A question that has plagued man ever since we became aware of our existence. I suppose the best place to start is to actually clarify the question.
Is each person alone? Yes and no. Humans are a wonderful contradiction. We are a tribal species yet also an individualistic one. We are each one person out of several billion others, yet we are each also very unique - even identical twins and identical triplets have a few differences that separate individuals - and therefore we are each also alone.
Are we alone as a species? Most likely. There is evidence to support the previous and simultaneous occupation of our little world by several other humanoid species. Neanderthals come to mind as a classic example. But these past neighbors appear to have each died out, leaving homosapiens as the only remaining heir to our current place within the world. We have many different tribes and customs, colors of skin and dialects of speech, sizes and shapes, but underneath all these surface differences we are all the same basic genetic creature.
Are we alone in general as a thinking species?
now here is the true meat of the question. UFO's and ET's and what-if's run rampant in our imaginations. More and more of our explorations both of our own planet and those beyond it seem to indicate that the answer might actually be yes, we are unique - we are alone. Conspiracy theories of hidden UFO evidence to the contrary, very little in the way of solid scientific evidence has surfaced. It is very possible that we, as living and thinking creatures, are truly 'alone' in the universe.
But there's usually more than one side to a question and its answers. Let's try a different tactic and take a look from a mathematical theory of perspective. How many solar systems are in our galaxy? (Millions?) How many galaxies are in the universe?(More millions?) We're told that there are literally billions, possibly trillions of other suns, planets and moons and other traveling bodies in the eternalness that is the universe. Out of those trillions and more of possibilities, doesn't it make sense that at least a few have the potential to harbor life? Maybe even something similar to ours?
The odds against our one tiny planet, when compared to this infinite sea of so many others, as being the only one to ever sprout life is almost absurd. The mathematical odds would suggest that the possibility for life elsewhere is actually quite high. Downright foregone in its conclusion.
Keep in mind that this does not necessarily mean that when we do find life elsewhere that we will recognize it as such. Even within our explorations of our own Earth, we have found a nearly infinite variation of life forms and working ecosystems. From the tiniest, single-celled creatures and plants to the largest whales and trees. Everything from hot arid deserts to cold Antarctic wastelands and all the lush forests in between all sport very different combinations of life. Yet each of these earthly ecosystems function within its own rules and its own niche. In previous eras, a human who had lived his or her entire life in the lush jungle or even temperate forest might not recognize a working ecosystem if suddenly transplanted into the middle of the Arctic Circle. The barest of lichens and bacterial colonies may look like so much paint or other lifeless material splattered across the glaciers and tundra. Yet we now know how incredibly adaptive and diverse the ecosystems truly are in these 'lifeless' wastelands.
The only problem now is gathering hard evidence to prove this theory - that life, especially 'intelligent' life, exists elsewhere. A daunting task indeed to prove that among the billions and billions of other worlds out there life abounds beyond our own tiny sphere. Especially since despite the good chance that there IS life elsewhere - possibly even as intelligent or higher than ourselves - the odds are still quite low that any other world would develop a species and/or mindset quite the same or even similar to ours. Just as we are each unique in comparison to others of our kind, each heavenly body is unique. Even 'twin' suns have differences. And the ecosystems that could develope from those different worlds could have absolutely no resemblance to anything we've experienced. So even if we find life elsewhere, the odds that it would in any way resemble or even be compatible with our own are still quite remote.
And then there's the problem of how to travel to find these worlds to prove our theories one way or another, once and for all. We are still in the barest infancy of our exploration of space. Hubble telescope pictures and other test results give us tantalizing hints as to how our universe functions, but can't focus nearly close enough to any particular heavenly body to give us more than the barest of evidence to work with.
So we are back to the original question - Are we alone?
Yes and no. The jury is still out because not all the evidence has been gathered. And only time and future generations will be able to tell us for certain, if ever.
But in the meantime, it's fun to imagine and wonder.