According to A Sound of Thunder, when the fabric of time and space is ripped due to timeline alteration, the effects are not immediate and irreversable, nor are any paradoxes created.
Instead, we get Time Waves.
Imagine, if you will, great big tsunami-like waves which sweep across the globe systematically assimilating everything into the new flow of time.
But don't worry, we humans won't be affected until last wave... because humans were the last species to evolve!
We shouldn't need to point out what's wrong with this.