Comparable system-level organization of Archaea and Eukaryotes

A central and long-standing issue in evolutionary theory is the origin of t he biological variation upon which natural selection acts'. Some hypotheses suggest that evolutionary change represents an adaptation to the surroundi ng environment within the constraints of an organism's innate characteristi cs(1-3). Elucidation of the origin and evolutionary relationship of species has been complemented by nucleotide sequence(4) and gene content(5) analys es, with profound implications for recognizing life's major domains(4). Und erstanding of evolutionary relationships may be further expanded by compari ng systemic higher-level organization among species. Here we employ multiva riate analyses to evaluate the biochemical reaction pathways characterizing 43 species. Comparison of the information transfer pathways of Archaea and Eukaryotes indicates a close relationship between these domains. In additi on, whereas, eukaryotic metabolic enzymes are primarily of bacterial origin (6), the pathway-level organization of archaeal and eukaryotic metabolic ne tworks is more closely related. Our analyses therefore suggest that during the symbiotic evolution of eukaryotes,(7-9) incorporation of bacterial meta bolic enzymes into the proto-archaeal proteome was constrained by the host' s pre-existing metabolic architecture.