Abiogenesis or chemical evolution is the theory which is used to explain how life on Earth arose from nonliving materials. It has attracted scientific and religious controversy, as there are some people who believe that abiogenesis is simply impossible or that researchers have failed to back up their claims with usable data. Ultimately, as with many controversies, there is no way to demonstrably prove that one side or another is right or wrong, although it is possible to muster significant evidence.
It is important to be aware that there are several theories about how abiogenesis occurred and the mechanics of the process. Some of these theories have since been discredited, but this does not discredit the theory as a whole. The basic idea is that natural chemical reactions on the Earth led to the formation of amino acids, a process which can be clearly demonstrated, and that somehow nucleic acids arose and began building proteins out of these amino acids, eventually leading to the development of living organisms. Researchers who study the history of life on Earth are interested in exploring how the nucleic acids showed up.
Criticisms of abiogenesis often grossly simplify the science; in part this is done to make it accessible to laypeople, and in part it is done in an attempt to discredit the theory. Researchers openly admit that the mechanics of abiogenesis are still a bit mysterious, but that in all likelihood, the process involved a lot of trial and error. The development of life on Earth probably took a number of wrong turns and encountered a few stagnant periods, and abiogenesis did not leap from amino acids to full-on proteins overnight.
Creationists who believe that life on Earth was created by God argue that this process is not possible because it is highly statistically unlikely. This is, in fact, true; the probability of having life arise from nonliving materials is extremely low and it would require a very unique and remarkable set of circumstances. But Creationists may neglect to mention that the circumstances which could have led to abiogenesis probably happened a number of times before anything happened, and that even when something is statistically unlikely, it can still happen. Statistics are not disproof, they are merely statements of probability.
Some scientists have critiqued the theory, arguing that it has logical holes or that no satisfactory explanation for the development of nucleic acids has been put forward. These arguments against current theories usually accept that abiogenesis is possible, but that it did not happen in the way that researchers have theorized that it did.
More research may provide more clues into what happened on Earth in its early stages to create life, and whether or not the perfect storm of conditions which led to abiogenesis could happen elsewhere. Some people have attempted to reconcile religious faith with abiogenesis by suggesting that perhaps God set up the circumstances, and let chance take its course.