There are a number of other hypotheses, many slight modifications of either the “gene first" or the “metabolism first" hypotheses. Some hinge on details regarding either how replication of a molecule like a nucleic acid started, while others hinge on where life started exactly. An example of the former is the “clay hypothesis", put forward by A. Graham Cairns-Smith in the mid 1980s, who was troubled by the absence of “scaffolding" for the early replication of molecules. He proposed that the first organic molecules needed some kind of platform, or template, to aid their replication; the platform was clay, or silicate crystals in solution. Scientists have researched the idea of crystals as templates and found that there may be some basis to the idea. As an example of the latter, there is the “deep biosphere" model of Thomas Gold, proposed in the 1970s, which states that life may have originated several kilometers below Earth’s surface and could have taken advantage of reactions between minerals and gases.
Finally, there are some scientists who subscribe to the idea that life, or at least organic molecules, arose somewhere in space and found their way to Earth by some means, usually an asteroid or a comet. The theories of panspermia and exogenesis are found in this category. Organic material is actually relatively common in outer space, having been found on rocks originating from Mars and on comets. Researchers have taken water, ammonia, carbon monoxide and methanol, have frozen this mixture and then have exposed it to UV radiation; these conditions would be similar to those found in extraterrestrial environments. Organic molecules were created and these molecules, when hydrated, formed bubbles. The simple fact that organic molecules (the basic building blocks of life as we know it, such as proteins, sugars and fatty acids) is found in outer space, though, does not provide the definitive answer to the question as to exactly how these organic molecules arose in the first place.
At this point, a number of scientists allow for the possibility that perhaps more than one of these “models" is true. In other words, perhaps life arose through a combination of mechanisms, or perhaps life “tried" to get started along different pathways. We may never know which one was ultimately the successful pathway that led to life as we know it. The important thing is that mechanisms are being discovered in which the basic building blocks of life are created and linked together by physical and chemical means, by processes that very well could have operated on early Earth. To wrap up the series, we’ll look at what these origin of life theories mean for the search for extraterrestrial life and what kind of clues could indicate that life may have been or may be present on other planets.