In recent decades, the documented fossil record has been extended to nearly 3,500 million years ago. Hundreds of fossiliferous units have been discovered that contain thousands of microbial fossils, and the rules for accepting ancient microfossil-like objects as bona fide have come to be well established. Of these, criteria for establishing biogenicity have proven the most vexing to satisfy. Three new techniques have now been devised to help answer this need: (1) Ion microprobe spectroscopy has been used to measure the carbon isotopic compositions of individual fossils in geologic units ~850(1), ~2,100(1), and ~3,490-Ma-old(2). (2) Laser-Raman imagery has been used to analyze the molecular compositions of cellular fossils and particulate organic matter in 25 geologic units ranging from ~400 to ~3,500 Ma in age, including specimens from several of the oldest microfossiliferous units known(3,4). (3) Atomic force microscopy has been used to reveal the nanometer-scale structure of the kerogenous components of single Precambrian microscopic fossils(5).

These new approaches hold promise not only for elucidation of the isotopic composition, chemical makeup, and submicron-scale structure of individual microscopic fossils, but for understanding of the geochemical maturation of ancient organic matter and for clarifying the nature of minute fossil-like objects of putative but uncertain biogenicity, whether Precambrian or extraterrestrial.

(1) House, C.H., Schopf, J.W., McKeegan, K.D., Coath, C.D., Harrison, T.M. and Stetter, K.O (2000) Carbon isotopic composition of individual Precambrian microfossils. Geology 28: 707-710. (2) Ueno, Y., Isozaki, Y., Yuimoto, H. and Maruyama, S. (2001) Internatl. Geol. Rev. 43: 196-212. (3) Kudryavtsev, A.B., Schopf, J.W., Agresti, D.G., and Wdowiak, T.J. (2001) In situ laser-Raman imagery of Precambrian microcopic fossils. Proc. Nat. Acad. Sci. USA 98: 823-826. (4) Schopf, J.W., Kudryavtsev, A.B., Agresti, D.G., Wdowiak, T.J., and Czaja, A.D. (2002) Laser-Raman imagery of Earth's earliest fossils. Nature 416:73-76. (5) Kempe, A., Schopf, J.W., Altermann, W., and Heckl, W.M. (In Press) Atomic force microscopy of Precambrian microscopic fossils. Proc. Nat. Acad. Sci. USA