We have two reasons for learning how life appeared on our planet.We have to know more about our origins and to understand the mechanisms that are implicated and whether they are reproducable on other planets.

After many theories based on spontaneus generation (3), Pasteur required the scientific community to change it's approach. It was the start of the evolutionary hypothesis.

This is based on two research factors :

• geological indications (tracks of primitive life)
• knowledge of the primitive terrestrial atmosphere

This is why there are many theories about an organic origin of life. The mineral origin seems improbable (3).

• The primitiv soup :
  • Molecules forms from elements present in the atmosphere and in the ground.
  • heat sources could have helped the apparition of organised structures thanks to underwater gaseous exchanges and thermal energy of the earth's mantle.
• The earthly origin
  • 60 organic molecules have been detected in interstellar clouds. These clouds could have spread these molecules on earth when it passed through them.
  • comet impacts : comets are very rich in organic materials.
  • meteoric and micrometeoric impacts : organic molecules and amino acids have been found in meteors, clear of any terrestrial contamination.

The goal of scientists is now to understand how the molecules that made a simple living being did appear :

• enzymes : proteins that copy the genetic information.
• lipids : they form the membrane and the isolation of the internal compounds, protect the cell from the immediate environnement and thus create individualisation.
• amino acids : DNA needs these bricks. They compose the genetic information that make possible reproduction and mutations.

A DNA or RNA world : the RNA is used as a messenger by enzymes: It is a DNA translation from which the proteins that appear in the genetic code are made. We arrive to the chicken and the egg : Does DNA come first, or is it RNA ?

We know that life appeared very soon after formation of Earth (14). So it can't have been very complicated at start.
This is why science wants to find what simple form is at life's origin. This missing being is called LUCA (Last common ancestor) as a tribute to LUCY, his palaeonthological equivalent.