There's evidence that dinosaurs existed in a higher oxygen environment, allowing their large bodies to exist.
There are claims that a large space object impacted our planet long ago.
Many religions record a great flood event in prehistory.
So. I suggest that a great hydrogen comet impacted earth. This event created mass quantities of water and soot, resulting in mass excitinction caused by mass flooding and oxygen drop.
Evolution works just fine for birds and turtles but doesn't apply to humans