It came to us from the Romans, who adopted many letters in it from the Greeks, in the sixth century B.C. The Greeks got their alphabet in the main from the Phoenicians, who had adapted the Egyptian and other methods of writing.
The letters are commonly supposed to be simplified pictures of objects, the names of which began with the sounds indicated by the various letters.
But it is also held that the very early scribes, when trying to find symbols that would suggest sounds, took those in which the hand most closely followed the movements of the tongue and lips. To take one example: when one makes the sound "oh," the mouth naturally becomes more or less circular, and O is the symbol which suggests this.