  An Earth-like planetary magnetic field has been widely invoked as a
requirement for habitability as it purportedly mitigates the fluxes of ionizing
radiation reaching the surface and the escape of neutrals and ions from the
atmosphere. Recent paleomagnetic evidence indicates that the nucleation of
Earth's inner core, followed perhaps by an increase in geomagnetic field
strength, might have occurred close to the Ediacaran period. Motivated by this
putative discovery, we explore the ensuing ramifications from the growth or
reversals of Earth's dynamo. By reviewing and synthesizing emerging
quantitative models, it is proposed that neither the biological radiation dose
rates nor the atmospheric escape rates would vary by more than a factor of
$\sim 2$ under these circumstances. Hence, we suggest that hypotheses seeking
to explain the Cambrian radiation or mass extinctions via changes in Earth's
magnetic field intensity are potentially unlikely. We also briefly discuss how
variations in the planetary magnetic field may have impacted early Mars and
could influence exoplanets orbiting M-dwarfs.
