Most scientists agree that an asteroid or comet put a dramatic exclamation point at the end of the Cretaceous period. It is also widely acknowledged that continents shifted, sea levels dropped, and volcanoes erupted across Earth's surface. Most paleontologists would even grant that mammals probably competed to some extent with dinosaurs.

With all of these pieces to the puzzle in hand, the question remains: What role did these phenomena play in the extinction of the dinosaurs?

While the answer to this question remains a mystery, the question itself is far more refined than the one scientists were asking just a few decades ago. We're getting closer to knowing what killed the dinosaurs.

And so, for now, we can be satisfied with maintaining several viable hypotheses. It's likely that one of them -- or all of them in combination -- holds the answer. In time, with more evidence, we will undoubtedly get closer to knowing for sure.