Some 65 million years ago, a big rock -- a very big rock -- slammed into the southwest portion of the Yucatan Peninsula in Mexico, creating a 110- to 180-mile crater and triggering a biological catastrophe that wiped out more than half the Earth's species, including the dinosaurs.
In 2010, an international panel of scientists ruled out alternative explanations as they coalesced around the theory that the space rock impact was responsible for this cataclysmic event. However, they debated whether the crater was produced by a comet or an asteroid.
New research now points to a comet as the culprit.
Scientists based their conclusion on evidence that suggests the object that left the large crater in Mexico was smaller than earlier thought. The research was presented at the 44th Lunar and Planetary Science Conference.
Still, the judgment is not unanimous, as many scientists continue to favor the thesis that the impact was caused by a large and relatively slow moving asteroid. The BBC has more on the controversy here.