Viviane the test would be applied to all ancestors of every human living today and the ancestors of last X(10,000) years. Since you would test all you would find the first human not just A human.
Why? You would also have to test every single human alive today, and by the time you were done, there would be more people alive to test. And by the time you captured all the new humans, you would have more yet again. And you are completely ignoring the scenario I proposed.
Thought experiments still have to be practical and account for various scenarios, such as, what if you find 1K humans from the same time period of 30K years ago that every modern human could mate with, you still have a population!