I think the apple was only ever valuable in the sense that it had some calories and could keep you alive a bit longer, long enough to keep on working a bit more

I think the apple was only ever valuable in the sense that it had some calories and could keep you alive a bit longer, long enough to keep on working a bit more and generating other forms of value for others. Value is a very strange concept, if there were no people then apples would be of no value. If there were people but they all agreed they didn't want apples and weren't prepared to work for them then they wouldn't have any value. They value of apples is defined by how many other people want them and how hard they are prepared to work in return for them. When you eat an apple you stay alive and thereby increase the value of all future apples.