Why Are There Still Monkeys?
Monkeys
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"Why haven't all the monkeys evolved into human beings?"
Short Answer
"You understand that all dogs have evolved from wolves, right?" (Even the most rabid creationists accept this - basic DNA confirms it)
So, why are there still wolves? For the same reason there are still monkeys - there is a habitat that supports them - there are still places in which they can live and breed.
Long Answer
Evolution occurs when a new opportunity arises - ie a new environment that can be exploited. If there is a food source available, there is an opportunity for an organism that can catch and digest that food. If a mutation occurs in an organism that lets it use that food source, it then has a survival and reproductive advantage.
Almost all humans from North West Europe have a mutation which allows adults to digest milk. This mutation probably arose in a single individual about 7 or 8000 years ago. Having an extra source of food in times when everyone else is starving is an obvious survival benefit. But that doesn't mean that everyone without this mutation has died out. The mutation is rare in groups which didn't have domesticated milk producers, such as Asians and most Africans.
Not saying that they are separate species. But this is the mechanism by which groups diverge. One part of the original group finds a way of exploiting a new environment and moves into it. If the old environment still exists, the founders will survive.
Sometimes the environment changes drastically. Rivers in America were heavily polluted by PCB's which are toxic to most fish. But some were able to tolerate it. It turned out that the genes controlling the way the toxin affected the fish had mutated, simply by way of the end of a chain breaking. The fish which had this broken copy of the gene survived to reproduce; those which didn't, died. Within a generation the gene was found across all the members of that species in the polluted rivers, simply because they were the only ones which survived to become parents.
Back to the monkeys. Because there is still an environment where existing monkey genes are good enough, they survive to reproduce. If the environment changed radically enough, then all the monkeys of today would die out. If the change happened slowly enough, enough favourable mutations might appear to allow monkeys 2.0 to evolve. If the change was too rapid, or if the mutations didn't appear, the species would go extinct.
There are two sides to this question: one is whether any modern view of evolution requires that there only be one instance of a "type" and once it has been evolved out of, it should go extinct. This is a silly belief - that each "position" on the "scale of nature" once occupied by a lineage, must become empty when that lineage moves upward. If we did evolve from monkeys, then monkeys do not all have to go extinct just because another kind of monkey (i.e. us) has evolved.
Evolution doesn't necessarily mean a ladder of one group succeeding another. More often, you start with one founder group, and others radiate out from that as new environments become available. Think of the cat family. They have radiated out into different niches. Cheetahs evolved extreme speed in an environment where prey ran fast. Lions in a niche where the prey didn't move as fast but was big and strong. Leopards in a dappled forest setting where stealth and camouflage got you a meal. Domestic cats live off tiny birds and animals which wouldn't keep a big cat alive long. You can see that these are all felines, even without DNA proof. But no one is silly enough to ask "Why are there still lions?"
The second side to the question though is this: our ancestors were not monkeys. Our ancestors, and the ancestors of modern monkeys, apes, chimps, bonobos etc, were primates - they didn't look like like any modern primate, but they shared most of their genes.
