Variations of such questions about awareness, consciousness, and Human development come up frequently on Quora, and are probably asked at some stage in their life by anyone interested in such things as consciousness, existence, and the curiosity of Humans and other species too.
Anyone who studies the topic through The Portal will find the questions thoroughly and comprehensively answered, to the best of current Human knowledge. You’ll also find a chapter or so dealing with such important matters in my book, CK’s Wisdoms On Loving Yourself Properly. You can’t love yourself properly unless you know what it is to be Human. Human consciousness is clearly an essential aspect of that.
This is just a short response that makes the obvious points.
Questions about yourself, your awareness, other people’s awareness of you, and you own existence become inevitable the moment you have a language that includes pronouns - I, you, he, she, etc.
Any language which has the dissociated inversion of subject / object between I and you, (that is my ‘you’, becomes your ‘I’ and visa versa), automatically means one has to start asking questions about:
What am I?
What are you?
How am I not you?
Add to that a conceptual model of time, that is an awareness of a flow of time from plu-perfect, to past, to present, to future etc. and the questions such as:
Where did I come from?
What was here before me?
Am I here now?
How do I know?
become natural, and pretty much inevitable.
If by Human one means homo sapiens sapiens, which on this site we do, then it is highly likely that we were asking such questions the moment our species came to be - around 200,000+ yrs ago. It is clear that hominids had been using fire and language well before that, and homo sapiens sapiens came into existence as fire users, cooked food eaters, and talkers.
So I would expect that the very first family of humans that ever walked this Earth asked this question.
The more interesting questions are:
What creature first asked this question?
Do cetaceans ask questions about their own existence?
Can Artificial Intelligences wonder about their own existence?
and such like.
You’ll find these questions addressed in the Oracle, and my books. Just occasionally I include a teaser in The Blog too.