"If most, if not all plant and animal life had been wiped out after an apparent asteroid collision with earth, then would that mean that life, post-extinction, would have to start from scratch, like it did billions of years ago? and if so, how would there possibly have been time to create the deiversity that we see today? "
--No, because it wasn't the impact itself that in an evolutionary time scale destroyed the dinosaurs, according to uniformitarian geologic time, it was a very fast process (in geologic time) though would have spaned many years I believe. As it was the blockage of sunlight warming the planet I would think that killed them off.
--Very simmilar to my position on the extinction of the Dinosaurs (and many other creatures ofcourse, it wasn't just Dinosaurs) an ice age that was the cause of the impacts and clouded atmosphere and a drop in climate.
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