Beginnings of terrestrial life: problems of the early record and implications for extraterrestrial scenarios

摘要

Abstract: With the paleobiological evidence from the currently known Archaean rock record at hand, the existence on Earth of microbial ecosystems as from about 3.8 Ga ago so firmly established as to be virtually unassailable. However, important details pertaining to the establishment and early history of life on this planet call for further elucidation. Residual questions primarily center around (1) the impairment of relevant information in the oldest record that bears a metamorphic overprint, (2) the apparent non- documentation in the Archaean record of an archaebacterial lineage expected to form the base of the phylogenetic tree of terrestrial life, (3) the possible role of impact interference with early biological evolution, and (4) problems of the time scale of early organic evolution as exemplified by the unheralded appearance about 3.8 Ga ago of microbial life on the organizational level of the prokaryotic cell. !37