Until I644 the legislature [in Massachusetts] was unicameral although the assistants had what was called a ” negative voice ” in spite of the fact that they constituted a minority of the Court. Being better educated and accustomed to leadership they ordinarily monopolised the debates. The irritation which resulted came to a head in I644 following a bitter series of suits between the Goodwife Sherman, a poor widow of Boston, and one Robert Keayne, a merchant of that city, who had killed her sow when it trespassed upon his property. Three times the suit came before the General Court; three times the deputies sided with the widow and the assistants with the merchant. The deputies denied the right of the ” negative voice ” ; the clergy and many of the colonists took sides in the dispute. The upshot was vindication of the ” negative voice ” and separation into two houses of legislation. Such is the origin of the bicameral system in America!
In short, Americans have a pig to thank for forever having two houses of Congress [OK, may be eventually the South would have adopted the UK system with an upper house to protect southern aristocratic interests….]
Peverill Squire confirms this story in his excellent book on the Evolution of American Legislatures (1619-2009)
kenopalo.com 
Bit odd to see a look at American political institutional evolution. Anyway the two house model might have come about anyway with the northern aristocracy, and probably would have at the federal level to balance the tension of sparsely populated states and heavily populated states.
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