Chemical markers

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An alternative to the morphological approach, to assess the presence of fossils, is the search for chemical markers specific to eukaryotes which could be preserved in the sediments. Sterols are good candidates for this approach (see Table 1). This is what was done on very old rocks, dated to 2,700 Ma ago, which therefore date from the Archean era. Chemical analyzes have shown, without ambiguity it seems, the presence of steranes, organic compounds derived from sterols. Like morphological data from primitive fossils, this finding suggests that eukaryotes probably have a much older origin than is commonly believed. However, we have seen that some bacteria, such as Planctomycetales, can synthesize sterols and some put forward this argument to reject the early appearance hypothesis and give a more recent date for the appearance of eukaryotes.

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