|MadSci Network: Astronomy|
The questions you pose are difficult to answer since we are only beginning to grasp the subtle concepts of the origin and evolution of life and the possibility of extraterrestrial intelligence.
To begin with, there is no solid evidence that any aliens exist, much less that they have landed/crashed anywhere on Earth. There is no reason for us to believe that such a species could not exist, but that certainly does not mean that they crashed in Area 51. When scientists study and form theories, we typically use what is called Occam's razor, which basically says that the simplest explanation is most likely the correct one. Also, for any theory to be accepted by the scientific community, it must be testable and results reproducible. No one has proven that the government is hiding aliens or alien technology. There is no doubt that the government is secretive about the activities surrounding Area 51. I find it far more likely that this is because of issues of national security (such as testing new types of airplanes or bombers) than because they are trying to hide information concerning alien visitors. So the answer to your question is no. Based on a lack of solid evidence and more likely possibilities to explain the secretive nature of the government concerning Area 51, I do not believe that the government is holding back information pertaining to UFOs or aliens.
Now for your second question. Current scientific studies seem to indicate that simple single celled organisms evolved very quickly on Earth. The first evidence for life is dated at 3.85 Gyr (billion years) old (Earth itself is 4.6 Gyr old). Single celled organisms are also found in a very wide range of environments from the frozen rocks of Antarctica to the boiling water in hot springs and volcanic vents to rocks a kilometer or more deep. Some species are highly resistant to radiation and can be revived after being exposed to the harsh environment of space. This would seem to indicate that bacterial life can survive in a wide variety of environments which are impossible for animal life to live or evolve in. From this reasoning, we could expect there to be bacterial life on a variety of planetary bodies throughout the Galaxy.
Making more complicated life is much more difficult. An excellent book that addresses the issues and conditions necessary for evolving animal life is "Rare Earth" by Peter Ward and Donald Brownlee. Animal life only evolved on Earth approximately 600 Myr (million years) ago. It took over 3 billion years to go from simple single celled organisms to more complex organisms! This would seem to indicate that it is much more difficult to evolve animal life, though certainly not impossible. Once this process started on Earth, there was an explosion in diversity that led to all the life forms currently in existence. Life spreads to fill every available niche on our planet. Humans are a very recent addition and it was not certain that we would evolve. For example, if the asteroid impact that wiped out the dinosaurs had not occurred, then it is entirely possible that dinosaurs (or their descendants) would still dominate Earth.
Darwinian evolution involves survival of the fittest, along with a bit of luck. Advantageous characteristics are passed down to succeeding generations and disadvantageous characteristics are eventually lost. This is not a directed process. It is not necessarily advantageous to develop intelligence or a humanoid form in order to evolve and survive. Even if animal life evolves on a planet, that does not mean that an intelligent species will arise. However, the fact that we exist is proof that intelligence can evolve on a planet and so could evolve elsewhere given acceptable conditions. The physical form that intelligent life form could have is, for now, unknown.
For more information on Astrobiology or Origins of Life, check out the Astrobiology web at http://www.astrobiology.com/ or NASA's Origins Program at http://eis.jpl.nasa.gov/origin s/index.html
Try the links in the MadSci Library for more information on Astronomy.