There are only three places on Earth with sedimentary rocks older than 3,300 million years: the greenstone belts of Isua in SW Greenland, the Barberton area east of South Africa, and the Pilbara area of NW Australia. Archean morphological fossils occurring in 3.3-3.4 Ga rocks from South Africa. The isotopic signatures of the organic carbon from Greenland bring indirect evidence that life may be 3.85 billion years old.
The carbon atom has two stable isotopes, carbon 12 and carbon 13. The 12C/13C ratio in abiotic mineral compounds is 89. In biological material, photosynthesis raises the ratio to about 92, indicating that living matter is enriched in 12C. The carbon isotopic composition of over 1,600 samples of fossil kerogen, a complex organic macromolecule produced from the debris of biological matter) has been compared with carbonates in the same sedimentary rocks. Those data showed that biosynthesis by photosynthetic organisms was involved in all the sediments studied.
In fact, this offset is now taken to be one of the most powerful indications that life on Earth was active nearly 3.9 Ga ago, because the sample suite encompasses specimens right across the geologic time scale. Although the origins of life were probably geothermal and chemo-synthetic, derivatives of photosynthetic and UV-protective pigments have been found in ancient sediments up to 3.8-3.5 Ga.
These biomolecules can be detected in situ by Raman spectroscopy. Some organic matter in ancient sediments has been measured as being even more enriched in the light isotope of carbon, which would suggest involvement of methane utilizing organisms.
Fig.: The photos show a stone with the earliest indication of life on Earth.