This has little to do with what is "human" and what isn't. "Father of all humankind" is just a figure of speech, not any scientific assertion about this charact

This has little to do with what is "human" and what isn't. "Father of all humankind" is just a figure of speech, not any scientific assertion about this character. Ignore species distinctions for a moment. Just tihnk about all the organisms that begat the people alive on earth today (that is, all their parents), and then the parents of those, and so on back. Eventually, you will find a organism that begat every person alive today in a direct line. The question is how far back in history you have to go to find such a person/thing. Note that there will be many such people (and this person was by no means the only person alive at the time, or anything like that -- just one of many, who had several children, who all had several children, who eventually interbred with all other humans). In fact, that person's parents are also the common parents of all mankind, and so on, so this isn't going to be a unique or special distinction; we're just curious about the most recent.