1) Contrary to Thomas' answer, we do know about the origins of humans, it's just that your question is worded in a way that is rooted in a misconception. If you were to trace any one person's ancestory back to the origins of life, you'd observe a gradual progression backwards from modern humans to the origin species. There was never any point at which one of our ancestors had a child that looked nothing like them. Speciation occurs very slowly, such that no living member of a species would ever be different enough from a different generation that they wouldn't be able to breed together. This is no more controversial a proposition than saying that you are just as human as your grandparents, although there doubtless exists some genetic variation between you. Thus, on our imaginary backwards review of ancestors, there's no set point at which you'd be able to say 'these are the first humans', but rather a fluid progression away from 'human'. If we could made a specimen of pre-humans with a modern human, we might perhaps be able to draw a more-or-less distinct boundary by seeing how far back we had to go to find non-reproductive pairings, but that's clearly not possible.
2) Muhammed's historical reality is more or less certain, Jesus' is fairly certain, though largely fabricated, elaborated and distorted, and Moses' is doubtful. Many of the supposed deeds, sayings and miracles of Jesus can be shown to be 'recycled' from other traditions, which would have been widely circulated in the cultural milieau at the time of Christianity's coalesence. Available historical evidence suggests that Jesus was a historical figure who is completely different that our modern conception of him, which was largely constructed by Paul of Tarsus some decades after his death. The real Jesus was probably an apocalyptic Jewish street preacher of a type common in that area of the Roman empire. As for Moses, there is no record of him or anyone like him outside Judeo-Christian sources, and almost all of the things attributed to him can be found in other religious texts predating the Torah.
3) Whatever. No one can answer that for you.
Absolutely! I don't care whether you can spell psychiatrist or not, you have my vote!