Abstract
In this paper, we examine a restricted subset of the question of possible alien biochemistries. That is, we look into how different life might be if it emerged in environments similar to that required for life on Earth. We advocate a principle of chance and necessity in biochemistry. According to this principle, biochemistry is in some fundamental way the sum of two processes: there is an aspect of biochemistry that is an endowment from prebiotic processes, which represents the necessity, plus an aspect that is invented by the process of evolution, which represents the chance. As a result, we predict that life originating in extraterrestrial Earth-like environments will share biochemical motifs that can be traced back to the prebiotic world but will also have intrinsic biochemical traits that are unlikely to be duplicated elsewhere as they are combinatorially path-dependent. Effective and objective strategies to search for biomarkers, and evidence for a second genesis, on planets with Earth-like environments can be built based on this principle.