I've heard that the actual reason for pork not being considered kosher is that in ancient times pork was more likely to spoil and become unsafe to eat, and that the rule against eating pork began as a more practical, secular law that eventually found its way into religious law. I was wondering if there's a similar practical origin for circumcision, or if it is purely religious.
I'm going to begin by disappointing you with this line from Montagu's Ritual Mutilation Among Primitive Peoples:
The original reason for this practice varies and is generally lost in history.
Circumcision had been taking place long before the Israelites adopted the practice. Figurines found in the Amuq Valley show three male warriors, and circumcision is evident. These figurines date from ca. 2800 B.C. The manner in which the foreskin had been removed is the same manner (there are different methods found in the ancient near east) which the Hebrews would employ a thousand years later.
Much more can be said about the who and when, but your question deals with why.
According to W.D. Dunsmuir and E.M. Gordon, the practice of circumcising males for medical reasons is a more recent phenomenon, coming into popular practice only with the advent of modern surgery. The ritual in more primitive times would have been religious/cultural in nature.
Circumcision in the Ancient Near East - Jack M. Sasson
The History of Circumcision - Dunsmuir, Gordon
History of Israelite Religion, Georg Fohrer
Ritual Mutilation Among Primitive Peoples, Montagu MF
Dr Guest discusses the overall history in this presentation including Judaism.
https://youtu.be/XwZiQyFaAs0?t=12m27s
Quick notes: It originated in many different places at the same time. The first historical evidence for circumcision is Egypt where it was practiced by the priests for as a right of passage, likely a substitute for complete castration. It spread through the middle east and was practiced for different reasons; The Amalekites used it as a sign of degradation on their prisoners of war, the Israelites used it as a tribal markings starting 500 BCE to unify the Jewish people (more in video). The Greeks viewed circumcision as barbaric as they viewed the body as perfect. The Romans adopted this view and often decreed circumcision illegal. To attract more converts Christianity did not require circumcision. It did not exist in China, the other great power at the time.
The presentation continues on to the Middle Ages.