  One of the longstanding debates in the history of paleontology focuses on the
issue of whether or not there have been long term cycles (operating over tens
of millions of years) in biodiversity and extinction. Here we consider the
history of this debate by connecting the skein from Grabau up to 2008. We focus
on the evidence for periodicity that has emerged thus far, and conclude that
there is indeed some evidence that periodicity may be real, though of course
more work is needed. We also comment on possible causal mechanisms, focusing
especially on the motion of our solar system in the Galaxy. Moreover, we
consider the reasons why some scientists have opposed periodicity over the
years. Finally, we consider the significance of this for our understanding of
evolution and the history of life.
