For decades, the human race has been trying to solve one of the most puzzling questions of our time: are we all alone in the Universe? Are we, as a species, the only specimens of intelligent life that exist in the cosmos? And, if we're not, where the hell is everyone else?
In the 1960s, Frank Drake formulated an equation meant to calculate the probability of ~intelligent~ (which here refers to advanced life that uses technology) extraterrestrial life in the known universe. The equation took into account such factors as the number of stars and star formations in space, the number of potentially habitable planets that surround those stars, and the number of habitable planets that could potentially host intelligent life. Based on the equation's results, there should probably be around 1000 intelligent alien civilizations in our own galazy...
And this is one of the great mysteries of the Universe. It's called the Fermi Paradox: the contradiction between the lack of evidence and high probability of the existence of extraterrestrial civilizations. If there is such a high probability of other intelligent life out there, why can't we make contact with anyone?
There's no solution (that we know of) to the Fermi Paradox, but economist Robin Hanson has proposed a theory which suggests that -- since it would appear that there is no evidence of lasting, intelligent life anywhere in the cosmos aside from ourselves -- the specific conditions needed in order for life to explode out of nothingness are exceptionally rare. The theory refers to what Hanson calls the "Great Filter."
Somewhere along the path from simple, single-cell existence to the advancement of intelligent life, Hanson argues, there is supposedly a hurdle that any kind of developing species should be unable to pass. If this is the case, then perhaps all of the other lifeforms and inhabited planets with which we should have made contact by now never made it past this impossible leap in the evolutionary process.
On the one hand, optimists contend that humankind has won the proverbial lottery. After all, it took over a billion years for any kind of life to form on Earth. Even then, it remained in the prokaryote or single-cell stage for two billion years before advancing into complex cells with a nucleus. ~Maybe~ the great leap from single-cell organism to complex nucleus-having cell is the Great Filter, and the entire known Universe is absolutely crawling with simple, unevolved single-cells, and we just haven't found any yet. If Hanson's Great Filter is in fact behind us, then mankind has surpassed all other attempts at life and is free to harness the power of the Universe.
We don't know where mankind is headed but, arguably, the end goal for our civilization is probably something along the lines of expanding beyond our solar system and ultimately colonizing the Milky Way galaxy. This would eventually make human kind a Type III ivilization on the Kardashev Scale, which basically means that we have been able to avail ourselves of all the energy in the galaxy for technological and communications advancements. If we are the rarity of the Universe and the Great Filter is behind us, then we are well on our way to becoming a Type III civilization.
On the other hand, we could be totally fucked.
So, all of this ultimately begs the question, "what's next?" What will be our Great Filter, if indeed we're not as special as we may think and it's still yet to come? Are we on the brink of imminent destruction? Is the human race soon to become just another potentially unique planet that never made it past life's greatest hurdle?
One possibility is that intelligent life simply cannot sustain itself once it reaches a certain level of advancement. Maybe one day we will have harnessed so much of the Earth's energy that there will be nothing left for us to continue on with, and we will eventually just run out of food, fuel, and resources and simply starve to death like the ~animals~ we are.