# [GolfScript], 24 bytes
A horribly long answer... golfed out 1 byte by using a hard-to-read output format

    ~[1]*{..2/{{+}*}%\,(}do;

[Try it online!][TIO-k57xqscq]

[GolfScript]: http://www.golfscript.com/golfscript/
[TIO-k57xqscq]: https://tio.run/##HcyxCsIwFIXhPU9xoAhSS4o6ugsuOjpohxBjE6xJSG8tEuKrx@hZ/4/Tu@E@ymA85fy5rLs6cr5pY1ylOi2uzTLd3C5XOLoGB4gnrCNQeBvbgxykVoKgVVCcc9ahwjkID9IKIwn5gLFFCQxmJBYZyjyKOk3kJ8JsSJdq1TwYq1hqf23vghJSw71U@D8ZW2zefgE "GolfScript – Try It Online"
## Explanation
```
~                        // Dump the contents of the input string
 [1]*                    // Create a 1-list with the length of the input string
     {              }do  // do ... while
                 \,(     // the length of the array is larger than 1
      .                  // Extra evolution step that we need to keep
       .                 // Create a copy of the input
        2/               // That splits into parts of 2 items
          {    }%        // For each over the splitted array:
           {+}*          // Reduce the item with addition
                         // e.g. [1] -> [1], [1 2] -> [3], etc.
                       ; // Discard the abundant copy
```