Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon

Evidence for carbon cycling or biologic activity can be derived from carbon isotopes, because a high ¹²C/¹³C ratio is characteristic of biogenic carbon due to the large isotopic fractionation associated with enzymatic carbon fixation. The earliest materials measured for carbon isotopes at 3.8 Ga are isotopically light, and thus potentially biogenic. Because Earth's known rock record extends only to ∼4 Ga, earlier periods of history are accessible only through mineral grains deposited in later sediments. We report ¹²C/¹³C of graphite preserved in 4.1-Ga zircon. Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes. Its ¹²C-rich isotopic signature may be evidence for the origin of life on Earth by 4.1 Ga.