been doing a bit of reading, Google is good for something, found references to Sumerian texts and their account of the flood and the fall of mankind. Which for anyone with a reasonable understanding of ancient history won't be surprised to learn are reasonably similar to later Greek and Hebrew texts.
So that takes the literal record of "yesterday was better" to c 3000 to 2000BC. ie 4-5000 years ago.
If we want to look sideways consider why so many of the existing tribal cultures practice some form of ancestor worship, it does suggest a belief that the ancestors are more knowledgable, that they are in the position to offer some guidance, I don't believe it is a huge step to take to argue that the idea of deference to the past generations might mutate into the belief that the world they lived in was better, that the lives were longer, the weather better, etc.
I think the idea of the past being better falls into the category of the summers of our childhood were hotter, the winters snowier. It is quite incredible how falible the human memory can be whilst being perfectly correct, by that i mean we recall exactly those things that suit our argument and forget those things that do not. So yes there are feral kids now and in the past there were times there were not, but there are times now when there are no feral kids and times in the past when there were. Your memories suit what you want to believe.