Across many religious traditions are similar symbols or depictions of straight staves (the plural of staff!!), poles, stakes, crosses, and the like. This includes the Christian cross, Kundalini rising, totem poles, and any other straight, vertically-oriented line, object, or religious fetish.
Some interpret this as symbolic of something that comes from and relates to the body. In fact, there is an entire school of thought often associated with George Lackoff and other cognitive scientists, that suggests nearly all knowledge is often just extended from body-knowledge. For example, "10" is an important number in math, simply because we have 10 fingers and toes!
So these vertical lines we see in religious symbol systems is thought to be represent the importance of a straight spine, which is maintained or achieved through sitting postures or meditation.
An ancient piece dug up from Harrapa and Mohenjo-Daro from the ancient Indus Valley Civilization (one of the oldest known), depicts a figure in what looks like a full lotus posture (asana).
Interestingly, as ancient as many of these are, they are decidedly different from religious fetishes from a more goddess-oriented religion, which depicts round shapes as in the Divine Yoni or rotund female figures, often featuring prominent and exaggerated genitalia.
So, this phallic symbolism seem to be associated with male-dominated civilizations, whereas, round shapes seemed more associated with female-centered societies that dominated our earliest human history.
I can only speculate, perhaps the deviating spine is symptomatic of increasingly complex and stratified societies that sees greater and greater amounts of sedentary and isolated cultural behaviors. Perhaps increased alienation leads to increasingly twisted spines (and minds) that is more typically of male-dominated heirarchical socieites, thus making straight spines an important enough issue for it to be permanently embedded into its symbology...