For a fossil hunter, there's not much glamour in the Precambrian era. Beginning with the birth of our planet and ending about 570 million years ago, this huge period of time bequeaths no dinosaur bones, nor even the corrugated imprint of an early fish. For all but the latest of Precambrian times, the fossil record offers little more than shapeless microscopic blobs, the remains of single-celled algae and bacteria.

But these smears of ancient life contain an amazing amount of information, a report in the journal Geology1 now shows. Christopher House of the University of California at Los Angeles and colleagues have deduced how fossilized Precambrian bacteria digested their 'food'.

If we are what we eat, then so too are bacteria. Some organisms, like humans, make their carbon-based molecules by breaking down food. Others, including plants and so-called autotrophic bacteria, build these molecules from carbon dioxide in the environment. Creating these carbon-based building blocks is part of metabolism, from which cells also derive energy.

Many Precambrian microfossils are autotrophic bacteria with diverse metabolic chemical processes found in many different evolutionary groups. Identifying these differences could provide vital clues for researchers striving to assign Precambrian microfossils to different groups (this is currently done by cell shape).

House's team use 'carbon isotopes' to help tell bacteria fossils apart. Natural carbon comes almost entirely in two forms (isotopes): carbon-12 and carbon-13. The isotopes are almost identical but have slightly different masses, so some biochemical reactions prefer one or the other. Carbon molecules churned out by metabolism can have a different proportion of isotopes to raw materials used. This gives fossilised bacterial cells an 'isotope signature'.

But individual microfossils do not contain enough isotope material to measure: so, until now, researchers determined an average for many fossils in a rock sample.

House's group now shows that accurate isotope measurements can be taken from single microfossils, using a standard 'ion microprobe'. The researchers studied autotrophic bacterial microfossils from two Precambrian rock strata -- an Australian deposit 850 million years old, and a Canadian formation 2,100 million years old. The isotope levels in these organisms matches that expected of the so-called 'Calvin cycle', a sequence of reactions featuring in human metabolism (but cycling in the other direction). The measurements ruled out two alternative metabolic pathways.

Finding evidence of the Calvin cycle strengthens the idea that these microfossils belong to the 'cyanobacteria' group, which are among the oldest of all known fossils. This association was already suggested on the basis of cell shape; but the new results demonstrate that such judgements no longer have to be made by eye alone.