Answer
The magnetic field of the Earth does protect us from
fast-moving charged particles streaming from the Sun,
but so does the atmosphere. It is not clear whether or
not the radiation that would make it to the Earth’s
surface during a polarity transition, when the
magnetic field is relatively weak, is sufficient to
affect evolution,
either
directly or indirectly, and cause extinctions, such as
that of the dinosaurs. But it seems that the radiation
is probably insufficient. This conclusion is supported
by the fact that reversals happen rather frequently,
every million years or so, compared to the occurrence
of mass extinctions, every hundred million years or
so. In other words, many reversals and, in fact, most
reversals, appear to be of no consequence for
extinctions.
