MICROFOSSIL BIOGENICITY: PITFALLS AND PROGRESS

In recent decades, hundreds of Precambrian fossiliferous units have been discovered that contain thousands of formally described microscopic fossils. Of the criteria required for acceptance of such ancient fossils, those for establishment of biogenicity have proven the most vexing to satisfy. Four techniques have recently been devised to help meet this need: (1) Ion microprobe mass spectrometry has been used to demonstrate the biologically characteristic carbon isotopic compositions of individual fossils in geologic units ~850-, ~2,100-, and ~3,490-Ma-old. (2) Atomic force microscopy has been used to visualize the nanometer-scale, evidently biologically distinctive structure of the kerogenous components of single Precambrian microscopic fossils. (3) Laser-Raman imagery has been used to demonstrate the carbonaceous composition of petrified microorganisms in 25 geologic units ranging from ~400 to ~3,500 Ma in age. (4) Raman imagery has been used to construct micron-scale three-dimensional virtual "chemical maps" of ancient microscopic fossils that show them to contain cell lumina.

Combined with studies of fossil microbial populations and their optically discernable cellular morphologies, these new techniques can show the isotopic composition, submicron-scale organization, carbonaceous (kerogenous) molecular structure, and three-dimensional chemical makeup of individual microscopic fossil-like objects. Although none of these indices -- by themselves -- can be regarded as "proof" of biogenicity, no mechanisms other than biology are known that produce populations of microbe-like objects that have cellular morphology and exhibit "biological" isotopic compositions, submicron-scale biologic-like organization, carbonaceous (kerogenous) molecular composition, and a three-dimensional structure that shows them to possess cell lumina. Thus, combined with morphometric data on cellularly preserved (including decomposed) fossil populations, these four indices -- taken together, as a suite of characters -- can provide compelling evidence of the biological origin of objects in question. Techniques such as these hold promise for clarifying the nature of minute fossil-like objects of putative but uncertain biogenicity, whether Precambrian or extraterrestrial.