ABSTRACT
According to the phylogenetic tree based on 16S rRNA (Woese et al., 1990), the organisms that branch closest to a common ancestor gain their metabolic energy from inorganic chemical reactions. This is permissive evidence for models of early ecosystems in which geochemical energy sources were tapped to fuel primary productivity rather than sunlight. It also suggests that metabolic systems emerged via energy transfer in coupled organic and inorganic redox reactions. Furthermore, we can infer from the central position of hyperthermophiles at the trunk of the l6S rRNA tree that early ecosystems were hydrothermal and that the emergence of life may have occurred in hydrothermal systems. If so, examining the geochemistry of hydrothermal systems should reveal the energy sources that supported early ecosystems, as well as the required thermodynamic drive for a metabolic system to emerge from a network of interdependent reactions.
