Unit 3:  The origin and evolution of life                          1   2   3   4   5   6 

1.2. The prebiotic evolution

In spite of all these discussions and controversies the fundamental question had not got answer yet. How did life originate?

The hypothesis about the origin of life can be classify into two different currents:

      - Exogeneous origin:

- Endogeneous origin:

This current suggest that organic molecules and living beings appeared in the primitive Earth

(Prebiotic synthesis or Abiogenesis)

Most likely is that life originated from organic matter present in the Earth which origin was in part extraterrestrial.

a) Physical-chemical origin of life 

In the beginning of 20^th century, the russian Aleksandr Oparin and the american John Haldane proposed his hypothesis about the origin of the first organic molecules. According with it, the organic molecules came from simple inorganic compounds which were present in the primitive atmosphere of the Earth.

In this early free oxygen atmosphere, gases (methane, ammonia, hydrogen and water vapour) reacted among them to form the first little biomolecules, thanks to the energy from solar radiation, electric storms and volcanic eruptions, constant in this time. The formed simple biomolecules were progressively accumulating and concentrating in the primitive oceans making a mixture to which he called “hot dilute soup” or “prebiotic soup”

These ideas were confirmed in 1953, thanks to the experiments realized by Stanley Miller, in collaboration with Harold Urey. They simulated the supposed conditions of the early atmosphere in their laboratory and obtained several organic compounds (amino acids, urea and fat acids). The hypothesis was so confirmed. Today, the composition of the primitive atmosphere is supposed different, but the hypothesis is equally valid.

Interactive: Miller's experiment (UCSD)

Animation: Miller's experiment (Freeman LifeWire)

b) Formation of macromolecules

     - Clay hypothesis

Once formed, the simple biomolecules (amino acids and nucleotides) began to react among them making bigger molecules by polimerisation (proteins and nucleic acids).

However, the little concentration of these simple molecules in the prebiotic soup would make very difficult forming polymers from them.

Cairns-Smith proposed the participation of clays in the process. Clay could act as catalytic converter, attracting and concentrating on its surface the little molecules and making easier that they joined to form the first biologic macromolecules. 

Recently it has been proposed two alternative minerals to clay that could do the same function:

- Mica (silica of aluminium, magnesium or iron)

- Pyrite (FeS₂)

     - Hydrothermal hypothesis

In 1977, Corliss discovered hot springs nearby the ocean ridges.  They supported a large variety of non-light dependent fauna. Producers of these ecosystems were chemosynthetic bacteria which obtained energy from the chemical elements that flows from these springs.

Later, Russell who investigated the simplest forms of life, suggested that conditions in this place are better for appearing of life than the primitive atmosphere:

- The depth of the oceans would give protection against the harder surface conditions

  (ultraviolet radiation)

- The environment is reductor (without oxygen)

- They are rich in clay minerals

- The flow of geothermic energy is continuous and the reactions involved are exothermic

  (spontaneous)

     1.3. What was the contribution of Urey and Miller to the debate about the origin of life?

     1.4. Name the main ideas of the theory of the prebiotic synthesis.

     1.5. What role could some minerals such as clays and micas play in the abiotic formation

             of the first macromolecules?

     1.6. What advantages have deep sea vents to be the scenario of the origin of life?