After reading Foundation's Edge I'm wondering if the Foundation series was the first literary work to consider the possibility of humanity (as a whole or most of it) forgetting Earth as its place of origin?
If this one qualifies, it has Asimov's May 1942 "Foundation" beat by nearly eight years. "The Living Galaxy" by Laurence Manning was first published in Wonder Stories, September 1934 (available at the Internet Archive); my copy is in Groff Conklin's 1950 anthology The Science Fiction Galaxy.
Here is Groff Conklin's editorial blurb for "The Living Galaxy":
How long the human race will exist on the surface of the planet is the subject of frequent discussion by geologists, physicists, psychologists, philosophers—and military men. Perhaps, some of them think, the time may come when our descendants will spread out among the stars of the universe, and this planet itself will be but a cold pinhead endlessly rolling around an equally cold orange which will be all that is left of the sun. Laurence Manning takes us, in this charming and highly imaginative tale, out beyond the years and the light-years, to give us a glimpse of possibilities so far beyond tomorrow that the human mind can hardly conceive of the distance and the day. The very memory of the home planet has completely disappeared, but the human ideal continues, untarnished and unweakened by time. That is the message of the story, if message it must have: mankind is more than it knows or dreams.
That sounds like what we're looking for. The story itself, except for a foreword and an afterword, is told in the words of a teacher, History Zeta Nine by name ("more than 100,000,000 years old"), lecturing to a class of 6-year-old children. Here is an extract from the foreword:
The date is very far in the future—more than 500,000,000 years—and the sun, Earth, Mars, Venus, and other ancient things have long since died and become as forgotten and legendary as the Garden of Eden.
Better and better. However, the old teacher knows Earth and the other planets of the solar system by name, and knows something about man's history on Earth. Here's how he starts his lecture:
As human beings, history must start for us upon a planet circling a small sun that has long since died. This sun was not located in our present universe, but very far away from here in a large cluster of stars known by courtesy as the "First Universe." In the Chart of Space it is known as Nebula X23G79 and is medium sized, slightly smaller than the one in which our sun and planets happen to be located. It will be the object of this first introduction to history to paint a brief picture of the progress of the race through space and to give some hints as to its final limits and their possible nature. When you have understood this general picture, we shall be in a position to go into more detail, but this is reserved for future lessons.
The planet on which the race first developed was called "Earth" and it possessed by nature a climate and an atmosphere suitable to human existence without any artificial aids. In all of Space, counting millions upon millions of Universes, such a condition has been noted only seventy-two times so that it may be considered extraordinary. Eight other planets circled the same sun and two of these called "Mars" and "Venus" (all ancient heavenly bodies were named instead of numbered) were colonized with great difficulty. This would have been the total distribution of humanity but for three prehistoric inventions that occurred among the ancients who inhabited the "Earth" planet. Let us examine these.
The "three prehistoric inventions" were atomic power, atomic synthesis, and the rejuvenation operation. "Universes" are what we today would call galaxies.