Experimental taphonomy of organelles and the fossil record of early eukaryote evolution

The timing of origin of eukaryotes and the sequence of eukaryogenesis are poorly constrained because their fossil record is difficult to interpret. Claims of fossilized organelles have been discounted on the unsubstantiated perception that they decay too quickly for fossilization. We experimentally characterised the pattern and timescale of decay of nuclei, chloroplasts and pyrenoids in red and green algae, demonstrating that they persist for many weeks post-mortem as physical substrates available for preservation, a timescale consistent with the known mechanisms of fossilization. Chloroplasts exhibit greater decay resistance than nuclei; pyrenoids are unlikely to be preserved but their presence could be inferred from spaces within fossil chloroplasts. Our results are compatible with differential organelle preservation in seed plants. Claims of fossilised organelles in Proterozoic fossils can no longer be dismissed on grounds of plausibility, prompting reinterpretation of the early eukaryote fossil record and the prospect of a fossil record of eukaryogenesis.