Actually, Darwin did not talk about the descent of Man in On the Origin of Species at all, but many of his contemporaries did, based on his ideas, wonder how Man fit into that history. It was not until ten years after the publication of On the Origin of Species that he published Descent of Man. At the time, many believed that the theory regarding Man meant that we descended in direct line from big, existing apes, meaning that Man was the immediate descendent of chimps, gorillas and orangutans. Still today, a 150 years after the publication of Descent of Man, you can meet this misunderstanding.
Darwin never wrote that Man descents from the apes. Instead he talked about a shared origin and used the image of The Evolutionary Tree to explain his theory. Ultimately all living things have the same origin – the most simple living organism you can possibly imagine. From this point life on earth branches off in a wealth of groups, species and examples. On The Evolutionary Tree Man is not only placed further away on the same branch as the apes, the two species each have their own branch. If you look back even further you can see how some of the branches have the same origin. Man and the chimp branched off about 4.1 million years ago. Two million years before that gorillas developed in another direction, and we have to go back 18 million years to see when exactly orangutans and man were divided into two different species.
In Darwin’s time orangutans were often portrayed as our very close relatives, for instance in zoos. It is true that we are related to the apes – just as we are related to all life on earth. They are even our closes relatives. But we are not a more advanced version of the apes or primates we know today.
In 2009 a number of articles about Ardipithecus ramidus in the respectable magazine Science unexpectedly bought life to the ape-discussion once again. here you can read more about the chimp-problem.