Symbolism of the footprint in Robinson Crusoe by Daniel Defoe?

The footprint symbolizes fear of the unknown. Crusoe sees the footprint and realizes that he is not alone.

It happen'd one Day about Noon going towards my Boat, I was exceedingly surpriz'd with the Print of a Man's naked Foot on the Shore, which was very plain to be seen in the Sand: I stood like one Thunder-struck, or as if I had seen an Apparition; I listen'd, I look'd round me, I could hear nothing, nor see any Thing, I went up to a rising Ground to look farther, I went up the Shore and down the Shore, but it was all one, I could see no other Impression but that one, I went to it again to see if there were any more, and to observe if it might not be my Fancy; but there was no Room for that, for there was exactly the very Print of a Foot, Toes, Heel, and every Part of a Foot; how it came thither, I knew not, nor could in the least imagine. But after innumerable fluttering Thoughts, like a Man perfectly confus'd and out of my self, I came Home to my Fortification, not feeling, as we say, the Ground I went on, but terrify'd to the last Degree, looking behind me at every two or three Steps, mistaking every Bush and Tree, and fancying every Stump at a Distance to be a Man