplets, and “reproduce” through fission into daughter droplets, and so have a primitive metabolism in which those factors which promote “cell integrity” survive, those that do not become extinct. Many modern theories of the origin of life still take Oparin’s ideas as a starting point.
Around the same time J.B.S. Haldane also suggested that the Earth’s pre-biotic oceans, which were very different from what oceans are now, would have formed a “hot dilute soup”. In this soup, organic compounds, the building blocks of life, could have formed. This idea was called biopoiesis, the process of living matter evolving from self-replicating but nonliving molecules.
Early conditions on Earth
There is almost no geological record from before 3.8 billion years ago. The environment that existed in the Hadean era was hostile to life, but how much so is not known. There was a time, between 3.8 and 4.1 billion years ago, which is known as the Late Heavy Bombardment. It is so named because many lunar craters are thought to have formed then. The situation on other planets, such as Earth, Venus, Mercury and Mars must have been similar. These impacts would likely sterilize the Earth (kill all life), if it existed at that time.
Several people have suggested that the chemicals in the cell give clues as to what the early seas must have been like. In 1926, Macallum noted that the inorganic composition of the cell cytosol dramatically differs from that of modern sea water: “the cell… has endowments transmitted from a past almost as remote as the origin of life on earth”. For example: “All cells contain much more potassium, phosphate, and transition metals than modern … oceans, lakes, or rivers”. “Under the anoxic, CO2-dominated primordial atmosphere, the chemistry of inland basins at geothermal fields would [be like the chemistry inside] modern cells”.
If life evolved in the deep ocean, near a hydrothermal vent, it could have originated as early as 4 to 4.2 billion years ago. If, on the other hand, life originated at the surface of the planet, a common opinion is it could only have done so between 3.5 and 4 billion years ago.
Lazcano and Miller (1994) suggest that the pace of molecular evolution was dictated by the rate of recirculating water through mid-ocean submarine vents. Complete recirculation takes 10 million years, so any organic compounds produced by then would be altered or destroyed by temperatures exceeding 300 °C. They estimate that the development of a 100 kilobase genome of a DNA/protein primitive heterotroph into a 7000 gene filamentous cyanobacterium would have required only 7 million years.
History of Earth’s atmosphere
Originally, the Earth’s atmosphere had almost no free oxygen. It gradually changed to what it is today, over a very long time (see Great Oxygenation Event). The process began with cyanobacteria. They were the first organisms to make free oxygen by photosynthesis. Most organisms today need oxygen for their metabolism; only a few can use other sources for respiration.
So it is expected that the first proto-organisms were chemoautotrophs, and did not use aerobic respiration. They were anaerobic
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