What is the Origin of Life on Earth? Main Theories.

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Since Darwin and his theory of evolution, we know that life has evolved over time and that in fact it has not always been like it is today. But how and when did life originate on Earth?

Do all forms of life on earth have a common origin?

The question concerning the origin of life on earth fascinates scientists who embark on different courses of explanation. Some suggest that life began in hydrothermal deep-sea vents about 3.5 billion years ago. While others theorize that comets brought to earth the elements essential for life from outer space.

All organisms currently living on planet Earth share a common genetic origin. However, the concept of a significant horizontal gene transfer during early stages of evolution has led scientists to question the monophyletic theory (single ancestry) of life.

The Last Universal Common Ancestor (LUCA), is hypothetically, the last common ancestor form which all cellular organisms evolved.

This theory of origin suggests that all living things on Earth can be traced back to a single “primitive” archetype. While all other, similarly primitive life forms that probably existed on early Earth have left no descendants.

Scientists estimate that Earth is about 4.55 billion years old. According to the most recent study (2018) from the University of Bristol, LUCA existed about 4.5 billion years ago, during the Hadean era.

According to the most recent estimates abiogenesis (evolution of living organisms from nonliving matter), occurred relatively close to the formation of oceans, about 4.4 billion years ago. The earliest known life-forms on Earth were present in hydrothermal vents, as early as 3.77 to 4.28 billion years ago.

Hydrothermal deep-sea vents

Four billion years ago volcanoes on earth emitted water vapor, toxic gases and rocks into the still thin atmosphere. Every now and then, asteroids hit the Earth, bringing the oceans to a boiling point. In the depths of the primordial ocean, hot liquid flowed out of hydrothermal vents, so-called “black smokers”.

Scientists agree that life began somewhere between 3.7 and 4.4 billion years ago in these oceanic “black smokers”. But it is difficult to find out how exactly complex life evolved from carbon in this primeval chimney.

According to the hydrothermal deep-sea vents theory the precursors of life used carbon dioxide and hydrogen, to create the building blocks of life such as amino acids and building blocks of DNA.

These chemical reactions require a source of energy. Researchers suggest that sea vents had the optimal conditions for life to originate in these hydrothermal vents. In particular, the rocky mineral walls in the shafts of the seabed may have provided the optimal source of energy.

These shafts contain gases and minerals. Basically, a chemical cocktail from which simple, then increasingly complex organic compounds were formed over time. These were proceeded by living cells which evolved in these conditions, multiplied and then moved out of the vents.

The environmental conditions in the porous hydrothermal deep-sea vents created a gradient in positively charged protons. These served as an energy source for the production of organic molecules and protocells. These primitive life forms then used a basic type of cell pump that pushes sodium out of the cell and at the same time absorbs positively charged protons. Scientists suggest that a precursor of this cell pump mechanism developed in the membranes of protocells.

The first microorganisms

The first bacteria that populated the Earth had to evolve in extreme conditions: intense volcanism and atmosphere of a different composition. These ancient organisms flourished in an environment in which other species would not survive very long. The first bacteria had to evolve in extreme conditions: intense volcanism and atmosphere of a different composition. These ancient organisms flourished in an environment in which other species would not survive very long.

Bacteria have been the very first living organisms to inhabit Earth. However, the strongest evidence for the hydrothermal deep-sea vents theory are actually archaebacteria (archaeans). Archaea are a sub-group of microorganisms which is the most ancient form of life. All archaea are anaerobes and they flourish in very inhospitable (low in oxygen) biotopes such as the water seepage of coal mines, geysers and deep-sea hydrothermal shafts.

Research suggests that these ancient organisms evolved separately from bacteria and blue-green algae. Initially classified as bacteria, archaea is more closely related to eukaryotes – life forms whose cells have a nucleus enclosed within membranes, similar to human cells, than to bacteria.

Photosynthesis is key to the origin of life

One billion years after the first organisms, cyanobacteria native to water decisively changed the living conditions all over the world. These tiny unicellular organisms use sunlight for photosynthesis and release oxygen as a waste product.

It is due to the cyanobacteria and its massive oxygen production that the oxygen has been able to accumulate in the atmosphere and life was able to emerge outside of the oceans. At present, the oxygen content is about one fifth of Earth’s atmosphere. Without oxygen there would be no higher life forms on earth today.

Cyanobacteria may have appeared 3.5 billion years ago. Capable of carrying out photosynthesis, they have transformed carbon dioxide into oxygen. It is partly thanks to them that life was able to emerge outside the oceans. Cyanobacteria may have appeared 3.5 billion years ago. Capable of carrying out photosynthesis, they have transformed carbon dioxide into oxygen. It is partly thanks to them that life was able to emerge outside the oceans.

Cyanobacteria may have appeared 3.5 billion years ago. Capable of carrying out photosynthesis, they have transformed carbon dioxide into oxygen. It is partly thanks to them that life was able to emerge outside the oceans.

The primordial soup

The primordial soup theory is one of the best known scenarios of the origin of life on earth. According to this theory, life originated in the ocean as result of a combination of chemicals and energy from the atmosphere which produced amino acids, the building blocks of proteins. This is thought to have happened at least 3.5 billion years ago; the earliest undisputed evidence of life on Earth.

In 1953 the chemist Stanley Miller (1930-2007) and the physicist Harold Urey (1893 – 1981) conducted a famous experiment in 1953 to test this theory. To recreate the seething primeval ocean in miniature, they boiled water in a glass flask. Then they mixed the water vapor (H20) with gases thought to be present on primitive Earth:

  • methane (CH4)
  • ammonia (NH3)
  • hydrogen (H2)

Then, they exposed the mixture to sparks generated by electrodes. These were designed to simulate thunderstorms in the primeval atmosphere. The electrical energy stimulates the gas mixture to react, from which amino acids, the basic building blocks of life, are formed.

The objective of Miller-Urey experiment was to lift the veil on the mechanisms behind the appearance of life on primitive Earth.  They did so by showing that it is possible to make amino acids, from an atmosphere of the time, which was similar to those of Jupiter and Saturn.

This ingeniously simple experiment was a decisive step forward, but it only brought science to the brink of life’s origin. The mystery of how the amino acids might have developed into biocells in the next step remains unsolved to this day. So far, researchers only found significant evidence that the amino acid arginine may has a more significant role in the chemical origins of life than previously thought.

Comets as the origin of life on Earth?

The theory of panspermia assumes that life did not originate spontaneously on earth, but came from outer space. Comets could have been an ideal means of transport for bacterial life. The comet’s nucleus consists largely of ice. With it, resistant bacterial spores could have reached the Earth and “infected” it with life, preserved and protected from cosmic radiation.

The main objective of upcoming space missions investigating the interior of comets is to clarify whether there is any truth to this theory. Scientists suspect that the comet’s nucleus contains matter from the time when the solar system and the Earth were formed, and thus might contain evidence of early life forms.

However, even this approach does not answer the question of how life originated in principle, but merely shifts the scene of the origin of life into space.

It all started in 1986. When Giotto space mission flew over comet Halley, it detected the presence of organic matter, without being be able to determine its nature. Then in 2006, in the samples collected by the Stardust space probe from the tail of comet 81P/Wild, scientists identified traces of glycine, the simplest amino acid, found in all living organisms.

Rosetta space mission

Finally, in 2016 the Rosetta mission has repeatedly and directly detected the presence of glycine in the comet. The other significant detection made by Rosetta was phosphorus, a key element of all living organisms. Mission’s main objective was to explore comet 67P/Churyumov–Gerasimenko and provide more definitive answers about the origins of the solar system and life on Earth.

The primordial gas cloud

But where do these compounds that are found in comets come from? And why were they absent on primitive Earth? This is because comets are the vestiges of the time that saw the formation of the solar system, over 4.5 billion years ago.

Because of their small size and the fact that they orbit in regions very far from the sun – beyond the planet Neptune – and therefore very cold, these frozen objects would indeed have kept the almost intact trace of the material contained in the cloud of gas and dust that gave birth to our sun and its procession of planets. Organic compounds needed for the development of life were probably generated in primordial gas clouds. Probably, under the effect of the low temperatures which reigned there then and the intense radiation of the surrounding stars.

Destroyed by heat on the forming planets, these organic molecules would thus have subsisted in comets. This is due to an enormous bombardment of the earth shortly after its formation. As soon as the water was stable in liquid state, comets were able to deliver these complex organic molecules. And some could then play a key role in the evolution of living organisms.

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