In case it wasn’t clear: many of today’s kosher laws are based on oral traditions that have developed over many hundreds of years.

For example, the basic prohibition on mixing meat and milk is extrapolated from Exodus 34: “...you shall not boil a kid in its mother’s milk.” This rather vague passage was later interpreted (or, at least relayed), first by Hebrew priests and later by Talmudic rabbis, as representing the complex body of rules governing the separation of dairy and meat that religious Jews observe today.

Thus it might be more accurate to say that the rules practiced today might be adaptations of original practice and not rooted in any particular beliefs or practices that existed before the writing of the Torah.

Putting that aside, I don’t think anyone has claimed a specific source that reliably indicates a pre-Torah justification. Rabbi Samson Raphael Hirsch thought that some animals were considered clean/religiously pious and some unclean/religiously evil. Certain Hasidic Jews share a similar view - that some animals can be considered holy and some not. Kosher laws may have been adapted from existing pagan beliefs about certain animals having “good” or “bad” spirits.

Maimonides thought the rules were related to health concerns (in Moreh Nevuchim). It may be possible that existing beliefs about health/safety of certain foods and combinations led to the kosher laws (although Maimonides would have ascribed the rules to God’s wisdom, not existing human practices). This is plainly spelled out in the (much later) Kitzur Shulchan Aruch, where certain foods and practices are frankly described as being dangerous to one’s medical or spiritual health.

But the majority of these complex rules were developed later, over centuries. It would probably be difficult to trace the real origins of each particular rule.