In my final Astrobiology lecture yesterday, our professor made a very interesting point. Humans with advanced technology (by our modern standards) have only been around for about 50 years. Humans in general have only been around for about 200,000 of the 4,600,000,000 years the Earth has existed for. Trilobites, on the other hand, were on our planet for roughly 300,000,000 years, and bacteria has been on our planet in some shape or form for at least 3,600,000,000 years. So it is actually far, far, FAR more likely that if intelligent aliens have ever visited our planet, they did so when the Earth was only covered in bacteria and, seeing as there was no intelligent life on our planet at that time, they probably would have checked off our planet as not being home to any intelligent life and went away, never to return again. On the small chance that they visited when there was more than just bacteria, they probably only encountered aquatic invertebrates such as trilobites and scooted on away.