There’s many theories about the Origin of life. Having rejected spontaneous generation, a new challenge on the horizon of biology appears. How was originated the first living being?
“Primordial soup” theory. The first one…
No new notable research or theory on the subject appeared until 1924, when Alexander Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for the evolution of life. In his The Origin of Life, Oparin proposed that the “spontaneous generation of life” that had been attacked by Louis Pasteur did in fact occur once, but was now impossible because the conditions found on the early Earth had changed, and preexisting organisms would immediately consume any spontaneously generated organism. As early as 1922, he asserted the following tenets:
1. There is no fundamental difference between a living organism and lifeless matter. The complex combination of manifestations and properties so characteristic of life must have arisen in the process of the evolution of matter.
2. Taking into account the recent discovery of methane in the Celestial body atmospheres of Jupiter and the other giant planets, Oparin postulated that the infant Earth had possessed a strongly reducing atmosphere, containing methane, ammonia, hydrogen, and water vapor. In his opinion, these were the raw materials for the evolution of life.
3. At first there were the simple solutions of organic substances, the behavior of which was governed by the properties of their component atoms and the arrangement of those atoms in the molecular structure. But gradually, as the result of growth and increased complexity of the molecules, new properties have come into being and a new colloidal-chemical order was imposed on the more simple organic chemical relations. These newer properties were determined by the spatial arrangement and mutual relationship of the molecules.
4. In this process biological orderliness already comes into prominence. Competition, speed of cell growth, survival of the fittest struggle for existence and, finally the natural selection determined such a form of material organization which is characteristic of living things of the present time.
Oparin outlined a way in which basic organic chemicals might form into microscopic localized systems possible precursors of the Cell from which primitive living things could develop. He cited the work done by de Jong on coacervates and other experimental studies, including his own, into organic chemicals which, in solution, may spontaneously form droplets and layers. Oparin suggested that different types of coacervates might have formed in the Earth’s primordial ocean and been subject to a selection process leading eventually to life.
Around the same time, J. B. S. Haldane suggested that the Earth’s prebiotic oceans—different from their modern counterparts—would have formed a “hot dilute soup” in which organic compounds could have formed. This idea was called biopoiesis or biopoesis, the process of living matter evolving from self-replicating but nonliving molecules
The underlying hypothesis held by Oparin and Haldane was that conditions on the primeval Earth favored chemical reactions that synthesized organic compounds from inorganic precursors. A recent reanalysis of the saved vials containing the original extracts that resulted from the Miller and Urey experiments, using current and more advanced analytical equipment and technology, has uncovered more biochemicals than originally discovered in the 1950s. One of the more important findings was 23 amino acids, far more than the five originally discovered.
The Miller – Urey experiment
Specifically, the experiment tested Alexander Oparin’s and J. B. S. Haldane’s hypothesis that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors. Considered to be the classic experiment on the origin of life, it was conducted in 1952 by Stanley Miller and Harold Urey at the University of Chicago and published the following year.
The experiment used water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2). The chemicals were all sealed inside a sterile array of glass tubes and flasks connected in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes. The liquid water was heated to induce evaporation, sparks were fired between the electrodes to simulate lightning through the atmosphere and water vapor, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle.
Within a day, the mixture had turned pink in colour, and at the end of two weeks of continuous operation, Miller and Urey observed that as much as 10–15% of the carbon within the system was now in the form of organic compounds. Two percent of the carbon had formed amino acids that are used to make proteins in living cells, with glycine as the most abundant. Sugars were also formed.Nucleic acids were not formed within the reaction.
In an interview, Stanley Miller stated: “Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids.”
As observed in all subsequent experiments, both left-handed (L) and right-handed (D) optical isomers were created in a racemic mixture. In biological systems, most of the compounds are non-racemic, or homochiral.
The original experiment remains today under the care of Miller and Urey’s former student Jeffrey Bada, a professor at UCSD, at theUniversity of California, San Diego, Scripps Institution of Oceanography. The apparatus used to conduct the experiment is on display at the Denver Museum of Nature and Science.
There is no “standard model” of the origin of life. Most currently accepted models draw at least some elements from the framework laid out by the Oparin-Haldane hypothesis. Under that umbrella, however, are a wide array of disparate discoveries and conjectures such as the following, listed in a rough order of postulated emergence:
- The Oparin-Haldane hypothesis suggests that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43-), with molecular oxygen (O2) and ozone (O3) either rare or absent.
- In such a reducing atmosphere, electrical activity can catalyze the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
- Phospholipids (of an appropriate length) can form lipid bilayers, a basic component of the cell membrane.
- A fundamental question is about the nature of the first self-replicating molecule. Since replication is accomplished in modern cells through the cooperative action of proteins and nucleic acids, the major schools of thought about how the process originated can be broadly classified as “proteins first” and “nucleic acids first”.
- The principal thrust of the “nucleic acids first” argument is as follows:
- The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis)
- Selection pressures for catalytic efficiency and diversity might have resulted in ribozymes which catalyse peptidyl transfer(hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. The first ribosome might have been created by such a process, resulting in more prevalent protein synthesis.
- Synthesized proteins might then outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer, relegating nucleic acids to their modern use, predominantly as a carrier of genomic information.
No one has yet synthesized a “protocell” using basic components which would have the necessary properties of life (the so-called“bottom-up-approach”). Without such a proof-of-principle, explanations have tended to be focused on chemosynthesis of polymers. However, some researchers are working in this field, notably Steen Rasmussen at Los Alamos National Laboratory and Jack Szostakat Harvard University. Others have argued that a “top-down approach” is more feasible. One such approach, successfully attempted byCraig Venter and others at The Institute for Genomic Research, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist John Desmond Bernal coined the term biopoiesis for this process, and suggested that there were a number of clearly defined “stages” that could be recognised in explaining the origin of life.
- Stage 1: The origin of biological monomers
- Stage 2: The origin of biological polymers
- Stage 3: The evolution from molecules to cell
Bernal suggested that evolution commenced between Stage 1 and 2