Not everything died, and prehistoric life had evolved for millions of years prior to human dominance. Dinosaurs were instinct long before the first primates arrived, for example. Other animals evolved throughout the years-birds actually had claws and scales, but the changing from heat and air condensation to what we know today had given them feathers instead, for the gravity of the Earth became weaker. (Another hint at planetary movement-a dinosaur could NEVER exist today, as it would be crushed under its weight-just like prehistoric insects which were thrice the size of those from today, the physical structure could not relate with the level of gravity today-but back then, it was said to be a lot heavier. Think Kakarott on King Kai's planet.
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If it was an ice age, many animals could have survived, such as the mammoth-or maybe it came with it-via evolutionary process-lizards live in tropical and dry climates, so automatically, the cold will kill them. It is hard to say of course-why did mammoths die? Perhaps it was that the climate, nearing the end of an ice age, no longer accommodated them.
If it was the slow movement of the planet shifting, then wildlife evolution and adaptation makes a lot of sense to me. Perhaps the ice age, if ever there was one, is only a small part of it. The Dodo bird died, but why? Perhaps its ecology no longer needed it.
Although keep in mind, I do not know much about this subject, and most likely, there are many legitimate elements missing to make this a decent speculation.
Lets talk about something important.....
(I don't actually have bright green numbers floating all over my room). 
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