SOME THOUGHTS ON BIOMARKERS, AEROBIC RESPIRATION, AND THE NATURE OF EARLY EUKARYOTES

The lack of steranes in Mesoproterozoic rocks—despite a body fossil record of total group eukaryotes extending to 1.65 Ga—has been interpreted to reflect limited abundance of early eukaryotes due to restriction of favorable habitats. Recent studies of fossiliferous Mesoproterozoic units, however, show that even in those environments most likely to be hospitable to eukaryotes, biomarker assemblages lack steranes. This suggests another possible explanation for the sterane–body fossil mismatch: early eukaryotes had not yet evolved the capacity for sterol synthesis. Evidence suggests this capacity was present in the last common ancestor of living eukaryotes (LECA) but when it was acquired in the eukaryotic stem group is not clear. If sterol synthesis appeared late in stem group evolution, then steranes should become common in the fossil record only when members of the eukaryotic crown group become common, and both molecular clocks and body fossils permit a scenario in which this didn’t happen until early Neoproterozoic time.

A similar scenario could apply to the evolution of aerobic respiration (via mitochondria acquisition) in eukaryotes. Though it is clear LECA possessed mitochondria, it is not clear when in stem group evolution they were acquired. If they appeared late in stem group evolution, then—as with steranes—we might not expect to find abundant evidence for aerobic eukaryotes until the crown group becomes widespread. Several studies of Mesoproterozoic units have found lower diversity of eukaryote body fossils in samples from oxygenated environments than in those from environments with dominantly anoxic water columns. Though there are too few studies to make firm conclusions, in at least some this pattern does not appear to reflect differences in preservation, and thus could indicate that most or all Mesoproterozoic eukaryotes were primitively amitochondriate. More broadly, eukaryotic body fossils, steranes, and occurrences in different redox habitats are signals of different crown group characters (e.g., resistant-walled cysts, sterols, and mitochondria) that may have appeared at different times in stem-group eukaryote evolution. In combination, these records can be used to constrain not only the timing of crown group diversification, but also the relative order in which these characters evolved.