The Oldest Fossil or Just Another Rock?

摘要

The history of the past 542 million years of life on Earth can be read from shells, bones, teeth, and leaf casts preserved in the geological record. For the preceding 4 billion years, more subtle clues, such as remnants of microbial cells, biomolecules, and the impressions left by soft-bodied organisms, may be preserved in sedimentary rocks. But in sediments deposited in the first billion years of Earth history, these traces have been destroyed by metamorphism (recrystallization without melting, often accompanied by reaction and chemical exchange). Here, the only widely recognized evidence for life comes from measurements of carbon isotopes in kerogen and graphite (1). Because metabolic carbon fixation discriminates against ~(13)C, the ~(13)C/~(12)C ratio in biogenic carbon is ～3% lower than in inorganic carbon. This signature can be preserved through metamorphic processes that destroy microfossils and biomolecules.