Big enough that that's a totally reasonable question! I don't think I can even put a number on it very confidently, but maybe something like 1.0 -0.7/+2.0 Ga?

Big enough that that's a totally reasonable question! I don't think I can even put a number on it very confidently, but maybe something like 1.0 -0.7/+2.0 Ga? One counterpoint that comes to mind is that the Ediacaran fauna is usually though to be marine, and water is a pretty good shield of radiation. There is one geologist who argues the ediacarans were terrestrial (Greg Retallack), but he interprets any sedimentary rock that's red as a paleosol, which is actually not completely silly, but is a bit more liberal than most others would be comfortable with. In any case, I don't know, but it's not impossible! edit: I should also say that the generation of Earth's magnetic field in the liquid outer core is a bunch of incredibly messy magnetohydrodynamics that is hard to simulate accurately, but we can probably have a pretty reasonable magnetic field with a purely liquid core (no solid inner core at all) -- it's the convection of the liquid outer core that provides the energy to sustain the magnetic field, so it's actually once the core becomes fully solid that we'll be in trouble