Volcanoes erupt, tectonic plates crash into each other, the magnetic north pole drifts, an ice age thaws, comets impact the earth, semi-molten planets collide, and the sun expands. The earth has lived some and changed some, with still more living and changing to come. Yet, as impressive and sometimes catastrophic as these changes seem, none is so entirely earth-shattering as the moment the earth went from being flat to being round.
The biggest physical change the earth has ever gone through was not actually physical at all.