Over the eons the sun has grown warmer and Earth's CO2 levels have dropped, keeping the overall temperature pretty much stable. (There have been fluctuations i

Over the eons the sun has grown warmer and Earth's CO2 levels have dropped, keeping the overall temperature pretty much stable. (There have been fluctuations in both directions that were major extinction events, though.) Warming increases the rate CO2 gets converted into limestone, thus lowering the temperature. 50 million years is when that pegs and the mercury starts rising. As for it being a puzzling coincidence--it might not be a coincidence at all. Think of the Fermi Paradox--where are the aliens? These days the astronomers tell us planets are very common. A look at the emergence of life on Earth says it happened about as fast as possible. Why are we alone? Enter the Great Filter hypothesis--*something* stops species from colonizing the galaxy. Looking at what has happened on Earth there are only three likely candidates: 1) The jump to multicellular life. It took billions of years. One data point isn't enough to determine an average but we can see it's not something that happens easily. 2) The emergence of intelligence. Same situation, again. 3) The lifespan of intelligent civilizations. The odds of successfully crossing these three hurdles must be billions to one. Either intelligent races destroy themselves before reaching the stars or the billions to one odds are in the first two hurdles. The most favorable interpretation for humanity is that the average time to cross those hurdles is well beyond the couple of billion years it took Earth, most planets fail to produce intelligence before their ecosystem gets fried or smacked with something substantially larger than the dinosaur killer. If this is the answer it's no surprise we squeaked in just under the wire. And if that's not the answer humanity isn't going to be around for too much longer.