If humans evolved from apes, then why do apes not evolve from humans?

It is quite true that marine mammals evolved from land mammals, and land mammals evolved from reptiles, which in turn evolved from amphibians, which in turn evolved from fish. Therefore marine mammals have gone back to the water, where their ancient ancestor once lived. It is also true that humans evolved from an ape, and the australopithecines are intermediate stages between apes and humans. There is a very good fossil record over the past 5 million years to show how humans evolved from a quadrupedal ape to a bipedal ape with a brain comparable in size to those of an ape, and finally the brain evolved larger in Homo erectus before it reaches its current size in Homo sapiens. Curiously, there is no 5 million year old chimpanzee fossils. Therefore we do not know how our closest relative looked like 5 million years ago.

Hence there are some scientists who suggest that chimps could have evolved from an australopithecine or a bipedal ape to its current quadruped form, which would be a form of evolutionary reversal. Evolutionary reversals are quite common and therefore this possibility cannot be discounted completely. However, most scientists believe that chimps haven't changed much during the last 5 million years, and that the lack of fossils is due to the difficulty of fossil preservation in forest and woodland habitats. These habitats are humid and hot, and dead animals will decay quickly before they have a chance to be preserved as fossils.

A third possibility is that chimps did evolve, but from a larger ape. Chimps and gorillas are the only apes that walk on their knuckles. Some believe that it is convergent evolution, but that is unlikely because chimps are small apes, and there is no reason for them to walk on their knuckles, which is a way to keep the hands flexible but still be able to support the weight of a large ape. Therefore it is quite possible that chimps actually evolved from a gorilla sized ape and inherited the knuckle walking habits of its ancestor even though its smaller body size no longer requires it to knuckle walk. If that is the case, the chimp also illustrates another well known principle of evolution, known as Dollo's principle, which states that evolution is irreversible. It says that an organism, once it has changed, cannot completely revert to its ancestral form. For example, when a mammal evolved to live in the sea again, it may look like a fish superficially, but it can no longer regain those features found in its fish ancestor, such as gills. The same parts of the embryo that would turn into gills in the fish have been changed into something else, and these structures cannot be turned back into gills again. Nor can mammals re-evolve pectoral and pelvic fins, because these fins have turned into limbs in amphibians, mammals, birds and reptiles. It is true that whales have flippers that look like fish fins, but they are different than fish fins.

Therefore, to answer your question, according to Dollo's principle, a human cannot re-evolve into an ape. Humans can indeed evolve into a quadruped again, as some dinosaurs have evolved from quadrupeds to bipeds and then back to quadrupeds again. How likely is that? The likelihood is low, because we are doing things that endanger the living apes, and many of them have been greatly reduced in number since the beginning of the 20th century. Therefore if a population of humans were to evolve into quadrupeds and look like apes, they would face the same difficulty that others apes will face, such as habitat destruction and poaching by humans for food, souvenirs and pets.

I dont think humans evolved from apes. Rather more likely is that we all evolved from a common lineage of ancestors.

It is quite true that marine mammals evolved from land mammals, and land mammals evolved from reptiles, which in turn evolved from amphibians, which in turn evolved from fish. Therefore marine mammals have gone back to the water, where their ancient ancestor once lived. It is also true that humans evolved from an ape, and the australopithecines are intermediate stages between apes and humans. There is a very good fossil record over the past 5 million years to show how humans evolved from a quadrupedal ape to a bipedal ape with a brain comparable in size to those of an ape, and finally the brain evolved larger in Homo erectus before it reaches its current size in Homo sapiens. Curiously, there is no 5 million year old chimpanzee fossils. Therefore we do not know how our closest relative looked like 5 million years ago.

Hence there are some scientists who suggest that chimps could have evolved from an australopithecine or a bipedal ape to its current quadruped form, which would be a form of evolutionary reversal. Evolutionary reversals are quite common and therefore this possibility cannot be discounted completely. However, most scientists believe that chimps haven't changed much during the last 5 million years, and that the lack of fossils is due to the difficulty of fossil preservation in forest and woodland habitats. These habitats are humid and hot, and dead animals will decay quickly before they have a chance to be preserved as fossils.

A third possibility is that chimps did evolve, but from a larger ape. Chimps and gorillas are the only apes that walk on their knuckles. Some believe that it is convergent evolution, but that is unlikely because chimps are small apes, and there is no reason for them to walk on their knuckles, which is a way to keep the hands flexible but still be able to support the weight of a large ape. Therefore it is quite possible that chimps actually evolved from a gorilla sized ape and inherited the knuckle walking habits of its ancestor even though its smaller body size no longer requires it to knuckle walk. If that is the case, the chimp also illustrates another well known principle of evolution, known as Dollo's principle, which states that evolution is irreversible. It says that an organism, once it has changed, cannot completely revert to its ancestral form. For example, when a mammal evolved to live in the sea again, it may look like a fish superficially, but it can no longer regain those features found in its fish ancestor, such as gills. The same parts of the embryo that would turn into gills in the fish have been changed into something else, and these structures cannot be turned back into gills again. Nor can mammals re-evolve pectoral and pelvic fins, because these fins have turned into limbs in amphibians, mammals, birds and reptiles. It is true that whales have flippers that look like fish fins, but they are different than fish fins.

Therefore, to answer your question, according to Dollo's principle, a human cannot re-evolve into an ape. Humans can indeed evolve into a quadruped again, as some dinosaurs have evolved from quadrupeds to bipeds and then back to quadrupeds again. How likely is that? The likelihood is low, because we are doing things that endanger the living apes, and many of them have been greatly reduced in number since the beginning of the 20th century. Therefore if a population of humans were to evolve into quadrupeds and look like apes, they would face the same difficulty that others apes will face, such as habitat destruction and poaching by humans for food, souvenirs and pets.

Because there's no environmental reasons for human to evolve back, and it'll take a few million years of evolution to do it.

Well that is just a silly question -- but humans do continue to evolve -- we are taller, we digest differently than we used to, etc.

It depends on the situation. People in cold northern areas have more body fat. If you had multiple generations of inuit people move to the middle east they would lose their body fat over thousands of years. You might actually want to study the whole thing before you talk.

In what period of tine would be comfortable for you to observe this?

Maybe We just haven't "Evolved" that far ( YET ). ;)

It hasnt happened yet. It easily could, humans developed over millions of years and we have only been alive for a small period. After another 10 million years we could evolve into apes we just havent hade time to.