The Nemesis idea came about from the combination of apparent periodicity of mass extinctions and the Alvarez model. David Raup was one of the early participants, and has a couple of books about Nemesis and mass extinction. Richard Muller here:
http://www.freerepublic.com/focus/news/1962278/posts?page=46#46
The Nemesis Affair:
A Story of the Death of Dinosaurs
and the Ways of Science
by David M. Raup
1st ed paperbackExtinction:
Bad Genes or Bad Luck?
by David M. Raup
hardcover
import paperback
interesting stuff- the orginal conversation doc seems to have disappeared
As for periodicity- at least one of the sources I’ve been reading speculates that when “planet X” ( or whatever) passes through our solar system, it pulls asteroids and comets in with it, and the earth is then buffeted for a period of years by the junk- about 7 years of “tribulation”? Possibilities of follow on collisions