Did stack overflow steal the idea from Yahoo Answers, did Yahoo steal from...
QA sites are as old as the internet and most features are known from forum sites or knowledgebases.
The only thing You could refer to on this site is the optics / the theme. There is some similarity. But due to the flexibility of the q2a theme, i would say it is of minor importance.
Everything else is based on totally different code and user input. You may have a look through the questions, where the author asks for the features the community want to have implemented.
The q2a script here can look like any other question/answer site, by trimming the theme. Like stack overflow, yahoo answers, blurtit, You name them...
For further reading I recommend one of the very first questions on this site where the author points out the most important differences in comparison to stackexchange: