Following up on my epiphany about the significance of historical happiness, I reanalyzed the data and developed an explanation that I call the "Half-Earth Hypothesis." Simply: there are a fixed amount of ecological resources, which include both producers and what they produce; and we, like other creatures, have been programmed by evolution to make sure that a critical minimum amount of those resources (18% of the total) is never used up.
In our natural state, we require a minimum amount of resources to survive. That same amount is used by a supporting group of producers, which means we use only half of the resources in our environment. To preserve our basic functioning, we've had to protect the supporting producers and enlist enough more to supply our growing population.
We would like to use more, and technology (physical and cultural) has enabled us to do so since the start of civilization, some 12,000 years ago. The motivation for doing so seems to be connected to a need to dominate our environment, and is clearly manifested by increasing both lifespan and happiness. Increasing our population has provided a labor pool for finding and converting resources into creations that are more of a match to our personal wants and needs than the minimum can provide. These "creations" also include byproducts such as pollution. Nature appears to have limited the amount of resources we can use for creations to 52% of the total, with our basic resources and supporters together limited to 30%. To use more reduces the producers available to sustain our supporters, which of course sustain us.
At present, we are essentially at the limit. Any increase in consumption, whether individually or due to added population will likely reduce the supply of resources we need for basic survival. This makes the growing threat of climate change particularly troubling, since it could have the same effect regardless of what we do, like being on the edge of a precipice and having someone pushing you toward the abyss.