Human Evolution

As someone who has dabbled into evolution for sometime I am now interested in really grasping and understanding the theory even better so I will bring up another topic of interest. Specifically: human evolution.I just have a few brief and simplistic questions to start, forgive me if they seem silly:1. Why are all other Homo's in the Genus (except us) extinct?2. As we continue to evolve will there eventually be another speciation event where a group of humans become too genetically incompatible with another? Or since we are so great in number and widespread is this no longer possible?3. Regarding human origin and ancestry specifically the relation of Humans and Neanderthals: what common ancestor do we share with them? And how long ago was the split?I have searched the internet regarding these various random questions but I was just curious if anyone here with experience in relevant fields could help me understand further. I know these are completely random questions heh.Thanks.

Thank you all for the responses so far they have been very informative and CosmicChimp, my 2nd question was the one I was the most curious about, thank you for the article. I skimmed it and when I have more time later tonight I will read it more in depth.