Fish or Fowl

One of the most conspicuous aspects of the Kosher rules is the prohibition against mixing meat and milk. The source is a single cryptic statement, repeated three times in the Torah -
"Don't cook a kid (goat) in its mothers milk" (Exodus 3:19 and 34:26, Deuteronomy 14:21). The modern understanding is to forbid the Israelites from emulating the ancient pagan fertility ritual that consisted of literally boiling a kid goat in it's own mother's milk. This may be compared to other specific injunctions against local superstitions, for example, the necromancy of "ov and yedoni" et cetera. Of course, if this was the extent of the prohabition as it was practiced today, it would hardly be noticible at all, since few contemperary farmers (at least, in Western countries) are trying increase thier crop yields with animal sacrifice. However, today's Orthodox Jews scrupulously avoid cheeseburgers, own at least two complete sets of dishes, and partronize separate resturants for meat and milk. So how did this happen?

First of all, the Torah prohabition was intrepreted much more broadly by the Talmudic Rabbis:
*Not only a "kid," that is, a young goat or sheep, but any mammal
*Not just it's mother's milk, but the milk of any animal (even of another species)
*Not just for ritual purposes, but for any reason
*Written three times means "Don't cook it, Don't eat it if they were cooked together , Don't benefit from it" (Notice that these are not disjoint catagories!)

So far, this was taken by traditional commentators to be the "original meaning" of the verse. During the Talmudic period, a large number of additonal "Fences" were added as Rabbic Enactments:

*Not only mammals, but even fowl (which don't give milk)
*Any mixture of milk and meat, even if not cooked at all
*Must have separate pots, dishes, and utensils for milk and meat
*Must wait some time period (1,3,5, or 6 hours) after eating meat before eating milk

So a large part of the Kosher rules are based enitrely on one statement, its expansive interpretation, and hugely expansive added safeguards, when a simple reading seems to imply that one one particular pagan practice was to be outlawed.

Why did this happen? Why so many protections for one rule, compared to almost any other commandment? This may relate to the whole point of food taboos in general, an enforced separation from other groups. Sociallizing with outsiders is severely limited if eating together is not possible. This is explicitly stated when it comes to enactments like Bishuil Achum and Stam Yanam, and the result is the same for other Kosher rules).
Another explanation for why it seemed plausible to have so many safeguards is that avoiding "tainted" or even questionable food is very adaptive from an evolutionary perspective, so humans are automatically conscious of the possibility of contamination.

For more see wikipedia article