"Carter was motivated by the timing of our emergence, which we can round off at 4.6 billion years after the formation of our planet. He reasoned that the upper

"Carter was motivated by the timing of our emergence, which we can round off at 4.6 billion years after the formation of our planet. He reasoned that the upper limit for habitability at Earth’s surface is on the order of 5.6 billion years after Earth’s formation, a suspicious fact – why would human origins require a time that approximates the extinction of the biosphere that supports us?" My absolute favorite Fermi's Paradox theory is that life on Earth was evolving too slowly to survive, so aliens sped up the process while hiding themselves, but that came with a variety of problems that humanity bumps into, and the aliens are trying to clean up the mess without destroying humanity's potential for a unique identity. The sci-fi novel practically writes itself.