11. Morpho- and Chemo-Fossil Evidence of Early Life

This chapter summarizes what is known about the timing of the emergence of life on Earth from the morpho- and chemo-fossil (chemical and isotopic signals remaining from the decomposition of living organisms) records. The geologic record back to ca. 3.5 billion years includes low grade sedimentary rocks in which organic residues of microbiota present during deposition have remained substantially intact. As different metabolic mechanisms variably fractionate carbon isotopes toward isotopically light values, a longstanding strategy has been to measure δ¹³C in these organic residues, or kerogens, for biologic signatures. When compared to carbon isotopes in inorganic carbonate rocks, a consistent offset is seen throughout the past 3.5 billion years with inorganic carbon averaging δ¹³C close to 0‰ and kerogens yielding δ¹³C of approximately −25‰. As the latter value is broadly characteristic of oxygenating photosynthesis, this relationship has been seen as evidence of past biologic activity. However, as metamorphic grade increases, kerogens are reacted to simpler hydrocarbons, ultimately yielding graphitic residues. The discovery of isotopically light carbon isotopes in microscopic graphite inclusions in rocks as old as ca. 3.83 billion years and in a 4.1 Ga zircon extends the possible emergence of life on this planet back into the Hadean eon. Although inorganic mechanisms exist that could potentially produce light δ¹³C signatures, these isotopic data are consistent with molecular clock calibrations of genomic mutations which suggest a lower bound for the time of life’s origin between 4.1 and 4.4 billion years.