If my memory is correct the original Constitution does even mention a Speaker of the House as an office. Keeping with the House of Commons Congress through its ability to make rules created the office. The Speaker is mentioned in regards to an amendment regarding Presidential succession or incapciation.
The Speaker is mentioned in the original Constitution (Article I, Section 2, Clause 5). Sections 3 and 4 of the 25th Amendment refers to the Speaker as one of two Congressional leaders who receives a written declaration regarding whether the President is disabled.