I was thinking about carbon sinks and the like in the universe. On our planet, it took about 5 billion years for life to get to the point that it could communicate with the universe. At this time, a spectacularly large amount of carbon is tied up in limestone and hydrocarbons. Consider what would have happened without humans. Eventually, the amount of carbon, naturally sequestured, would have reached the point that photosynthesis would no longer be possible. I think, prior to man, the planet was very close to that point. If CO2 levels drop to the point that photosynthesis stops, then life on the planet is hooped. Especially the more evolved forms of life, such as man. Could this be why there are no other civilizations in the universe? Death from lack of CO2?