From the online etymology dictionary (boldface mine):
c. 1400, "backbone," later "thornlike part" (early 15c.),
from Old French espine "thorn, prickle; backbone, spine" (12c., Modern French épine),
from Latin spina "backbone," originally "thorn, prickle" (figuratively, in plural, "difficulties, perplexities"),
from PIE *spe-ina-, from root *spei- "sharp point" (see spike (n.1)).
Meaning "the back of a book" is first attested 1922.
I do not comprehend the underlying semantic notions for the entitled question; the human spine does not resemble or feel like a “thorn, prickle”.