Your question is a little confusing; it asks about your debt to the original authors, while suggesting you're the only original author. Nevertheless, you've written enough to make me wonder about contributors, and the licence under which they contributed.
Contributions to code under GPL generally don't cause problems. The normal workflow (contributor copies codebase, modifies codebase, then conveys modifications back to the original project) is enough to ensure that a contribution can only be lawfully made if it's made under GPL. A Contributor Licensing Agreement is often helpful, but without it there are still legs to stand on if a contributor later decides to cause trouble about the terms under which their contribution can be used.
This is not so for all the permissive free licences. I can copy the original codebase and modify it, and as long as I preserve the original copyright notices and some additional text, I can re-distribute those modifications without incurring any particular obligation with respect to the licensing of my contribution. As amon points out (to whom many thanks!) Apache2 s5 does make provision for this, but it's only advisory as an opt-out is permitted.
If you are the project leader, and you don't clarify the terms on which you are accepting my contribution, such as by using a CLA or (in the case of Apache2) having a procedure to ensure nobody's opting out of s5, then they may remain ambiguous, and I could later cause a fuss if you did something with them that I personally did not approve of.
I'm not suggesting this is common, or even likely, but if your project accepted contributions and didn't have a clear CLA in place, then to be on firmest ground it would be useful to make a good-faith effort to contact those contributors and have them clarify that their contributions were also made under Apache2.
Once it's clear the whole codebase is under Apache2, then I concur with the interpretation of my colleague D. SM.