A good friend of mine once asked me a good question:
If humans evolved from chimpanzees, why are there still chimpanzees around today?
This causes a lot of confusion, and I completely understand why. If you think about evolution in linear terms, this doesn't make any sense. The truth is that evolution is not a goal-oriented process that is always moving forward. Evolution happens when chance mutations cause a group of organisms to change. This happens very rarely, actually. Most groups of organisms go extinct or change very little over time.
Evolution is not guaranteed. Some species have remained largely unchanged for hundreds of millions of years. Some, like us, have changed dramatically in a few million years. There are many factors that can cause a species to change, and a big one is chance mutations.
Without getting bogged down in all the details, here is how it went:
A few millions years ago there were several types of early primates. An individual in one of these groups, let's call her Jane, was born with a mutation that caused her brain to be slightly larger, so she was slightly more intelligent. Jane survived longer and had more children than some of her non-so-smart friends. Her children inherited her successful traits and grew up to have many children as well. You can see that over time, just one mutation can increase in a population if it causes those individuals to survive and have more kids than others. Remember that survival and having babies is not guaranteed in nature. So those that don't have "good" traits will not survive or will not reproduce and pass those traits on.
After many more generations of the smartest primates having many children, you now have a whole population of smarter primates. The group of primates on the other side of the hill didn't experience this smart mutation, so they stayed the same during all of this.
This is essentially how we have ended up with humans evolving while other primate species stay the same. We say that they are our ancestors, but they are really distant cousins. We share a ancestor with chimpanzees, but we as a group experienced a few key mutations that made us who we are, while the group of chimpanzees a few miles away did not get those mutations. I wonder if they're jealous.