Organisms at the base of the food chain, autotrophs that synthesise biological compounds directly from carbon dioxide, water and other fundamental materials in their environment, favour incorporating the lighter of the two common isotopes of carbon, 12C, as opposed to 13C. Consequently, one of the prime signatures of life in the carbon found in rock is a depletion in 13C, usually expressed as d13C with a negative value. It is this signature that has allowed the origin of life to be pushed back almost to the age of the oldest rocks on Earth (around 3.9 billion years ago) from carbon isotope studies of carbonaceous compounds (kerogen) in ancient sediments.
Different organisms alive today, particularly among the ecologically diverse bacteria, use different biochemical reactions in synthesising living material. Each of these have different effects on d13C. Potentially these differences could be used to identify roughly the kinds of bacteria that lived in the distant past. Up to now, however, isotopic studies of organic carbon have only been possible for bulk extracts from rock. That enables some bold conclusions, such as the current suggestion that oxygen-producing blue-green bacteria were around 3.5 billion years ago, but whole-rock results are ambiguous because of mixing of carbon originating from different metabolic pathways.
Being able to analyse carbon isotopes from individual fossil cells is a major breakthrough, and a team of palaeobiologists from the universities of California and Regensburg, Germany has done just that (House, C.H. et al., 2000. Carbon isotope composition of individual Precambrian microfossils. Geology, v. 28, p. 707-710). They used an ion microprobe that allowed the discovery of biological carbon encapsulated in resistant materials from 3.8 billion-year old metamorphosed iron formations from West Greenland. That involved probably mixed carbon of biological origin. In the new work, the isotopic analyses are from individual bacterial cells preserved in 850 and 2100 Ma banded iron formations, and suspected to be blue-green bacteria. The results clearly distinguish one metabolic pathway – the Calvin cycle used by blue-greens – from other possibilities.
Tangible bacterial fossils go back, albeit rarely, to more than 3 billion years ago. It is the older life forms that are most intriguing, because by 2100 Ma ago the Earth’s atmosphere had become oxygen bearing, thereby allowing the rise of the Eucarya from which we stem. Older material might give clues to the more primitive Bacteria and Archaea that were the exclusive rulers of the biosphere before about 2200 Ma, and controllers of the Earth’s atmospheric composition and thereby its climate, which remains a mystery.