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Do Aliens exist? That question has railroaded the discussions about xenobiology for decades. The ancient greeks discussed how planets might orbit other stars with life and even men of some sort but in the following millennia little further discussion has been had on the subject. We now know that billions of Earth-like planets probably exist in our Galaxy and this shouldn't really be a surprise. Venus is almost identical to Earth in size but has a runaway greenhouse effect. So there are two Earth-like planets just orbiting our star the sun.
We've learned a great deal through astronomy and the next generation of telescopes will teach us much more. We now know there are oceans under the icy surface of several moons orbiting Jupiter and Saturn being warmed by geothermal energy and the tidal forces of the gas giants they are circling. Ganymede holds more water than all of Earth's oceans combined. So much for water being rare in the universe. Battle Star Galactica, the old tv series V and other scifi drama's have depicted Aliens coming to Earth to steal our H2O but as Spock would have said 'that would be highly illogical'.
http://news.sciencemag.org/space/2015/03/huge-ocean-confirmed-underneath-solar-system-s-largest-moon
As water is the prime requisite for life as we know it this is great news for the prospect of Alien life. If we evolved then life should evolve on other worlds with a similar environment and scientists are discovering that terrestrial planets with a similar day/night cycle to earth and a thick atmosphere are far more common than suspected.
The process of life evolving from inanimate matter is called abiogenesis and we still don't fully understand it. That isn't to say we haven't learned a lot. With a petrie dish the size of the earth and millions of years for nature to experiment it turned basic elements into amino acids which are the building blocks of proteins and life. Researchers have done that in the lab and more. Trying different techniques such as heat, cold and electricity to imitate lightning which was common in the dense early atmosphere many stages of have been replicated right up to chains of RNA. These findings suggest life might have started in ponds where a chemical soup was alternately heated by the sun and dried, cooled and diluted by cold nights and rain then subjected to lightning and other stages to finally cook up complex self replicating DNA chains. Simple virus like packages of mutating genes which would eventually become wrapped in a membrane and turn into cells. RNA precursors have even been found in space which supports idea of panspermia which suggests life can travel to other worlds in the debris kicked up by asteroid impacts.
Life appears in the fossil record almost instantly once the Earth cooled down enough for life to be possible here (geologically speaking). It could have taken millions of years but the Earth is 4.6 billion years old so that's a comparatively short wait. It took about 3 billion years for life to reach the level of bacteria which happened about 1 billion years ago. The first fish appear over 500 million years later and then there is an explosion of new life forms. So life was slow to reach a multi-cellular stage here but once it did evolution went into overdrive.
We also know that planets form a lot faster than previously thought which further increases the potential number of exo-planets out there that might be suitable for life.
Could other forms of life not based on carbon or DNA exist? Theoretically yes but it depends on what elements are most available. Life on Earth is composed of the common elements found in the universe and if anything different had tried to formed after carbon based life it would likely have beaten eaten by the more advanced existing fauna which had a headstart. While its fun to contemplate such exotic life being there first on other worlds the best candidate is Silicon and it can't form chemical bonds with diverse sorts of atoms as Carbon does and this chemical versatility is necessary for metabolism.
Astronomers can even detect the components of alien planetary atmospheres now using spectral analysis. Discovering an Earth-like planet with oxygen would be a good indicator of life.
Many leading researchers are predicting we will find ET or even an alien civilization within 20 years using either reflector telescopes to study planetary atmospheres or radio telescopes to pick up alien radio messages.
The universe is immense and we reside in one of the outer arms of a major galaxy so it shouldn't be too surprising we haven't been visited or sent any messages yet. If other intelligent life forms exist our weak modern day radio chatter probably hasn't reached them yet assuming they have radio receiver dishes advanced enough yet or haven't moved on to something else like laser transmissions.
This might be a good thing. As Stephen Hawking has pointed out numerous times there is no reason to assume advanced Alien life would be friendly. They might see us as a threat or competitor. If they are more advanced they might see us the way we see Chimpanzees. As primitive and perhaps amusing neighbours too dumb to be trusted.
Asimov was fond of saying wimps don't become top dog and we ourselves wouldn't be the dominant life-form on Earth if we hadn't killed off a lot of competing carnivores.
We see a lot of parallel evolution in nature. Life forms from diverse groups developing similar methods of solving similar problems. So Aliens might not look totally strange to our eyes. They might even resemble animals we know to some degree. Land vertebrates on Earth all evolved rom bony limbed fish so have four limbs and two eyes etc. Aliens from another planet might be descended from a squid like creature that learned to crawl on land. The number of possibilities is seemingly infinite.
One of the most interesting questions is how long alien civilisations survive and this is one of the unknown quantities in the famous Drake Equation designed to calculate the potential number of intelligent alien species in the Galaxy. Do they mostly wipe themselves out the second they invent an anti-matter bomb or do they survive billions of years by colonizing nearby stars and maintaining strict controls over weapons of mass destruction?
Do they go to war with other advanced species they encounter and eventually go extinct by meeting someone bigger and badder than themselves or do they hide?
Humanity has been obsessed with the idea of immortality for thousands of years and thanks to religion and mythology has largely come to the conclusion it is an impossible dream but modern science is now dismissing those entrenched ideas.
Ageing is a collection of degenerative diseases caused by several factors which we can reasonably hope to find solutions to in the near future. In fact mice were rejuvenated only last year in several experiments which replaced the blood proteins discovered to be lacking in older animals.
Survival is part of the genetic programming found in all life and the drive to find ways of defeating death can be expected in other sentient species. If they have been experimenting around with medical science centuries, millennia or even millions of years longer than us then they have probably solved the problem of growing old.
Another major driving force found in living things is the will to reproduce and successful life forms tend to do this a lot. Overpopulation is the cause of most of our problems here on Earth and could even lead to our extinction but if we survive we may find that other species in the galaxy are in a similar quandary. Their solution might be colonization, revolutions and war with other species.
Heightened population pressure stresses animals out and leads to conflict. We've known this a long time. The conquest of America and Australia was at the expense of indigenous populations who suffered a reduction in numbers to make way for the new arrivals. Visitors from the Stars might come here for similar reasons.
I've heard many people argue that everything necessary for survival can be found in space by milking asteroids for resources. Technically yes but that's like saying the first fleet could have stayed at sea and fished for the rest of their lives. Creatures used to living on a planet with dirt under their feet and a sky overhead are unlikely to want a life in a space habitat forever.
Perhaps the reason why we haven't met any other galactic citizens yet is that the top dogs are all at war. The communication difficulties, distrust and repugnance of strange other beings with perhaps objectionable practices is a powder keg waiting to happen. What if the first encounter between two species has a misunderstanding and it quickly escalates into an interstellar war lasting millions of years? Little wonder they haven't set an exploration mission out here. All their resources would be needed to avoid extinction from an unknown unquantifiable enemy.
But even if Sentient Aliens are close enough to visit us, come under friendly terms and we somehow get along it still probably wouldn't end well. The fact that they could reach us means we are within their growing sphere of influence. Star travel is extremely difficult and slower than light ships taking decades or centuries to reach the next star are likely to be the only possible means of exploration.
So if they can reach us already then it won't be long before they can reach the next star and the next after that. Soon all the stars around us will be colonized before we' ourselves have even built our first Starship. There will no longer be any point in doing so because there will be nowhere to go. We'll be living inside the Empire of another species and forced to beg for handouts.
