Abstract
Abstract The last universal ancestor (LUA) represents a relatively accessible theoretical intermediary between extant cellular organisms and early, precellular ‘life’. In a previous study, the expectation-maximization (EM) approach was used to infer ancestral amino acid frequencies, where in each iteration expected counts were derived from posterior distributions at each site. Applying this approach to estimate the amino acid composition of 65 proteins in the LUA showed that composition was more similar to that of extant thermophiles than mesophiles. This chapter examines whether the previous result is robust with respect to the OGT of the taxa used to infer the amino acid composition of proteins in the LUA. It is shown that even if only mesophilic species are used to derive the estimated ancestral amino acid composition, that composition is most similar to that of thermophiles, as measured by Euclidean distance. The relative mean Euclidean distance between the amino acid composition in any one species and that of a set of mesophiles or thermophiles can be used unequivocally to classify it. Thus, the inferred amino acid composition in the LUA allows its classification as a thermophile.
Published Version
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