WestlawNext offers several sources that may be helpful in determining the history of federal statutes.
Statues begin as bills, before being passed and codified into law.
Look at the end of the code section for the Public Law number (abbreviated PL or Pub. L.) and the Statutes at Large number (abbreviated Stat.)
Once you know the Statutes at Large or Public Laws number, you can locate the text of the bill as passed.
Consult Statutes at Large (digitized volumes from 1951-present) arranged by Stat. and/or PL number,
or THOMAS: Bills, Resolutions (101st Congress/1989-90 to the present). Searchable by keyword, bill number, date, and more.
If legislation came out of a committee, the committee's reports may shed light on its intended goals.
Look in the 'Legislative History' section of United States Code Congressional and Administrative News (USCCAN), 5th Floor KF 48.A 5 (1980-2014). USCAAN is arranged by Congress, then by Public Law number.
Or, check THOMAS: Committee Reports (104th Congress/1995-96 to the present)
Look in the Congressional Record:
The Congressional Record is arranged by Congressional session, then by day. Indexes are published every 10 days (on 3rd floor, filed at end of section). The online version at THOMAS is searchable by keyword. See also GPO index to the Congressional Record, 1994-present.
For floor debates prior to 1873, consult the American Memory site, A Century of Lawmaking for a New Nation: U.S. Congressional Documents & Debates, which includes: