"But if we evolved from monkeys, why are there still monkeys?"

First, I am not going to give you the common brush off to your question: "We didn't evolve from monkeys, monkeys and humans share a common ancestor". That's just avoiding the question, and frankly, I don't agree with it. I'm going to assume that when you say "monkey," you not only include those animals called monkeys that are alive today, but also animals that looked and behaved like monkeys that lived millions of years ago. An animal that lived about 40 million years ago, known as Aegyptopithecus, is believed by scientists to be a direct ancestor of humans. If I saw that animal swinging through the tree branches today, I'd certainly call it a monkey.

I'm going to go one step further and assume that you are using the term monkey even more colloquially, and include chimpanzees and gorillas under the general umbrella of monkeys. Technically those are apes, but since they are non-human primates that are indeed decended from monkeys, let's go ahead and let that one by. So below, I'm going to cover a scenario whereby humans might have evolved from apes, while leaving apes still existing.

Before I continue, though, let's clear one thing up. It is natural to think of humans as "more evolved" than other animals, but this isn't true in any scientific sense. We are differently evolved, simply adapted to a different environment. It so happens that our intelligence, and the culture and technology that it spawned, has turned out to allow us an unprecedented degree of success, and the ability to live in environments that our ancestors couldn't. But evolution didn't somehow anticipate this.

The point is, evolution is only "directed" in that it favors survival, it does not favor high intelligence or walking upright or use of tools, unless those features aid in the survival and passing on of genes to the next generation. Other animals, for instance mosquitos, don't seem to be suffering for lack of these human-like capabilities.

interfertile: two animals are interfertile if they would be able to produce an offspring, if they were allowed to mate or were artificially inseminated. If they are both the same sex, they are interfertile if each could produce an offspring with the appropriate sex parent of the other. See this essay for an elaboration on the concept of interfertility, and why I use the word rather than the ambiguous expression "the same species."

Around 7-10 million years ago, there was a population of apes in the forests of Africa, all of which were interfertile with all the others. These apes probably roughly resembled modern day gorillas, chimpanzees or bonobos ("pygmy champanzees"), but were not identical to any of them. Nor were they interfertile with any ape or human alive today.

The forests eventually reached the maximum capacity of these apes per square mile as it could support. Like modern chimpanzees, it is likely that various bands of the apes fought with and killed members of adjacent bands, with winners taking over the territory from losers. Even in the absense of fighting, there is a limit as to how many animals an environment can support, so the number of apes stabilized at this number.

Some of the apes lived near the edge of the forest, where the thick trees give way to the drier grasslands, or savanna. There were still trees, but they were much less dense than in the forests. While these apes were less adapted to this environment than they were to the forest, at least there was less competition from the other apes. The apes who lived in this transitional area gradually adapted to the different environment. Tree climbing ability and upper body strength was less important than ability to move about on the ground, and capabilities like carrying things long distances and throwing rocks at prey or predators were valuable. The savannah-compatible attributes and skills increased in number and degree among the populations living in the transition area between forest and savannah.

Over hundreds of thousands of years, more and more of the apes spread out farther and farther into the savanna. They continued to adapt to the savanna, becoming different in appearance and behaviour from those in the forests, much as coyotes differ from gray wolves today. The apes that lived near the border of forest and savanna looked and behaved like a cross between those living deep in the forest or far into the savanna, much as red wolves look and behave like a cross between coyotes and gray wolves. Like coyotes, gray wolves and red wolves, all of the apes -- whether living in the forest or the savannah -- were interfertile with all the others, as they were still fairly close relatives and therefore genetically similar.

A million or so years later, some of the apes were living over one thousand miles apart, and were separated by more than 10,000 generations -- 10,000th cousins, so to speak. Some were so distantly related that they were no longer interfertile with one another. Had they somehow been put together and tried to breed, they would not have produced offspring. However, they remained interfertile with the apes closer to them.

Typically, those less than 6000 generations apart remained interfertile. So they all were still "connected" by a chain of interfertile pairs. While it might be impossible for two apes that were 10,000 generations apart to have children together, they could have still theoretically shared grandchildren or great-grandchildren, by mating with apes that were only 5000 generations apart, which in turn mated with ones 5000 generations apart. Keep in mind, though, that in reality the degree of interfertility was rarely if ever tested: apes interbred only with apes that lived nearby, which were rarely separated by more than a dozen or so generations.

As time went on, the apes living in the forest became separated by more and more generations from the apes in the savanna. No longer were any of the apes interfertile with all of the others, not even the ones living in the transition territories. As populations ebbed and flowed, various bands moved about, took over territory of others, suffered from diseases and starvation and predation and changing climates -- eventually there became a day that the last "interfertility link" between the forest apes and savanna apes died. This day came and went without fanfare, as interfertility is a hypothetical concept: it is unlikely that any of forest apes had even tried to breed with any of the savanna apes for thousands of years prior to the point it became theoretically impossible.

From this point on, the two lines were completely diverged. There was no turning back, as it was now impossible for any of the forest apes to mix their genetic code with the savanna dwellers. The apes living in the forest continued to thrive in the forest, and were the ancestors of modern chimpanzees and bonobos. The apes in the savanna were our ancestors, and by this point, there is a good chance they looked a lot like primitive humans. They were far more intelligent than the apes in the forest, since intelligence had an especially high value in the savanna. Being able to accurately throw a rock, sharpen a stick to use as a weapon, make a fence or other structure to protect themselves from predators, and make use of animal furs were all extremely valuable skills here, and those that more readily acquired these skills tended to be the ones that produced the most surviving children.

One final point I'll make to avoid upsetting the scientists: the more we learn about the path from ape to human, the more we know just how messy the process really was. I described it as a fairly neat process, one group evenly and gradually splitting into two over the course of a couple million years or so. In reality, there appear to have been numerous partial splits, and numerous different fluctuating populations with various degrees of interfertility (and actual interbreeding) between them. Most splits led to eventual extinction, Neanderthal man being a well known (and relatively recent) example. Certain chance events happened along the way, for instance there is evidence a jaw mutation that reduced biting power allowed the brain to grow bigger. In the forest that mutation was a disadvantage, but in the savanna the additional cognitive power given by the larger brain outweighed the loss of bite strength.

It hasn't even been conclusively proven that we are not still interfertile with chimpanzees: some people speculate that a hybrid may be possible, although personally I would bet against it. All indications are that modern humans would be interfertile with Neanderthals. Likewise, modern chimpanzees are interfertile with bonobos, although there is no interbreeding in the wild between the two species as they are separated by the immense Congo River.