Symbolism

[FTVS]

i was recently reading a website about the Dogon. On that site was a statement along the lines of "... and these 10 (somethings) are thought to symbolise the strands of human DNA..."  

Another similar statement seen on this website today "Adam and Eve are symbols for the early humans"

 

My question then is: firstly, how and who recognises certain things as symbols; and, secondly, how did those things come to be represented symbolically? 

[Kingaroo]

An artist can intend a symbol -- a rose to symbollize love, 13 stripes to symbolize the 13 colonies.
A person can assign symbolic meaning to something -- a mother may feel her caesarean scars symbolize her sacrifices for her children.
A culture can imbue symbolic meaning when everyone shares the same understanding of the symbol being used. In preliterate societies, a myth about a coyote or a raven or a sacrificial lamb would be understood as having metaphorical meaning. When everyone knows that lambs are sacrificial animals, stories about a lamb of god have clear meaning. If everyone knows bread is a fundamental food, a parable about feeding a loaf to a multitude is an easily understood symbol for sharing life and sustenance with the whole world.