FWIW ...
Senate Rules from 1789 to 1806 permitted calling the question with a simple majority. See http://rules.senate.gov/history.html, Rule IX. This rule was removed in 1806, and in its place was a requirement to obtain unanimous consent to move to the vote. One objecting Senator could stifle the vote.
The cloture rule was implemented in 1917, on a bipartisan 76-3 vote. (p226). With the concurrance of 2/3rds of the Senators voting, debate would be limited and taking the vote would be set for a time certain.
In 1949, on a 63-23 vote, the threshold was modified to 2/3rds of the Senators duly chosen and sworn. (p229).
In 1959, a 77-22 vote made cloture possible with 2/3rds of the Senators present and voting. (p247). Also, cloture was broaded to include rules changes - this is where the "2/3rds are required to change the rules" rule comes from. The 1959 changes are referred to as the "Johnson 9LBJ) Compromise."
In 1975, Senator Pearson introduced a proposal to change the threshold to 3/5ths of Senators present and voting. (p257). That proposal did not pass. In the same year, Senator Byrd's proposed revision to 3/5ths of all Senators passed on a 56-27 vote. (p259).
http://www.law.harvard.edu/students/orgs/jlpp/Gold_Gupta_JLPP_article.pdf