For vertebrae it started with fish. Four fins, head and tail.

A good book about the line of fish to vertebrate is Your Inner Fish by Shubin

For insects it started earlier.

It probably originated simply from practical reasons... You want your head at the front to sense where you're going, and you want waste behind you so you don't run into it/eat it again. The limbs on the sides therefore is the best spots (of course there are exceptions to this body plan... Sponges, jellyfish, etc)