Now, I am certainly no phraseologist, ..., well, maybe I am somewhat of an armchair phraseologist, in that occasionally I will speak or hear some colloquialism or another and think to myself something along the lines of, "I wonder who first said that, why, and exactly what it originally meant?" And usually it turns out to be from either Shakespeare or The Bible, but I still usually use Google to help me track these things down.
In the case of "scum of the earth", as far as I can tell, it's a Bible one. Well, sort of, anyway... The phrase "slime of the earth" appears in Genesis 2, verse 7, but only in the Douay-Rheims translation of the Bible, which, as many people who profess to know about that sort of thing would point out, is neither terribly accurate nor modern. In pretty much every other translation, in place of "slime of the earth" appears "dust of the ground", as in, "And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul."
So, to sum up, the phrase probably entered into the English language in 1609 (which was when the Douay-Rheims translation was first published), at some later point "scum" replaced "slime", because let's face it, "scum of the earth" just rolls off the tongue more nicely than "slime of the earth", but since it really means "dust of the earth" anyway, can probably be expressed as the roughly-equivalent "dirtbag".