If it is believed that humans descended from monkeys and apes. Then why do we still see monkeys, I mean they should all have been evolved into humans?

• To begin with, it is important to note that human beings did not evolve from modern-day monkeys. Instead, monkeys and humans share a common ancestor from which both evolved around 25 million years ago. This evolutionary relationship is supported both by the fossil record and DNA analysis.
• The idea of sharing a common ancestor leads us to a major misunderstanding that evolution is a linear process where one species evolves into another. In fact, evolution is a branching process where one species can give rise to two or more species.
• The fallacy of linear evolution is most clearly illustrated by the analogy of asking, ‘How can you share common grandparents with your cousins if your cousins and your grandparents are still alive?’ The answer is of course that your grandparents had more than one child and they each went off and started their own families creating new branches of your own family tree.
• The same thing happens in evolutionary families. A species can split into two or more descendant species and they can split again and again across the generations.
• New species usually arise when a relatively small sub-population of an existing species becomes isolated from the rest of the species in some way, and diverges genetically from the parent population. If this population continues to diverge, and remains isolated long enough to prevent inter-breeding, it will eventually become so different that it can no longer be considered to be the same species.