I figure a maximum of 5 million intelligent humans (Haida, perhaps), in small bands, distributed far apart, is just about sustainable. Of course the planet can't support humans the way we currently multiply, fight, pillage and waste. Try to calculate how much of the earth's bounty has been turned into implements dedicated exclusively to the destruction of other humans. Even the peaceful applications of technology are mostly insane. I lived in LA briefly - in Anaheim Hills. The fake waterfall in our garden foamed blue bleach day and night, the swimming pool was always heated, the mini-jungle was sprinkled automatically at 7am and the pavement outside was scrubbed down every Wednesday night - two blocks downslope of a sagebrush desert! Anthropocentrism (not humanism) is insane. That insanity produces ideas like organized religion, monetarism, nationalism and militarism. Still, hungry people are even more dangerous than content people, so feeding them intelligently until they reduce their own reproduction would be a good idea. I speculate idly on various impossible ideas. Not gonna happen, obviously, as only the least sensible humans aspire to be world leaders.