Unlike panspermia, this argument presents the idea that simple, primarily single-celled organisms are likely to have developed independently of life elsewhere in the universe. Instead of life forming in one single place, this idea requires life to be a far more common occurrence in the universe relative to hypotheses that rely on panspermia.
It is thought that life on Earth developed in a "Primordial Soup" of basic proteins, organic(carbon-containing) molecules and water mixed together with the input of energy from sources such as the Sun and lighting. In the Miller-Urey experiment; a closed system of the basic materials we think were contained on earth, are mixed together and have both thermal and electrical energy applied, when left, even for a relatively short period of time were found to have Amino Acids form. These are the chemicals considered to be the essential building block for life, and the molecule that DNA is built up of form. It is thought that if this was left for sufficiently long, these molecules would begin to form far more complex chains leading to single-celled organisms capable of replicating. Earth had approximately 1Gyr to do this. None of the elements and chemicals used in the Miller-Urey experiment is unique to earth. They have all been found in vast quantities within even the Milky Way. Amino Acids have been found in giant molecular clouds that are themselves capable of containing planets around a habitable star. It is a natural progression to believe that the same process that caused life to begin on Earth could occur thousands, if not millions of times throughout the Universe.
Though Amino Acids have been seen to naturally develop in the lab using what we think was contained within the "primordial soup", not only have we never seen these Amino Acids develop into far more complex molecules. Not only this, but we do not know for sure what was contained within the "primordial soup", it is purely speculative. Even in space within the giant molecular clouds, we have not observed anything more complex than simple Amino Acids. We have also never directly observed Amino Acids on any planets, let alone on planets contained within an orbit that we classify as suitble to life similar to ours
It is believed life formed on Earth in a basic soup of organic molecules and water, with an abundance of electrical and thermal energy. None of this is unique to earth and is, in fact, common throughout the Universe. The most basic building blocks of DNA, Amino Acids have successfully been created in the lab in simulated early Earth condition, and have also been found in giant molecular clouds in space, showing them can form with artificial intervention.
Rejecting the premises
Though there is strong evidence, we do not know for certain the specific composition of what we term the primordial soup. Though the individual elements have been observed to be fairly common throughout the universe, it is significantly rare to find the combination we hope to find that matches the primordial soup. Though simple Amino Acids have been observed in Molecular clouds, it is though that a planet is important for life to develop, as it provides a far more stable environment relative to a cloud, which is typically very turbulent, making it difficult for life to form within.