I am currently brainstorming the history and logistics of the dragon species in my urban fantasy story about immortal mythical creatures. The way I currently envision them, dragons have their stereotypical signature hoards of treasure because they essentially use gold like solar panels: the celestial energies of the earth can either be dangerous to a magical creature or be harnessed for magical sustenance depending on their wavelength, which changes depending on what solid or liquid substance it most recently and predominantly passed through. In dragons' cases, they can absorb the planet's energy most effectively when it's recently passed through a large amount of gold.
Furthermore, in my setting, all mythical creatures were once humans who were subject to an extremely rare, spontaneous magical transformation that magically activated them and turned them into their own species, which could then reproduce with humans and override mundane human DNA (incidentally, yes, dragons are capable of shapeshifting back into a human form for this and other purposes). So it stands to reason that the ex-human who became the progenitor of the dragon species already had a hoard of gold while they were human, otherwise they wouldn't last long enough to breed when they became a dragon.
So now I'm wondering what that implies about how old the dragon species could be.
Given what we know about the history of the development of human technology and culture, what is the absolute earliest that a human being could have amassed a pile of golden treasure, big enough for a human to lie down in, that could have become the first dragon hoard?