Because if it wasn't balanced, it wouldn't exist. The reason everything fits so well into its niche is because it's spent millions upon millions of years adapting to fit into it...evolution isn't a blink and you'll miss is process.
Let's not forget that we're not the only planet in the universe Every other planet in our solar system is a barren wasteland...if that's the norm, then all of this occuring by chance isn't actually so surprising...even if it is a billion to one, earth is only one among billions of planets.
All species only evolved from a single common ancestor in the strictest possible sense of the world...the fact is that evolution has branches off so many times that any commonalities are very much lost. Generally speaking, we don't (and probably can't) trace that far back. Usually we can determine the common ancestor of any given family of animals however (primates for example).
However, since you asked...yes, it is a valid scientific theory. One that has withstood the test of time, and countless attempts to bebunk it. The fact is that evolution is pretty much rock solid, and just because you either don't understand it, or refuse to accept it, doesn't change the fact that at the moment it's certainly the closest thing we have to hard truth.