Sooner or later, the human race is doomed to extinction: the only real question is why this will occur. Although insisting that extinction is inevitable does seem morbid, even gruesome, this need not be the case. As individuals, we know our lives will one day come to an end - although we hope that end will not come too soon, and most of us avoid taking unnecessary risks for just that reason. The same thing is true of our species: we should sidestep risks, but we should realize that sooner or later our time in this universe will come to an end. The only real question is how - and it is over this matter that, fortunately, we do have a measure of control.
- War, Famine, Pestilence, and Death -
The most immediate threats to humanity are those of our own making: nuclear annnihilation or environmental collapse. The first of these seems to be a relatively minor threat today, but just a few decades ago, the most advanced societies on Earth were technically just minutes away from near-total obliteration, barring a sufficiently extreme political crisis. We should be thankful that tensions are lower than they were in the Cold War, but it would be foolish to believe that at this point we have solved the problem of war forever.
Second, and perhaps more seriously, there are myriad ways we are placing a greater strain on the Earth environment than it can plausibly bear: chemical pollution that poisons the biosphere, carbon dioxide and methane emissions that clog the atmosphere, intensive agriculture that exhausts the topsoil, overfishing, deforestation, desertification, and so on. So long as the Earth is our only home, using it in an unsustainable fashion can only be a threat to our species's future. Even if any one of these processes did not cause extinction on its own, it could certainly reduce our society to a group of impoverished, postapocalyptic survivors huddled in the ruins of a greater age.
Physicist Enrico Fermi once remarked that, given what we know of the natural world, we ought to see alien civilizations everywhere in the night sky - and we don't. One reason could be that few, if any, intelligent species like ours are able to escape the consequences of their own genius.
- Asteroids and Gamma Rays -
If we don't destroy ourselves, nature might do it for us anyways. Two of the most important threats which face us from the night sky are near-Earth objects like comets and asteroids, and distant stellar explosions releasing deadly gamma ray bursts. Large asteroid impacts have occurred before in Earth's history; one, at Chicxulub, is believed to have caused the global devastation that wiped out the dinosaurs 65 million years ago. An asteroid several miles in diameter would cause a comparable amount of damage today.
Fortunately, there is a reasonable chance that we would detect such an object well in advance. As they are identified, near-Earth objects are tracked, plotted, and placed on the Torino Scale, which assesses the likelihood of a global impact. RIght now no object rates higher than 1 on the scale (an extremely low possibility of an impact but not in the immediate future, and which is based on incomplete information about a newly discovered object), although asteroid Apophis reached level 4 after brief fears it could strike the Earth in 2037. Once detected, an asteroid could also be deflected off course, although the technology to do so has never been tested.
In contrast, there really is nothing we could do to fend off a gamma ray burst. These intense blasts of radiation are given off from the poles of an exploding star during a supernova, when it gives off more energy in the intense explosion than it has during the entire rest of its lifetime. If Earth happens to lie in the course of that burst, we would be exposed to a deadly blanket of extremely high radiation - and if the star is close enough, this radiation would be intense enough to devastate our atmosphere and destroy all life on the surface. We would probably receive a little warning, since the radiation from the gamma ray burst travels slightly slower than the light from the explosion itself, but we really have no way of building a radiation shield to protect the entire planet. Fortunately, it is relatively unlikely that a close-by star will blow up while one of its poles is pointing at the Earth - there's about a one-percent chance of this event happening, spread over the entire multi-billion-year history of the Earth.
- End of the Solar System -
Assuming that we solve war and environmental destruction here on Earth, and that we are not killed off by asteroids, comets, or gamma ray bursts, there is no reason to believe that life on Earth, if not humans themselves, could not survive until the end of the solar system. However, this end, like the gamma ray bursts, is one that we really have no feasible way of preventing. As the Sun ages and uses up its hydrogen fuel supply, it is gradually beginning to burn hotter and hotter. Within about two billion years, Earth will have heated up to the point where the oceans boil and life perishes; essentially, our blue planet will turn into a twin of Venus. By that time, hopefully, we will have moved off to somewhere else, perhaps Mars or one of the moons of Jupiter and Saturn.
Even there, however, we will not escape the fate of the Solar System. A few billion years after life on Earth is destroyed, the Sun's core will run out of hydrogen fuel altogether. At that point, the Sun will initially puff up into a red giant star, cooler than its current yellow self but so large due to internal expansion that the surface of the Sun will lie somewhere around Earth's current orbit. From there it will shrink again - not to its former self, but to a glowing ember, a white dwarf. The planets may be enveloped by the star's outer layers, shed to form a planetary nebula; if they survive, they still will not be able to receive more than a dim glimmer of light from the dying white dwarf. At that point, if humans have not invented interstellar travel, we will surely die along with our solar system.
- End of the Universe -
If we can escape from the solar system and settle on planets elsewhere in the Milky Way Galaxy (which is the stuff of science fiction today, but could be a simple matter a few thousand years from now), that buys us a considerable amount of time. Even then, however, the end is all but certain. Current models indicate that the universe is expanding and that it is doing so at an accelerating rate due to a mysterious force known to physicists as dark energy. As it expands, however, no new regular matter or energy is being created - which means that everything that exists is being spread out more and more thinly over more and more space. Sooner or later, our own galaxy will be so isolated from everything else that the most powerful telescopes may struggle to find evidence of other galaxies. By the time that happens, the Milky Way will also be running out of the free-floating dust and gas necessary to form new stars. In hundreds of billions of years, our galaxy will have been reduced from its current haven of stars and light to a dark cluster of dead stars, faint red dwarfs clinging to life, and black holes swallowing everything else.
Even if the universe's expansion does not continue to the point where we freeze to death alone in our dying galaxy, the universe's fate is still sealed. Ultimately, all work is done on the basis of heat exchange, and stops when the objects in question reach an equilibrium, i.e. are the same temperature. Sooner or later, however, everything will end up the same temperature, and no more work - and no more life - will be possible. This scenario is known as the heat-death of the universe. If it turns out we are wrong about dark energy, then the remains of this dying universe will eventually be tugged back together by gravity, and it will all fly back together into a singularity, creating a cataclysm referred to as the "big crunch." (This might be followed by another Big Bang.)
The only hope that cosmologists can offer is a solution that sounds truly like one straight out of science fiction: if humans are still alive as our universe comes to an end, hopefully we can invent some way of creating a wormhole to another universe where life is still possible, and flee there as refugees.
- Biological Evolution -
All of those wild scenarios, of course, assume that human beings will survive - even to the end of time - in their current form. Given what we know about biological evolution as well as about technological progress, it would be somewhat foolish to believe that this would actually happen. Human physical evolution is not a significant part of our lives today, nor should it be: in every way that counts, we share the same genes and live more or less the same types of lives. However, if humanity goes on to survive millions or billions of years and to settle elsewhere in the galaxy in the process, it is plausible that the "humans" of one billion years from now would be genetically almost unrecognizable compared to those of today.
Moreover, in the meantime, we might devise some sort of relationship with computer technology that made our current organic bodies seem obsolete. In the future humanity might simply choose to upload itself to a giant computer system. This idea, too, is one drawn from the heady realm of science fiction - but again, if technological progress continues, the distant future would possess capabilities we would find to be indistinguishable from magic. Either way, from a biology perspective, Homo sapiens sapiens would technically be extinct, even if humanity lived on in some modified form.
We have a long way to go before reaching any of those fates, however. There are more immediate threats to the future of our prosperity and survival on Earth, and it is best to remember not only that most civilizations in the history of humanity have failed and fallen over time, but that all of the dominant species prior to ourselves have ultimately fallen from the heights, as well. Ultimately, the human race is doomed to extinction - the only real questions are how, and when.