An age old question to which there are many theories, but no real rock solid answers to how did humans come to occupy planet earth. Even though this may seem like a difficult question to answer, it pales in comparison to the question, WHY are humans or life for that matter here?
I'm not even going to pretend to answer the latter question, but I do have an alternative theory for the former.
Recently, I was reading an article in a Popular Science magazine that talked about some real world issues facing us today. An article debating each of these could be discussed, but to stay focused I'll suffice it to say that the resources this planet has to offer cannot sustain the entire population in the way they would like to live. In other words, Americans enjoy a lifestyle and an appetite for food and other natural resources that most of the rest of the world do not. For example, with only 5% of the worlds population Americans consume 60% of the worlds meat. With the worlds affluence increasing, there is a corresponding increase for the desire to consume like Americans. If you do the math the numbers don't add up to the earth's resources being able to support that kind of consumption.
So, the article surmised, that it would likely be required for humans at some point to move off the planet to ensure the long term survival of the human race. Technology exists today to construct huge self sustaining space stations that humans could live on, orbiting the earth and visiting the planet much like someone would visit Hawaii or some other exotic place.
It would be generations before these space stations would be capable of space travel since we don't have good enough rocket technology to get anywhere very fast. So they would just orbit the earth.
Problem with building these space stations is that they are very expensive and governments are having crippling money problems right now. And private industry hasn't yet figured out a way to profit from building these mammoth buildings in space so they haven't committed any funds towards these projects yet.
At some point, however, man may be forced to build these stations for survival. When that happens all bets are off for the expense of it, it will become a priority and it will happen.
So just for arguments sake let's say humans build some of these space stations and people actually begin living on them. Whether it be boredom or advances in technology, someone is going to want to begin exploring space and looking for other inhabitable planets.
This journey would take generations and people would be born and die in space without ever stepping foot on soil. Other evolutions may occur like happen when animal organs and senses change to adapt to their environment. In the low light environment of space maybe peoples eyes begin to enlarge and bodies begin to shrink or change due to minimal amounts of available food and low levels of gravity.
Hundreds, maybe thousands, of years pass and many potential planets are discovered before one is finally found that can support life. Anxious, but cautiously, people explore their new-found world and begin to develop it. Some stay behind, others jump back in the space ship and continue their journey. Maybe with improved technologies that enable them to travel ahead and back in separate space ships, the mother ship 'checks in' with the humans that stayed behind colonizing their new planet to make sure they are ok.
While this dream is many years into the future, if it is in the least bit possible, then maybe it happened before. Maybe that's how this planet was colonized too.