Yes small shifts can have catastrophic results. And that turns out to be a good thing in creating diversity.
If you think it's all impossibly improbable, consider that not a single one of your own ancestors from the very first single celled creature was unlucky enough to get killed, too slow to be eaten, too ugly to find a mate before they reproduced. They all survived long enough to reproduce. How many 1000's or 1,000,000's of them have there been? Petty improbably no? Yet here you are. It's almost beyond imagination right? Evolution seems just as (more) improbable if you think about it in one way, but once you think about it in the right terms it's just a likely and obvious as the fact that you exist now.
nothing you have said differs from my beliefs, except I believe a higher power had a hand in guiding those improbable events.