Rank structure in Western armies comes from the old class system in Europe. Centuries ago aristocrats were the officers, lower class guys with a head on their shoulders became sergeants (a word which once had the connation of "servant"), and the rest were enlisted. The word infantry has the same origin as infant, because the "better" classes way back then were mounted.
In spite of what the Marxists might want to say, I don't believe that during the last 300 years these class lines were as rigid within societies or military organizations as they'd have us believe. Incompetent officers fall by the wayside, talented NCOs move into the officer corps, good enlisted guys become NCOs, and so forth.
Other factors are at play too -- in a way Robert E. Lee was an aristocrat on a downward slide until he married because his family fortune was gone; though his social and financial status had declined I doubt anyone would criticize his abilities as an officer. Occasionally the fallen nobility make the best officers because they are determined to redeem lost honor.
To get back to the topic of this thread, today the minimum for being an officer is having a college degree, to an extent having the "good family" thing and having strong letters of recommendation with your application. The system isn't rigid though, and guys of modest origins can and do get in and rise to high places.
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