At the present time, I believe the most plausible hypothesis for the origin of life is that it originated within the Earth in environments where hyperthermophilic organisms could thrive (beneath the seabed or near hydrothermal vents). I think that these earliest organisms passed on genetic information (and were instructed to operate via) RNA based mechanisms. I do not, however, feel that the RNA was the first polymer to contain genetic information, but that it was slowly put in place over a period of time while a “scaffold” system allowed the organisms to operate. In Paul Davies paper The origin of life II: How did it begin?, Davies mentions Cairns-Smith’s analogy of a stone arch, which seems to be an inexplicable structure simple by looking at it. However, the arch is explained by noting that a scaffold was in place while it was assembled (1). I think it is likely that some form of inorganic crystal contained the initial information required for a chemical system to thrive and, as organisms began to evolve from increasingly complex chemical systems, a more complex information carrier evolved as well. RNA would be the result of this system, in my opinion, until further evolution pushed for a more stable information carrier, the end result of which would be DNA.
I think this is the most plausible hypothesis as the geological record of Earth indicates that the Earth may have been habitable for organisms resembling hyperthermophiles for sometime near 3.9 billion years ago (2). There are areas of the Earth, namely locations well beneath the seabed, that could have been cooled enough to allow life while also allowing for a flux of nutrients by sea water movement at this time. The noted location in Davies paper The origin of life I: When and where did it begin? is at least a kilometer below the seabed of the Pacific Ocean (2). Additionally, discoveries were found on land that could indicate life at similar distances below sea level, but I believe these emerged at a later period than those below the seafloor. As Davies also mentions, existing below the seafloor would shield any existing organisms from heat and vapor produced by meteor collisions, allowing for these organisms to thrive once the environment had become habitable (likely several millions of years later, presumably when more complex machinery had evolved to allow organisms to adapt to conditions above the seabed). During this time, organisms may have utilized crystal defects for containing genetic information. Many years later, after organic bases had either been delivered by meteor collisions or formed because of the environment created by these collisions, the assembly of nucleic acid systems to form RNA may have occurred as an evolutionary drive for more stable, portable storage mechanisms (3). In this hypothesis, pores in the rock would have served as rudimentary cells until the need for compartmentalization (when pore sizes became too large or an organism escaped a pore altogether) was realized by evolutionary driving factors.
The above mentioned hypothesis seems largely different from other proposed hypotheses, such as the “prebiotic soup” theory or the panspermia theory (2,4). The prebiotic soup seems like an unlikely theory, as Davies points out: amino acids are rather easy to assemble, but proteins and DNA/RNA are not. There are an extreme number of combinations of these building blocks, only a miniscule amount of which would be biologically meaningful. The theory that life formed on Mars and may have traveled to Earth by riding on masses ejected from collision events is considerably more likely than the prebiotic soup theory. However, life would have had to originate on Mars under similar conditions to my proposed hypothesis above: in small compartmentalized areas (non-membranous) with some inorganic method of passing on information. While some discredit life formation on Earth due to the unsuitable conditions caused by collision events, I believe that it is more likely life survived on Earth through these events (by existing well below rock beds), than that life traveled on ejected Martian rock, survived space-travel for a period of time between months and millions of years, and survived reentry to Earth's atmosphere. A theory counteracting the proposed RNA-based hypothesis is the protein-world hypothesis, in which proteins served as the initial storage mechanisms for DNA while also acting as enzymes. However, I find this unlikely for the same reason as the prebiotic soup theory: the combinatorial nature of nucleic acid peptides allows for such a large amount of combinations, only a minute amount of which would actually have appropriately folded structures for the required activity.
Assuming life starts according to my hypothesis, then the best place to look for it would be any depth up to 1 kilometer below the ground. Unfortunately, this is an unrealistic method for finding life elsewhere in our solar system, but it may lead to new discoveries (perhaps of multiple biogenesis events) on our own planet. Additionally, since there are no longer large meteor collisions on most planets in the solar system, it is possible that life has percolated up on some planets (towards the soil surface) and may soon be reaching a period of development wherein it may breach the surface and become detectable to human observational methods. If this is the origin of life, the best method for determining its timeline, statistical probability, and possibly even its initial form would be the attempted recreation of deep-Earth environments and conditions in the lab.
Davies, P. (2001) The origin of life II: How did it begin?. Science Progress, 84(1), 17-29.
Davies, P. (2001) The origin of life I: When and where did it begin?. Science Progress, 84(1), 1-16.
Callahan, M. P., Smith, K. E., Cleaves II, H. J., Ruzicka, J., Stern, J. C., Glavin, D. P., House, C. H., Dworkin, J. P. (2011) Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases. Proceedings of the National Academy of Sciences of the United States of America, 108(34), 13995-13998.
Wesson, P. S. (2010) Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space. Space Science Reviews, 156(1-4), 239-252.