Life in and of itself can only be described as unlikely, being that nearly 14 billion years ago, only basic light elements existed. Over time, gases like hydrogen and helium were fused into heavier elements, which later made carbon-based life-forms possible. As Edward R. Harrison allegedly said, "Given enough time, hydrogen starts to wonder where it came from, and where it is going"
The odds of certain things existing seem insurmountable, yet the strangest, most improbable things happen anyway. Take you, for example, you only exist because an infinite series of choices — most of which, are beyond improbable individually — made it possible.
Say your grandparents happened upon each other at a restaurant. If one of them chose a different restaurant, or arrived a few minutes too early or too late, he or she would have lessened the odds of ever meeting at all. Consequently, if they never met, your parents would never be born; they in turn, wouldn't have you.
These things don't take biological factors into account — like the specific sperm and egg that gave you your identity long before you even emerged from the womb. Simply put, some scientists calculate the probability of you existing exactly as you are, at this very moment, at one in 400 trillion [or 4×1014]).
In this infographic from Visual.ly, researchers break down many of the probabilities: