Stable+carbon+12+isotopes+evidence+for+the+first+presence+of+life

**Outline stable isotope evidence for the first presence of life in 3.8 x 109 year-old rocks**
 * Stable Carbon 12 isotopes evidence for the first presence of life**
 * Known examples of rocks older than 3 500 million years have experienced intense metamorphism. This would have obliterated any fragile microfossils they might have contained.
 * Measurement of the proportion of the stable isotopes of organic carbon and carbonate preserved in a rock can be used to indicate if the rock was composed of material produced by living organisms. During photosynthesis, cells preferentially build carbon-12 atoms into their tissues, leaving carbon-13 to accumulate in the environment. So carbonates that come from living things should have a higher concentration of carbon-12. The result is that the carbon in living cells and the carbon in the sedimentary carbonate have isotopic compositions that can be matched.
 * Using this method evidence for life dating to 3800 million years has been found in rocks from Greenland, Australia and South Africa.

Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P. Friend, C.R.L. (1996) //Evidence for life on Earth before 3,800 million years ago,// Nature 384, (6604): 55-59 (see below)

Evidence for life on Earth before 3,800 million years ago

**Abstract** IT is unknown when life first appeared on Earth. The earliest known microfossils (˜3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued1,2. But the known examples of crustal rocks older than ˜3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measure-ments of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences—a ˜3,800-Myr-old banded iron formation from the Isua supracrustal belt, West Greenland35, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr (ref. 3). The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present. S. J. Mojzsis*, G. Arrhenius*, K. D. McKeegan†, T. M. Harrison†, A. P. Nutman‡ & C. R. L. Friend§