In 1060, the Danish invaded England, which started
a war. In the early 11th century, if somebody was walking along and
they turned up a skull from the invasion, they would start kicking it
around, to start the game of futballe. Later, America changed the
rules and name to football.
Also, other things have been used for a football like a pig's bladder blown up, rocks, and other things put into a leather hide.