The first signs of decline are physical. Citizens don’t grow as tall. They don’t live as long. They start killing each other in large numbers.
Sounds like the post-mortem for a society that disappeared long ago, a conclusion that archaeologists deliver after sifting through bone fragments and pottery shards. Why, the puzzled scholars ask, did such a vibrant society, which produced beautiful art and remarkable scientific advances, fall apart so rapidly and leave so little behind in the unforgiving rainforest?