There is indirect evidence of an extensive land biota starting ~800 -850 Ma. This includes:
1. The clay example I started class with—implies the fixation of enough organic carbon to form humic acids; these weather feldspars and other minerals into clay.
a. But this claim has recently been challenged by the argument that the particular kind of clays could also be generated by abiotic processes; not humic acids.
2. δ13C in nearshore organic carbon suggesting fixation of carbon by land plants;
a. However the isotope evidence is also consistent with freshwater algae or intertidal/esturine algae; not necessarily land plants, per se.
3. Still another argument concerns meandering rivers—here the claim is that you cannot make meandering rivers until there is enough plant cover to hold the soil in the floodplain effectively;
a. the counter argument is that there are meandering systems in abiotic places, like Mars and in the deep sea (like the La Jolla Canyon); also it is not clear that algae, without roots, really could hold soil enough to make meandering river systems.