The history of chess can be traced back nearly 1500 years, although the earliest origins are uncertain. The earliest predecessor of the game probably originated in India, before the 6th century AD.

Chess evolved from an ancient Hindu game of chance called “caturanga,” which was played using carved ivory or teak game pieces as well as dice. Originally, the game pieces included a king, his counsellor or minister (later his queen), two elephants, two chariots or boats, two horses or camels, and eight foot soldiers. These represented the four traditional Indian army units: elephants, cavalry, chariots, and foot soldiers.

As the game spread west though the Islamic world to Persia and then Europe and China, the dice disappeared, the elephants became bishops, the chariots became rooks, the horses became knights, and the foot soldiers turned into pawns.

It’s not exactly clear why the elephants turned into bishops, although some scholars think it may have happened after the more abstract Persian chess pieces included bumps to symbolize tusks that may have reminded some people of a bishop’s miter.

Possibly it was simply because the Bishop was easier to carve.

More Info: en.wikipedia.org