Biosignatures and abiotic constraints on early life

Arising from: Y. Ueno, K. Yamada, N. Yoshida, S. Maruyama & Y. Isozaki Nature 440, 516–519 (2006); Ueno et al. reply Ueno et al.1 contend that methane found in fluid inclusions within hydrothermally precipitated quartz in the Dresser Formation of western Australia (which is roughly 3.5 Gyr old) provides evidence for microbial methanogenesis in the early Archaean era. The authors discount alternative origins for this methane, suggesting that the range of δ 34 C CH 4 values that they record (−56 to −36‰) is attributable to mixing between a primary microbial end-member with a δ 34 C CH 4 value of less than −56‰ and a mature thermogenic gas enriched in 13C (about −36‰). However, abiotic methane produced experimentally2,3 and in other Precambrian greenstone settings4,5 has 13C-depleted δ 13 C CH 4 values, as well as δ 13 C CO 2 -CH 4 relationships that encompass the range measured for the inclusions by Ueno et al. — which suggests that an alternative, abiotic origin for the methane is equally plausible. The conclusions of Ueno et al. about the timing of the onset of microbial methanogenesis might not therefore be justified.