I haven't read the whole thread besides this opening post, so I haven't seen what has already been discussed already. Excuse me if I repeated something that someone has already said.
As a biochemistry/microbiology major, I have a pretty strong background in this area. First, evolution is a FACT. This cannot be argued. Evolution is the change in inherited traits of a population through successive generations. The changes occur from a variety of things, such as mutations and genetic recombination. The changes that that stay today are the ones that are especially helpful for the organism to survive. Again, this is a FACT. You can easily see this with the changes seen in pathogenic microorganisms. Because they reproduce in a fast rate compared to humans, there are a lot of changes in relatively short period of time. This is why many of the drugs that worked before against certain microorganism are no longer useful, such as penicillin. In other words,the microorganisms have evolved with drug-resistance. With the luck of the draw, some microorganisms have genes that allow them to escape harm from drugs. These genes are then selected for because the other microorganisms cannot survive as well under the new environment where drugs are used.
The only argument you can make about evolution is that humans did not evolve from another species. In other words, humans has it's own evolutionary path completely separate from the other millions of species out there today. Of course, I don't believe this, but this is the only argument I can see against the evolution of humans. Really, the only argument creationist have is that God created a world in which evolution exists, instead of it arising spontaneously.
About your ape question and why they are still here, speciation (the evolutionary process by which new biological species arise) does not require the original species to die out for the new species to arise. For example with
Staph infections, there are new stands of it nowadays that are resistant to methicillin. These are called the methicillin-resistant staphylococcus. So there's a new strand, but the old strand of staphylococcus aureus still exists today. I know this isn't a perfect example because we're talking about different strands and not species. But the same principal applies.
If you have any questions, let me know. I couldn't go to much detail now because I have class early in the morning.