Nature evolves by accidents

It is very important to realise that nature is basically conservative which means that the processes repeat in the same way over and over, but from time to time (also random!) there is an "accident" which may change the way things are done. This is the way evolution happen.
If you have difficulties to accept this, just look at what happen around you. Look how plants deliver millions of pollens around and of them only a very small number succeed in building a new tree.
And do not underestimate the huge amount of time that nature has at its dispositions!