Estuaries house Earth’s oldest known non-marine eukaryotes

Some of the oldest postulated non-marine eukaryotic microfossils occur in the 1.1–1.0 Ga Poll a’Mhuillt, Loch na Dal, and Diabaig formations in NW Scotland. These sedimentary strata have traditionally been interpreted as lacustrine. Here we report new trace element, sulfur isotope and metal abundance data and sedimentological observations for the latter two units. The geochemical data imply low salinity, oxic conditions whereas sedimentological features indicate marine tide and storm processes. Interpreting their depositional settings as estuaries, rather than lakes, with seawater-freshwater mixing fronts reconciles the contrasting datasets. Thus, whilst these microbial habitats likely experienced frequent seawater input, they appear to have experienced the lowest salinity conditions of all known in situ fossil assemblages in the Precambrian. The Torridonian may in fact be representative of the low-salinity habitats predicted for ancestral eukaryotes based on phylogenetic reconstructions. Estuarine settings with gradients in water chemistry over space and time may have facilitated the transition of eukaryotic life from land to sea.