The Origin of Life
Questions and Answers

Know the Hypotheses on the Origin of Life

It is believed that the earth is approximately 4.5 billion years old.

earth

From analysis of data collected by the Hubble telescope the age of the universe is estimated to be about 12 billion years.

Hubble telescope

It is estimated that life on earth emerged about 3.5 billion years ago, thus 1 billion years after the formation of the planet.

The most recurrent explanation for the phenomenon of life on earth is the mythological. People from various parts of the world developed explanatory myths about the origin of animals and human beings. Some of those myths were incorporated into religions and almost all religions have metaphorical or transcendental explanations about the origin of life on the planet.

With the development of science new explanatory attempts have emerged. Notable among them are the spontaneous generation hypothesis, or abiogenesis, that asserted that living beings were created from nonliving material, the cosmic panspermia hypothesis, theory that life on earth is a result of seeding from the outer space, the autotrophic hypothesis, according to which the first living beings were autotrophs, and the heterotrophic hypothesis, the most accepted nowadays, that affirms that life emerged from heterotrophic cells.

At the end of the 1980s decade a new hypothesis known as the RNA world hypothesis was presented. This hypothesis asserts that primitive life had only RNA as genetic material and as structural molecules that later turned into DNA and proteins. The RNA world hypothesis is strengthened by the fact that RNA can play a catalytic role, like enzymes, and by the finding that some bacteria have ribosomes made only of RNA without associated proteins.

The spontaneous generation hypothesys, or abiogenesis, asserts that life on earth has come from nonliving material. For example, the fact that with time rats appeared around waste was considered in the past a confirmation of this hypothesis. Some supporters of spontaneous generation associated it with the existence of an active principle (the vital elan) that would be the source of life, a theory known as vitalism.

Origin of Life - Image Diversity: abiogenesis

To refute the spontaneous generation hypothesis many experiments were performed. Francisco Redi, in 1668, verified that maggots appeared on meat only when there was exposition to the environment; within closed environments, they did not appear. In 1862, Louis Pasteur working with swan-neck flasks refuted the abiogenesis hypothesis definitively. In this experiment Pasteur demonstrated that boiled (to kill microorganisms) nutritive soups put in swan-neck flasks (with a curved down mouth so microorganisms could not enter easily) did not contaminate with microorganisms while the same soups within flasks with open upwards mouths were contaminated in a few days. The fact that both flasks were open refuted the argument of the vitalists that the vital elan could not enter the flasks. Pasteur broke the swan-necks of the flasks to demonstrate that proliferation of microorganism could happen if these beings were able to reach the broth.

Origin of Life - Image Diversity: Redi's experiment Pasteur's experiment

Panspermia is a hypothesis that describes life on earth as not originated from the planet. The idea is that the first living beings that colonized the earth came from outer space, from other planets or even from other galaxies by traveling in meteorites, comets, etc. According to this hypothesis even the type of life now existent on earth could have also been seeded intentionally by extraterrestrial beings in other stellar and planetary systems.

The autotrophic hypothesis on the origin of life asserts that the first living beings on earth were producers of their own food, just like plants and chemosynthetic microorganisms.

According to the heterotrophic hypothesis the first living beings were very simple heterotrophic organisms, i.e., not producers of their own food, which emerged from the gradual association of organic molecules into small organized structures (the coacervates). The first organic molecules in their turn would have appeared from substances of the earth's primitive atmosphere submitted to strong electrical discharges, to solar radiation and to high temperatures.

The heterotrophic hypothesis is the strongest and most accepted hypothesis about the origin of life.

The spontaneous generation hypothesis has been excluded by the experiments of Pasteur. The panspermia hypothesis is not yet completely refuted but it is not well-accepted since it would be necessary to explain how living beings could survive long space journeys under conditions of extreme temperatures as well as to clarify the manner by which they would resist the high temperatures faced when entering the earth's atmosphere. The autotrophic hypothesis is weakened if one takes into account that the production of organic material from inorganic substances is a highly complex process requiring diversified enzymatic systems and that the existence of complex metabolic reactions on the primitive earth were not probable.

The earth's primitive atmosphere was basically formed of methane, hydrogen, ammonia and water vapor.

The present atmosphere of the earth is constituted mainly of molecular nitrogen (N2) and molecular oxygen (O2). Nitrogen is the most abundant gas, approximately 80% of the total volume. Oxygen makes up about 20%. Other gases exist in the atmosphere in a low percentage. (Of great concern is the increase in the amount of carbon dioxide due to human activity, the cause of the threatening global warming.)

The presence of molecular oxygen in the primitive atmosphere was probably at a minimum and extremely rare. Oxygen became abundant with the emergence of photosynthetic beings, approximately, 1.5 billion years after the appearance of life on the planet.

3.5 billion years ago the water cycle was faster than today, resulting in hard storms with intense electrical discharges. There was also no chemical protection from the ozone layer against ultraviolet radiation. The temperatures in the atmosphere and on the planet surface were very high. Electricity, radiation and heat constituted large available energy sources.

In 1953 Stanley Miller arranged an experimental apparatus that simulated the atmospheric conditions of the primitive earth. The experiment contained a mixture of methane, ammonia, hydrogen and circulating water that when heated was transformed into vapor. He submitted the mixture to continuous bombardment of electrical discharge and after days obtained a liquid residual within which he discovered organic molecules and among them surprisingly the amino acids glycine and alanine, the most abundant constituents of proteins. Other researchers reproduced the Miller experiment and noted also the formation of other organic molecules such as lipids, carbohydrates and nucleotides.

Origin of Life - Image Diversity: Stanley Miller experiment

Coarcervates are small structures made of the aggregation of organic molecules under water solution. By electrical attraction the molecules join into bigger and more organized particles distinct from the fluid environment forming a membrane-like structure that separates an internal region of the coacervate from the exterior. The coacervates might divide themselves and also absorb and excrete substances. It is believed that these structures may have been the precursors of cells.

Origin of Life - Image Diversity: coacervates

Phospholipids are amphipathic molecules, i.e., they present a polar portion and a nonpolar portion. In contact with water these molecules tend to spontaneously unite and organize themselves forming membranes that create a closed interior space separated from the exterior environment. Polypeptide chains in their turn can attract water (by electrical attraction) forming a surrounding water layer and also creating an organized structure with delimited interior space.

Coacervates probably provided a nitid separation between an internal and an external environment and thus the organic material within was not lost to the ocean. The enzymatic action inside that internal environment could develop in different manners increasing the speed of specific chemical reactions. Coacervates also allowed the molecular flux across its membrane to be selective. Since containing different molecules and differently organized from each other, coacervates could have promoted a competition for molecules from the environment setting out an evolutionary selection.

It is accepted that the internal membranous organelles of the eukaryotic, like the Golgi apparatus and the endoplasmic reticulum, appeared from invaginations of the external membrane of primitive cells.

According to the most accepted hypothesis aerobic eukaryotic cells emerged from the association of aerobic prokaryotes engulfed by primitive anaerobic eukaryotic cells. This would have been the origin of mitochondria that thus would have primitively been aerobic bacteria engulfed by eukaryotic anaerobes. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria.

Chloroplasts would also have appeared by endosymbiosis from the entry of photosynthetic prokaryotes into aerobic eukaryotes, both establishing a mutualist ecological interaction.

The fact that chloroplasts are the organelles responsible for photosynthesis in plants leads to the supposition that before symbiosis they were autotrophic prokaryotes. For the reason that mitochondria are the center of the aerobic cellular respiration, the powerhouse of the eukaryotic cell, it is supposed that they were once aerobic prokaryotes.

The endosymbiotic hypothesis to explain the emergence of aerobic and autotrophic eukaryotic beings is strengthened further by the following evidence: chloroplasts as well as mitochondria have their own DNA, similar to bacterial DNA; chloroplasts and mitochondria reproduce asexually by binary division, like bacteria do; both organelles have ribosomes and synthesize proteins.

Origin of Life - Image Diversity: endosymbiotic hypothesis

The heterotrophic hypothesis asserts that the first living beings were the fermenting heterotrophs. Fermentation released carbon dioxide (CO2) and then the atmosphere became enriched by this gas. By mutation and natural selection organisms capable of using carbon dioxide and light to synthesize organic material appeared. These would have been the first photosynthetic beings (that were also fermenting beings since there was no abundance of oxygen).

Since photosynthesis is a reaction that releases molecular oxygen, with the emergence of fermenting autotrophs this gas became available. Some organisms then developed aerobic respiration using O2, a highly efficient method to produce energy.

It is more probable that photosynthetic prokaryotes appeared before the aerobic eukaryotes because without photosynthesis the earth's atmosphere would not be enriched with molecular oxygen, and without oxygen the existence of aerobic beings would not be possible.

Ultraviolet radiation from the sun was not disallowed to reach the surface of the primitive earth. Therefore the development of life on dry land or even near the aquatic surface was impracticable. Probably the first living beings lived submerged in deep water to avoid destruction by solar radiation. Only after the appearance of photosynthetic beings and the later filling of the atmosphere with oxygen released by them the formation of the atmospheric ozone layer that filters ultraviolet radiation was possible.




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