The development of the underclass has been a long-term sociological process that is a direct by-product of both technological advancement and human nature. There is a great difference between today's underclass, and the serfs who represented the lowest economic class of feudal times. We can see several of the important societal changes that have caused such regression by exploring There Are No Children Here.
The original "underclass" (which I will define simply as the least-affluent social group in any give place and time) seems to have appeared during the horticultural phase of the pre-industrial age, when humans began claiming large tracts of land as their own - either by physical force or persuasion. As a result of these large-scale claims, other humans were no longer able to own their own land; these landowners (henceforth known as the bourgeois) would offer land-less people (the proletariat) the necessities for living in exchange for work. This allowed landowners to maintain massive estates that commanded respect from other landowners, although they did not typically take advantage of their workers in an exploitative fashion (i.e., pay them less than the value of their work).
But this system continued to evolve. A system of labor division was established to create more efficient work environments - with each worker concentrating on one specific task, they could accomplish more over the course of a day. As we see in There Are No Children Here, division of labor has created a system in today's economy where job requirements have become so specific that previously sufficient amounts of education are no longer enough to guarantee employment. As an example, LaJoe's niece Dawn Anderson became one of the family's few high school graduates - a level of general knowledge that far surpasses that of any proletariat member of an agrarian culture. (pg126) The family had great hope for her, given this achievement, but it soon became obvious that this would not be enough for Dawn to fully enter the job market (pg244).