Abstract
The criticism by Pasteris and Wopenka of our use of laser–Raman imagery to investigate the carbonaceous make-up of extremely ancient fossils1 focuses only on their Raman signature; however, our interpretation that the carbonaceous matter that makes up these specimens is biogenic is based on several lines of evidence, of which Raman spectroscopy is only one.
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Schopf et al. reply
We did not state, nor did we imply, that Raman spectroscopic analysis can by itself be used to establish the biological origin of geochemically highly altered carbonaceous matter present in ancient sediments. We believe that the biogenicity of such matter, whether in fossil-like objects or sapropel-like detritus, should be demonstrated by a combination of data drawn from independent but mutually reinforcing lines of evidence.
For fossils in each of the four geological units we analysed1 — including those of the roughly 3,375-million-year (Myr)-old Kromberg Formation and 3,465-Myr-old Apex Chert, which are among the oldest fossils known — three lines of evidence are most compelling. These are their cellular morphology2,3, their carbonaceous molecular-structural make-up1,2,3,4, and the carbon isotope composition of such fossils5 and/or of co-existing particulate kerogen6,7, which have been shown by replicate analyses5,6,7 to be well within the range established for Precambrian biological organic matter on the basis of over 1,200 measurements from hundreds of fossil-bearing units7.
Our study1, which focuses on the first two of these lines of evidence, is centred on the use of laser–Raman imagery (rather than on more conventional single-point measurements), a technique new to palaeobiology1,4. We showed that there is a one-to-one correlation of cellular morphology and carbonaceous make-up in individual microscopic fossils from each of the four units investigated. Our claim is that such an analysis based on a combination of morphology and chemistry together provides a powerful new means to investigate the biogenicity of putative fossil-like objects, a problem that for many decades has plagued the search for evidence of early life8.
References
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Schopf, J., Kudryavtsev, A., Agresti, D. et al. Images of the Earth's earliest fossils?. Nature 420, 477 (2002). https://doi.org/10.1038/420477a
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DOI: https://doi.org/10.1038/420477a
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