About 2.4 billion years ago the Great Oxygenation Event began and, for the next 600 million years, oxygen from photosynthetic blue-green algae entered the oceans and the atmosphere. Life on Earth then had to adapt to the then poisonous effects of free oxygen. Some organisms persisted even to this day in compartments where oxygen does not reach. Others had to evolve to withstand and eventually to exploit the new conditions.
One problematic condition facing life was the disappearance of soluble iron as the soluble ferrous form was driven to the insoluble ferric form as oxygen entered the oceans. Iron then precipitated out and formed the banded iron formations that supply much of modern iron ores. Iron, which before was plentiful, suddenly became scarce and, even to this day, algae cannot thrive in the central oceans for lack of it. Near shore where iron is available from land runoff and from cycling of shallow sediments photosynthetic life persisted, principally by evolving the protein ferritin so that iron could be stored in a compact mineral form in the cell. 24 ferritin molecules interact to form a hollow sphere into which ferritin precipitates iron atoms by reversibly removing an electron to form the ferric form from the soluble ferrous form. A single sphere can then store up to 4500 iron atoms. So successful was this approach that ferritin has remained structurally identical in nearly every organism on Earth for 2 billion years.
These filled ferritin spheres are ferromagnetic and could then have served as the basis for sensors of the Earth's magnetic field. Also because of their density they could serve as the basis for sensors of gravity and motion as well. Perhaps, thanks to ferritin, cells could then evolve to sense their macro environment. Motile cells could then move with deliberation and animal life could become possible.