↣ The origin of life

Even though there are still a few questions to resolve, modern theories explain th. life began from molecules that existed in an early stage of the Earth's past. However throughout history, there have been many other explanations for the origin of life 

1.1. Theory of spontaneous generation 

In ancient times, humans thought that life began from inanimate objects. People thought that this process did not only happen at the beginning of life on the Earth, but also continued to happen under special conditions. In ancient Egypt, people thought that the Sun's heat on the sediment in the Nile produced snakes and crocodiles. In the Middle Ages, there were many recipes to create living things from all sorts of materials such as mud, animal faeces and food remains. 

In 1667, a Flemish doctor named Jan Baptist van Helmont wrote: 'If we pu sweaty underwear and wheat in a wide bottle, after 21 days the smell changes ad penetrates the wheat grains, changing the wheat into mice.' Today, statements like the one made by Van Helmont seem irrational. However it is possible to understand how people thought that living things appeared through spontaneous generation. If we observe some phenomena in daily life, it could seem possible that inanimate objects produce living things. For example, after a rainy day. we can see many microscopic organisms in puddles; or if we leave a piece of meat in a warm and damp place, it will be covered in flies and maggots? after a few days. Spontaneous generation could not be questioned until the first controlled experiments were carried out during the 17th century. However, for several hundreds of years, people still thought that microorganisms were formed in this way until some experiments carried out during the 19th century proved them wrong.

1.1.1. Redi's experiment

In 1668, the Italian doctor Francesco Redi demonstrated that the maggots found in rotting met came from flies' eggs.

1.1.2 Pasteur's experiment

In the second half of the 19th century, the French chemist Louis Pasteur demostrated that microorganinsms were not created from spontaneous generation. At this point, the theory was finally rejected.

1.2 Oparin's theory

 At the beginning of the 20th century, the Russian biochemist Alexander Ivanovich Oparin developed an abiogenic theory about the origin of life on the Earth. His theory is still the basis for the hypothesis accepted by most scientists today. We now know that in order for inorganic matter to create organic matter, energy and reducing agents', which are rich in hydrogen, are needed. According to Oparin, life appeared from physico-chemical processes that took place in the Earth's primitive atmosphere. The processes can be explained in four stages:

1.2.1 Miller's experiment

The North American scientist Stanley Miller put a mixture of gases, which were believed to be present in the primitive atmosphere, into a container and applied high-voltage electric shocks. The reaction produced several organic compounds, including amino acids. 

However, nowadays we know that the primitive atmosphere was not made up of the gases described by Oparin. This is why the majority of experts think his hypothesis is unable to explain the origin of life.

1.3 Panspermia theory

This theory, which was supported by scientists such as Svante Arrhenius, daims that life on the Earth arrived from outer space as microorganisms trapped inside meteorites and that all other living things evolved from these microorganisms. A similar theory, known as molecular panspermia, claims that what arrived from space were organic molecules, which are believed to be the building blocks of life on the Earth.

1.4 The present-day hypothesis

Today, we believe that the gases that formed the primitive atmosphere, mostly from volcanic eruptions, were carbon dioxide, nitrogen, water vapour and, in smaller quantities, sulphur dioxide and hydrochloric acid, There was no hydrogen or any other reducing agents. It is also believed that the strong ultraviolet radiation, due to the lack of oxygen and therefore a protective ozone layer, made organic molecules unstable. 

There are several abiogenic theories that try to explain the beginning of life by basing their hypothesis on two key requirements that all living things must have: the ability to reproduce and to have a metabolism that is able to exchange energy and matter with the environment.

 One of the possible places in which living things could have first appeared are deep-sea hydrothermal vents. According to some scientists, these places release inorganic matter that, along with water and carbon dioxide, formed the first organic molecules. The energy needed for this process could have come from the oxidation of reducing agents, such as hydrogen sulphide. 

According to this hypothesis, there are two possible mechanisms that could have led to the creation of the first cell: 

ºSelf-replication: molecules that were able to self-replicate (possibly RNA) and that were surrounded by a lipid layer were formed, creating the first prokaryotic cells. 

ºMetabolism: lipid bubbles that were able to carry out chemical reactions were formed and later developed the ability to self-replicate. 

Whatever their origin, ancestral cells must have been structures made up of a primitive membrane and an aqueous interior with metabolic and self-replicating abilities. 

The traces of biological activity that have been found in Australian stromalites, which O million years old, suggest they belonged to cyanobacteria, photosynthetic are over 3 organisms responsible for the existence of oxygen in the atmosphere. 

The formation of the ozone layer created a shield against ultraviolet radiation and as a result, life on the Earth bloomed.