Editor’s note: A version of this story appeared in CNN’s Wonder Theory science newsletter. To get it in your inbox, sign up for free here . CNN  —  Woolly mammoths , giant sloths and saber-toothed cats were a fact of life for early humans. But today’s researchers can only hope to find fossils that will shed more light on the existence of these fascinating prehistoric mega-creatures — and why they went extinct. New clues found in Earth’s northernmost regions are helping piece together portraits