The genomics of LUCA.

To understand the nature and evolution of LUCA, or Last Universal Common Ancestor, the minimum genome of LUCA has been identified based on the genes common to the eight primitive Euryarchaea and Crenarchaea species Methanopyrus kandleri, Methanothermobacter thermautotrophicum, Methanococcus jannaschii, Pyrococcus abyssi, Pyrococcus furiosus, Pyrococcus horikoshii, Aeropyrum pernix and Pyrobaculum aerophilum, together with the methanogenesis genes of the primitive methanogens. The 424 protein encoding genes in the minimum LUCA genome exceed significantly the 150-340 genes estimated to be present in a minimal proteome compatible with life. Thus LUCA was not a minimal organism but the first modern organism equipped with a DNA genome and the universal genetic code. The hyperthermophilic, Methanopyrus-proximal LUCA is consistent with a Hot Cross Origin of life which proposes that early heterotrophic life forms in the cooler temperature zones invented methanogenesis and a DNA genome upon their adaptation to the hydrothermal vents, where life flourished massively on lithoautotrophy supported by carbon dioxide and hydrogen, thereby leading to the rise of LUCA.