MAGNETIC RADIATION SHIELD. “Atmospheric oxygen is known to have been increasing in the relevant era but this is poorly quantified. A newly-formed, weak dipolar magnetic field originating about 600 m.y. ago, strengthening at about 550 m.y., could plausibly have shielded against ionized radiation, enabling the Ediacarian start and Cambrian acceleration of multicellular evolution.”
https://www.sciencedirect.com/science/article/pii/S0012825219302636?via=ihub