Only D forms of sugars are used, and only L forms of amino acids. This feature is called homochirality. Homochiral molecules are required for life. This means that self-replication must be stereospecific, preserving the homochirality of the molecules. How such a system could have arisen is the subject of this paper.
Life requires both homochirality of polymers and very high stereospecificity in self-replication.
Calculations show that the formation of homochiral polymers requires one of two types of conditions. If the stereospecificity is not very high, then the medium must be essentially chirally pure. If the medium is not essentially chirally pure, then the stereospecificity must be extremely high. The authors conclude that homochirality is necessary for extremely high stereospecificity, so stereospecificity could not have arisen first. This means that a chirally pure medium must have been present in order for life to originate.