There has been avid speculation for centuries as to the existence of alien life. Most of this can easily be dismissed as hoaxes, direct lies, hallucinations, or mental instability, yet it absolutely cannot be definitively proven that life does not exist on other worlds. So this question has been left mainly to the realm of philosophy. Though it still cannot be proven either way, we can examine the evidence and make certain conclusions that can sustain the very strong probability that alien life exists.
EVIDENCE FOR LIFE
Evidence #1: Suggestive Data that Mars once contained Life
The first evidence came from a Martian rock that NASA released evidence for on 06 Aug 1996. This rock, known as the SNC meteorite had been ejected into space by an asteroid impact on Mars about 15 million years ago, and orbited the sun until it impacted with Antarctica about 13,000 years ago. This rock contained fossilized micro-organisms. Not absolute evidence, but decent. Recent data collected by the twin rovers currently operating exceptionally (and well beyond their original scope) from rocks on Mars itself shows mounting evidence that life on Mars was probable in the past.
Evidence #2: Mathematical Probability
We are living in a star system with less than a dozen planets. Now, assuming that life does not exist elsewhere in our solar system (to do this, we must discard the potential on Mars, Titan, Europa, Venus, etc.), that leaves us in say a 1 in 10 chance (rough constant, but for arguments sake, lets say this is a general value). Consider that astronomers currently estimate 200 billion star systems in our Milky Way Galaxy (a good ballpark figure), and a minimum of 200 billion galaxies in the known universe (probably a very conservative estimate), we have 40 sextillion (40 billion billion, or
40,000,000,000,000,000,000,000) star systems in the known galaxy. With such radically gargantuan sized numbers like this, how would it be that this is the only planet in the entire universe to have life on it? To say so is ridiculous and blind.
Evidence #3: The Earth was changed by Life itself
In Planetary science terms, our world is composed of the Lithosphere (rock), Hydrosphere (all water), Atmosphere (all gasses), and a 4th part, the Biosphere (all life). Though many naysayers contend the Earth is precisely the perfect distance from the sun, orbital and revolutionary frequencies, contains a single light moon, etc. for life to exist, they perpetually ignore the fact that life itself has changed the conditions on this planet. Look at the atmosphere. On Mars and Venus (our closest neighbors, both 'terrestrial' planets, both similar in size and composition) the atmosphere is mostly CO2 (Carbon Dioxide). Doesn't it seem a bit strange that ours is mostly Nitrogen with strong showing by oxygen?
Easily explained. All life here on Earth is what is known as carbon based, meaning the carbon atom is the basic building block by which other atoms can attach themselves into organic molecules. Start off with an atmosphere of similar composition as Venus or Mars, take the carbon out of the atmosphere, put it into the biosphere (life), release the attached couble oxygen atoms, and VOILA!, you have the atmosphere of modern Earth. Its not magic, its called photosynthesis, and plants do it all the time. In other words, life itself (collectively) has modified its own conditions to those which better serve it as a whole.
BIOLOGY, EVOLUTION, and COMMUNICATION
Taking these 3 arguments, it becomes well established that life is indeed highly probable outside Earth (ie. alien life). So why haven't they called to say 'hi'? To understand this we really have to examine biology. The extensive fossil collections on Earth detail an extravagant evolutionary history covering hundreds of millions of years. In fact, simple organisms are likely to have been around for billions of years.
From humble roots as simple, single celled bacteria, more complex, multi-cellular forms of life sprang up. With time, these became increasingly complex, giving rise to many diverse kingdoms and families of life (using the taxonomic system developed by the Swedish genius Carolus Linnaeus). Just imagine the vastly different forms of life on our own world (such radical differences as jellyfish, fungi, redwoods, tigers, etc.) and one can start to comprehend the potential that life has to adapt and change.
Given over 2 billion years of evolution, we can only ascertain that our species, Homo Sapien is capable of sending radio waves into the depths of space. Thus, 100 years out of 2,000,000,000 has our planet had life with this ability, not a very extensive legacy. Further, this is on a planet with a decent time span of complex life. It is fairly safe to assume that a strong majority of worlds either have shorter durations of a biosphere or contain only microbes, which diminishes our chance to make contact even more. And to kick it in the ass one step further, the radio transmissions were sending out, even today are so weak compared to the solar wind generated by our own modest sized sun, that it probably wouldn't be received by another civilization within our galaxy should they even be listening.
UFOs? Bah. Outside of the realm of science fiction, there are certain laws in physics that would make an actual journey to even the closest star system (Alpha Centauri) extraordinarily problematic. Limitations such as:
1. a nearly inexhaustible energy source (doesn't exist) to propel the starship
2. life support, systems advanced enough to sustain the occupants over a huge amount of time (possible, but think about how many resources we use each day)
3. shielding materials and technology to weather lethal cosmic energy, space dust (a grain of sand striking a satellite can cripple it), etc.
I am not contending that it is impossible to project humans this distance, but the sheer logistics of it would be a global undertaking that would consume us for centuries. Thus, I cannot have any credence in those who claim to have seen a UFO (much less got an anal probe by its occupants).
What if I am being shortsighted about the potential for technology to overcome these obstacles and allow us to travel the stars? That is possible, and that brings me to my final point. Given that life should exist on billions of billions of planets, should 1 in a thousand give rise to complex life, and 1 in a thousand develop civilizations, and 1 in a million exist at this time, and 1 in a million be in our cosmic neighborhood, we are still left with thousands of civilizations technologically capable of saying hello. So why are we not being contacted or visited by any of them?
My original premise could be in error, and we somehow are the only beings to have ever come into existence in this infinitely large and old universe. Or, perhaps there is a reason why they don’t pop in for afternoon tea. This reason is what I like to call the 'Technological Threshold'. Simply put, this means that anytime a civilization reaches a certain advanced stage in its development, the species that created it will inevitably destroy itself.
This could take many forms. In our case, the following scenarios are all quite possible. First, simple Nuclear Annihilation. The sheer number of nuclear weapons we humans have at our disposal is easily enough eradicate not only our species, but every mid to large sized animal on the planet. The initial blasts would be destructive enough, but the radiation would ultimately be the killer. The Cold War may be over, but nuclear proliferation is increasing rapidly, and the nations in possession of these arms are no closer to peace than at anytime in history. All it takes is one nutball demagogue to end us all.
Second, Environmental Disaster. The ‘use and throw’ away industrial culture is growing massively with each passing decade. Instead of trying to develop more ecologically friendly ways to produce, distribute, and dispose of goods, short-sighted capitalist tycoons and Wall Street investors are more interested in profits. Think of the billions of tons of extremely hazardous toxic waste that industries spew into the air, land, and water every year. This is poison, and it is quite literally killing the biosphere of our planet.
Third, Overpopulation. When I was born, 3 billion humans lived. Today, roughly 30 years later, that number is over 6 billion. The most conservative estimates by the United Nations contend that at least 12 billion people will be alive before we can level off the birth and death rates (translation: zero population gain). That number is not far away either, many undeveloped nations are doubling their numbers every 20 to 40 years. With every new human inhabitant on this world, more land and resources are needed, leading to greater competition, instability, pollution, warfare, etc. We may simply reach the point where the resources and food do not exist for the people alive, then civilization will simply implode.
Fourth, Disease. We have already caused the third worst extinction in our planets long history from invading every habitat imaginable due to our burgeoning population. But we cannot lose sight of the simple wisdom of the chain of life. What occurs when a predatory species is abolished? Its prey explodes. If we continue to kill off everything larger than a shrew, we will find ourselves surrounded by smaller, more adaptive organisms that evolve much more rapidly than our species can possibly contend with. I am speaking in particular about bacteria and viruses. We already see evidence of bacteria becoming resistant or immune to antibiotics (superbugs), and despite a strong effort by the World Health Organization and other bodies, plagues seem to be always just around the corner from getting out of hand. Add to this actual biological warfare programs in effect by numerous covert government programs, and the problem is only compounded.
No matter the cause, whether it is one example from above or one not listed, it becomes pretty apparent that our species is likely to kill itself off within 1000 years. Societal suicide via the Technological Threshold. I do not believe that a species can get past this threshold without destruction as well, it is just in the nature of life to expand, conquer, grow, and worry about consequences only after it is too late. Granted, this is conjecture, but I do not believe it is a pessimistic view, rather a realistic one.