If evolution is an ongoing process, and all living things start at the same point, then by now there should be not only sentient humans, but fish, birds and reptiles, etc. Why would just one of the myriad species evolve to a superior position, and not the others......
Your definition of "superior" is biased. Intelligence is not necessarily an advantage. If deer devoted a higher percentage of their bodymass to their brains (and became smarter), then a lower percentage of their bodymass would be devoted to muscle and digestion.
That may not seem like a big deal, but a smart deer that can't outrun a wolf is dinner. Moreover, a smart deer that doesn't have a gut complex enough to digest cellulose isn't going to survive. If you look, you'll see that extant species have evolved to reach "superior positions" -- biologists call that finding a niche. But that niche can take any number of forms.
And if early humans evolved from apes, why are there still apes? Why did they not evolve even slightly over the millions of years it supposedly took humans to evolve?
"If evolution is an ongoing process, and all living things start at the same point, then by now there should be not only sentient humans, but fish, birds and reptiles, etc. Why would just one of the myriad species evolve to a superior position, and not the others......"
I will try to explain the conventional knowlege through a hypothetical.
If you take 1 species (Population A) and put half of that species in a different environment (with a lot of time) the population in a new environment (population B) will either die out or change. If this change is to such a degree that population A can not make viable offspring with population B, you now have two different species.
The two primary conditions are isolation between the populations, and a different environment for each population. Both of which occur on earth.