Observations and experiments have proven conclusively that all recognized life forms are produced by preexisting organisms or life forms, called the law of biogenesis. Despite the absence of substantiation for the theory, abiogenesis has become accepted by nearly all practicing scientists. The hypothesis of abiogenesis remains virtually unchanged since its inception in the 1920s, and assumes that life originated at some point in earth's past under conditions no longer present. This particular tenet of evolution can be summed-up by the phrase "abiogenesis at first, biogenesis ever since". It is taught today as a certainty, although the exact mechanisms remain theoretical. Discussions in evolutionary biology textbooks go to great lengths to demonstrate how abiogenesis could have occurred under multiple primordial scenarios