It works something like when you look at clouds and think you see something. The first painting on a cave wall was probably made because some shaman thought he saw something that looked like a horse in a smudge on the wall. A good example of this mentality is also here;

The Venus of Willendorf, it is the most famous of the many fertility figures. It has a roundness that resembles an egg-shaped "sacred pebble". Her navel, the center point of the design is actually a natural indentation in the rock, and not a carved feature.

There’s a lot of debate about the old girl. Some believe that there is not enough evidence to suggest that she is in fact a fertility figure. It cannot be denied that it is a possibility however. Mother figures are common symbols for many of what a society, especially at these stages needs. Birth, nurturing, they represent fertile fields. This ties in to the next piece: