BY LOOKING at carbon isotope ratios in 2.1 billion-year-old fossil traces,
researchers have been able to show they made their living by photosynthesis.
Because similar looking microbes can be very different, identifying fossils
by their appearance is tricky, says William Schopf of the University of
California at Los Angeles. But bacteria that use different metabolic pathways
end up with different ratios of carbon-12 to carbon-13. Of the 30 microfossils
Schopf studied, all but 2 had carbon-13 levels in the same range as modern
photosynthetic cyanobacteria (Geology, vol 28, p 707).
Schopf now wants to try the technique on 3.5…
