another asteroid hits the planet causing the second mass extinction event ,and throw the planet into it's second ice age.Most mammal species survive ,and certain plant life survive the impact.Not to mention a small number of the derpy life forms survive ,and now face a cold period of time on the planet. P.S. The first extinction event being the gamma ray burst causing much of surface lifeforms being wiped out ,and I believe we should call the derpy life forms derpo mamilian or something.