Actually tapioca is a starch. It comes from the casava or yuca root. The mayans were the first people who figured out how to eat it. The casava contains lots of cyanide which they extracted as poison for darts, leaving the root to be used as a thickener. The root is ground into a flour and then forced through a seive to make the pearls. Large bubbles for bubble tea just use a larger mesh than small pearls. The starch is also used in making paper and textiles.

For its use as a poisoning agent in paper read Umberto Eco's Foucaults Pendulum. Except that gives away the ending so disregard that part.