Two schools of thought address differently the principles of life's origin. The first school assumes that life began with a small repertoire of molecules similar in structure and function to those found in today's life, e.g. very specific subsets of nucleobases and amino acids, as well as linear polymers thereof . This top-down realm seeks prebiotic synthesis paths that could yield such life-like molecules . According to this school, the first entities that conformed to the NASA definition were membraneenclosed, rather complex reproducing protocellular entities, containing replicating RNA and encoded RNA or protein enzymes .