If humans emerged from apes or monkeys, then why there are monkeys around us?

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.