carbon atoms. Carbon has heavier (13C) and lighter (12C) sta-
ble isotopic atoms, and this old carbon, dated to 3.85 Gya, has
the carbon-isotope rations (enriched in 12C) characteristic of
biochemistry. Even more vivid evidence is provided by stoma-
tolites, layered hassock-like structures built by
colonies of
microorganisms in shallow water and fossilized to display a
characteristic layered rock signature in their cross sections.
These can be dated by isotope measurements to 3.45 Gya. However, because these stromatolites were built by relatively
advanced orgnism it seems likely that life, at least becterial
life, had appeared toward the middle of teh first billion years
of Earth history.
This chrolonogy is consistent with apparent