The Main Contenders…
To understand the origin of life on Earth remains one of the greatest scientific quests, and understanding it properly means not to see its emergence as a singularity, but rather to unravel the physical and chemical mechanisms that set the context. In recent years significant progress has been made based upon three hypotheses. (These are listed in chronological order and the proposer does not favour one hypothesis over the others):
firstly, the ‘panspermia’ hypothesis, maintains that life was made in the vastness of space (Hoyle, Arrhenius) and delivered onto the Earth ready-made (Wainwright). This is in part supported by recent discoveries of prebiotic molecules (methanol) and ions (eg cyanate ions) within the interstellar medium and in dust from a meteorite (eg glycine) (Elsila) as well as in the meteorites themselves (Pizzarello). In addition, evidence for panspermia also relies mostly on continued research involving extremophiles and resilient spores (Horneck).
the second is the ‘RNA world’ hypothesis (Crick), which promotes that RNA molecules were the pre-cursors of life on Earth - predating DNA, as evidenced by horizontal gene transfer via conjugation, transformation, transduction as well as the recently discovered gene transfer agents (GTA) and membrane vesicles transfer (MVT) (Forterre). Broadly, RNA can self-replicate (Ferris); it can act as a repository of information; and it exhibits enzymatic activity (Altman). In essence, the RNA world hypothesis presupposes a ‘genetic first’ hypothesis (Gilbert); and
the ‘alkaline hydrothermal vents’ hypothesis (Russell) argues that the first molecules were those belonging to the Fe-S complex (and possibly NiFeS) which initiated life. This being largely supported by the fact that the vents are rich in certain types of clays which contain Fe-S mineral complexes. Biologically, these complexes are important in redox reactions (eg ferredoxins). Clays also formed minuscule bubbles, which encapsulated reduced chemicals (eg formaldehyde) that went on to form life. Broadly this hypothesis presumes that metabolism came first (Morowitz).
In trying to appraise these hypotheses we simply do not know how many avenues we need to explore before we arrive at answering the ultimate question of the origin of life. For example, when assessing these hypotheses, the following questions need to be addressed: where and how were the necessary molecules made? Were these molecules made on Earth (using lightning within the atmosphere (Miller) and volcanic gaseous lightning as well as UV irradiation of molecules present in the interface between low/heavy density atmospheric layers); in the interstellar medium (Gerakines); or the hydrothermal vents? These hypotheses are just the tip of the iceberg, as there are innumerable other possibilities to discover and contemplate…