The conditions favourable to the development of life are the conditions that allowed the synthesis of the first molecules of living things, as well as their arrangement into living cells.
Life, as we know it, is only present on Earth. No trace of life has been found on any other planet. Scientists have studied the causes of the presence of life on Earth and the impossibility of it being present on other celestial bodies. They have drawn up a list of conditions that a planet must meet to support the development of even a primitive form of life.
A planet must be located at an optimal distance from the star around which it orbits. Given the energy released by the star, the planet should not be too close since it would receive too much energy, which would make its surface temperature uninhabitable. Conversely, it should not be located too far away either, since it would not receive enough energy and its temperature would therefore be too cold. The area surrounding the star is referred to as the habitable zone because the temperature conditions are optimal for a planet to have liquid water on its surface. The size and distance of the habitable zone vary depending on the diameter and power of the star concerned.
The shape of a planet's orbit affects the temperature variations that can be found there. For example, when the orbit of the Earth is closer to the Sun, then the planet receives more energy. Conversely, when it is farther away, it receives less energy. Thus, the shape of the orbit determines the amount of energy that reaches the planetary surface. If the Earth's orbit were more elongated, its distance from the Sun would be too great at times, which would cause the Earth's surface to freeze. Large temperature differences between hot and cold periods would then influence the Earth’s climate. Such fluctuations are not favourable to the development of life.
Life cannot develop without the presence of an atmosphere. This is because the atmosphere filters the Sun's harmful rays, such as ultraviolet rays and x-rays. It, therefore, acts as a protective shield for life forms. Also, the atmosphere allows the water cycle to take place, since the water in gaseous form condenses into clouds. Finally, we find molecules essential to life in the atmosphere, such as oxygen.
The mass of the stars is a factor that influences the gravitational force exerted by a planet (the second factor being the distance between two stars). Thus, the greater the mass of a planet, the greater the gravitational force exerted. The gravitational force of a planet is what holds together the particles forming the atmosphere. However, if the force of gravity is too strong, very light gas particles, such as helium, will also be strongly held back. Very light gases are unfit for life. The mass must therefore be ideal so as to retain only the gases allowing the development of life such as oxygen.
Water is an essential element in the reactions leading to the emergence of the first forms of life. Moreover, it is the main component of living beings. For life to develop, the presence of water is an essential condition. Water seems to promote the emergence of life in three ways.
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As the water molecule contains hydrogen and oxygen, it provides two essential constituents for the development of complex molecules in living things.
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Life first appeared in liquid water. It formed a "primitive broth" in which the complex molecules were able to come together to form living cells.
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Water offers protection for the first living beings against the harsh climate on land.
For the development of life to be made possible, a planet must offer solid support. Therefore, the planet must have a lithosphere on its surface so that the first molecules can settle and develop. The so-called telluric or terrestrial planets (Mercury, Venus, Earth, and Mars) have a lithosphere.
Earth combines all the six conditions favourable to the development of life. In addition, about |3.5| billion years ago, other conditions were present that made possible the emergence of the first living cells.
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The chemical elements essential to the complex molecules of living things were present on Earth. These are mainly carbon (|C|), oxygen (|O|), hydrogen (|H|), and nitrogen (|N|), in other words, the four elements which are the basis of all so-called organic molecules. These elements come together to form various molecules such as water |(H_2O)| , carbon dioxide |(CO_2)|, and molecular nitrogen |(N_2)| .
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The presence of energy sources enabled the chemical elements to be arranged into complex molecules. It is assumed that the primary energy sources were the ultraviolet rays emitted by the Sun, electric discharges caused by lightning, and heat given off during volcanic eruptions or by geothermal energy via hydrothermal springs.
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A very long period of time, because molecular arrangement does not take place overnight. Thus, the longer the time available for chemical reactions, the greater the possibility that the arrangements will give rise to organic molecules. The conditions for the development of life therefore had to be present for a very long period of time for the process to occur.