Hypothesis about the origin of life

The humans have always wondered about what could be the origin of life. Many hypotheses have been put forward about its origin, some scientifically based, and others influenced by the religious ideas of the time.

Some of these hypotheses about the origin of life have been:

• Spontaneous generation.
• Panspermia.
• Abiogenesis or prebiotic synthesis.

Spontaneous generation

The theory of spontaneous generation affirmed that some forms of life (animals and plants) arose spontaneously from organic or inorganic matter, or a combination of both.

It was known that reproduction occurred in the most common animals, which came from the body of a female or from eggs, but it was believed that the simplest beings could arise from non-living matter, such as worms that "appeared" in the decaying meat.

This belief was deeply rooted in antiquity, admitted by important thinkers such as Aristotle, René Descartes, Francis Bacon or Isaac Newton. For example, Aristotle affirmed that living beings came from mud, manure and other inert matter.

This idea was maintained until the end of the Middle Ages, where the divine origin of life was also believed and those who tried to question the origin of life were accused of being heretics.

Currently it is totally refuted, this theory began to be questioned in the seventeenth century, when the Italian Francesco Redi, in 1668, devised a simple experiment to try to refute this idea.

Redi's experiment

Redi tried to show that insects did not arise from rotting matter. He wanted to show that if the adult flies did not come into contact with the meat, no fly larvae would develop.

He put meat in an open jar and in a closed jar and let the time pass. Flies went in and out of the open bottle, not the closed one because they couldn't get in. After a while, there were only maggots (fly larvae) in the opened jar, so she concluded that maggots only appeared in decaying meat if the flies had previously laid their eggs in the meat.

His opponents criticized the experiment, claiming that the absence of worms was because air had not been allowed to enter the jar, preventing spontaneous generation.

Redi redesigned his experiment and used gauze to cover the jars, allowing air to enter but not for flies. In the end, no worms appeared in the meat of the jar covered with gauze, so it was shown that spontaneous generation was not true.

By CNX OpenStax (https://cnx.org/contents/5CvTdmJL@4.4) [CC BY 4.0], via Wikimedia Commons

Redi's demonstration was not enough for the defenders of the generation, who had to wait for Louis Pasteur to be able to demonstrate that no living being, nor microorganisms, arise by spontaneous generation, but that they all come from another living being.

Pasteur's experiment

Pasteur demonstrated with his experiment, in addition to the non-existence of spontaneous generation, that microorganisms were everywhere and that they were the cause of food spoilage and many human diseases.

Pasteur used two gooseneck flasks, which are S-shaped, into which he introduced meat broth. The liquid in each flask was boiled to eliminate possible microorganisms present. Since the flasks were S-shaped, air could get in, but the microorganisms stayed in the lower part of the tube. After allowing them to stand for a while, he found that neither of the two broths had any microorganisms, and he cut the S-shaped tube from one of the flasks.

After some time, the broth in the neckless flask had decomposed, while the broth in the flask that had its neck intact still did not decompose.

Thus Pasteur showed that microorganisms did not appear by spontaneous generation either, and that microorganisms were in the air, attached to dust particles. When the air passed through the S-shaped neck of the flasks, the powder settled on the first elbow and the microorganisms did not pass, but when it was broken, the microorganisms reached the broth and decomposed it.

By Carmel830 [CC BY-SA 4.0], via Wikimedia Commons

Discarding the theory of spontaneous generation, the main modern theories about the origin of life are:

• Panspermia.
• Abiogenesis or prebiotic synthesis.

Panspermia

Panspermia is a hypothesis that proposes that life can have its origin anywhere in the universe, and does not come directly or exclusively from the Earth, that life on Earth probably comes from outside and that the first living beings would have possibly arrived in meteorites or comets from space to Earth.

This hypothesis has gained strength by confirming the existence of organic matter, mainly carbon, in some meteorites or comets.

It should be emphasized that, currently, there is no evidence of the existence of life outside of Earth.

Abiogenesis or prebiotic synthesis

Currently, these theories about the origin of life are the most accepted. All affirm that life arose in three phases:

• Formation of simple organic molecules from inorganic molecules.
• Formation of complex organic molecules.
• Formation of coacervates, which would be the precursors of the first organisms.

The main abiogenic theories are as follows:

Theory of the primordial soup or broth

This theory was proposed, independently, by the Russian Alexander Oparin and the English Jonh Haldane in 1923. 

At first, when the Earth was formed about 45 billion years ago, our planet was an enormous fireball in which the elements were grouped according to their density, the densest being deposited inside, forming the nucleus, and the lighter, on the outside, surrounding the solid part.

Oparin and Haldane proposed that the primitive atmosphere was not like today. It lacked oxygen, and was made up of gases like methane, ammonia, hydrogen, and water vapor.

The intense ultraviolet radiation from the Sun, together with the electrical energy from strong storms and the torrential rains that occurred, caused these gases to react with each other, originating simple molecules that were deposited in the shallow waters of the first seas that were being formed.

Those primitive seas, still very hot, allowed the molecules to continue reacting, appearing more complex molecules. Oparin called these seas laden with molecules, the primitive broth or primordial soup. Some of these molecules associated to form spheres called coacervates .

Coacervates continued to form until a molecule, possibly a nucleic acid, emerged that was able to create copies of itself. The coacervates that had this molecule were isolated from the environment, avoiding reacting with other molecules, until they began to exchange matter and energy with the environment. The primitive cells had originated.

In this way, more than 3.5 billion years ago , anaerobic bacteria emerged, the first cells, which did not use oxygen to breathe.

About 2 billion years ago, eukaryotic cells arose from prokaryotes. Eukaryotic cells are much more complex and from them arose, by evolution, multicellular beings.

It still follows the process of biological evolution, which causes more complex living beings to emerge from simpler ones, and is the cause of biodiversity.

It is now believed that the early atmosphere had a lot of carbon dioxide and nitrogen, due to intense volcanic activity.

Miller and Urey experiment

Miller and Urey, in 1953, designed an experiment that tried to verify what was proposed by Oparin and Haldane. They tried to simulate the environmental conditions on Earth when life originated. They circulated gases such as those that made up the Earth's atmosphere for a week in a circuit in which they received an electrical discharge similar to that of lightning. As a result, they obtained amino acids and other organic molecules, which would confirm the hypothesis of Oparin and Haldane.

Hydrothermal vents theory

Another theory says that life could have arisen in hydrothermal vents. The fumes or hydrothermal springs are inhabited by organisms such as worms and bacteria, whose source of energy is not sunlight, but sulfur compounds emitted by these cracks associated with the ocean ridges.

Hydrothermal vents could have provided the energy and nutrients necessary for life to emerge.

It is believed that methane, hydrogen sulfide, and iron were present in hydrothermal springs, and bacteria arose that would feed on these compounds and are resistant to these temperatures.