sentence2_structure: Woolly mammoths<>began<>because of hunting by humans
sentence1: Recent studies have bolstered the hypothesis that human hunting contributed to the extinction of Pleistocene animals ranging from the woolly mammoth to the giant wombat to the North American camel.
sentence2: Woolly mammoths began to go extinct thousands of years ago because of hunting by humans.
question: Why did woolly mammoths began to go extinct thousands of years ago?