Original: One impetus for this change came from the publication of a historic paper by Walter and Luis Alvarez in 1980. This paper suggested that a 10 k asteroid struck Earth 65 million years ago at the end of the Cretaceous period.
Simplified: Walter and Luis Alvarez suggested in 1980 that a 10 k asteroid struck Earth 65 million years ago at the end of the Cretaceous period.