h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think the fish symbol also has its origins in sacred geometry, as it is a stylised section of the twin interlinked circles of the 'vesica piscis', another christian symbol.
The vesica is the basis of all sacred geometry forms. All the basic 2D Platonic forms (triangle, square, hexagon etc.)can be generated from the vesica using nothing more than compasses and straightedge.
This technique was certainly used by Pythagoras and his followers, but is undoubtedly older than that as the Vesica form can be seen in the geometry of the ancient Egyptians where is is very predominant in the layout of the Great Pyramid.
Sacred Geometry using the Vesica perhaps reached its peak in the design of the great gothic cathedrals of Europe.

~Enki