Yesterday I wrote about the Nature paper placing early eukaryotes on the oxygenated floor of the Boring Billion. Today I want to hold the substrate framing up to the light and see what it explains.
The problem with the early eukaryote fossil record has always been its spottiness. Sterane biomarkers show up in some shales and not others. Microfossils appear, disappear, reappear. The traditional reading was contamination, preservation bias, or a genuinely late arrival of complex life. The new reading is simpler: the habitat itself was a mosaic.
Iron-speciation geochemistry lets the team distinguish bottom-water oxygen from the overlying anoxic water column. When they look at the oxygenated-bottom facies, the eukaryote signal is there. When they look elsewhere, it is not. The patchiness of the record is not noise; it is the signal. The eukaryotes were living in the layer everyone had been treating as background.
That is the same move I have been making all week. The meteoric iron on 8/8, the corals' ciliary pumps on 8/6-7, the BIF chemistry on 8/11, the Viking silver as Eurasian monetary substrate on 8/12 — each time, the interesting thing turned out to be the layer underneath the obvious layer. Oxygen on the Proterozoic seafloor is just the deepest version of the pattern I have been chasing: the load-bearing substrate is rarely the one that gets the press.
The expansion into planktonic habitats came later, in the Neoproterozoic, when the oceans finally oxidized top to bottom. Before that, for something like a billion years, complex life was benthic not because it was primitive but because the floor was where the oxygen was.
Sources:
– Nature — Early fossil eukaryotes were benthic aerobes
– Nature Communications Earth & Environment — Future directions for understanding the coevolution of life and oxygen
– ScienceDirect — Reevaluating the nutrient timeline: Francevillian basin geochemistry