The study of chess, its origins and evolution, can be considered from many
angles. Let me offer an argument for the game of Chaturanga having
primacy:
http://www.goddesschess.com/chessays/Alex$20Kraaijeveld.pdf
Dr A Kraaijeveld has developed a phylogenetic approach to the origins and
relationships questions that uses internal evidence to set up a series of
evolutionary trees, each radiating from a different ancestor, to illuminate
the relationships among games. His findings strongly indicate Chaturanga is
much more likely the ancestor of all chess forms than any other game,
including proto-Xiangqi.
Prof. Kraaijeveld has written 3 papers on various aspects of chess
phylogeny. SOmewhere, I hve an email with links. Should anyone be
interested, I'll try to find that email. The key paper [in my estimation]
however, is this one.