Fire, scientists agree, helped give rise to a successful, thriving human population by providing heat for cooking and protection from the cold. But they don't agree【C1】______when humans began using fire. Some researchers argue that it【C2】______more than a million years ago when early humans【C3】______their way to Europe from Africa, and【C4】______say it happened much later. Now, a new study reveals that humans did not【C5】______fire until about 400 000 years ago. Two archaeologists(考古学家)in the US report their【C6】______in the current issue of the Proceedings of the National Academy of Sciences. They looked at excavation(发掘)reports and studies from 141 sites in Europe that were【C7】______1.2 million and 35 000 years old. They think there is no clear evidence of【C8】______fire use until about 400 000 years ago. In Africa,【C9】______there are several sites where there was evidence of fire, those may have been【C10】______fires that occurred in the African grassland.【C11】______, there are also sites older than 400 000 years in Europe that indicate the【C12】______of humans, with the oldest, in Spain, more than a million years old. This means that【C13】______Europe's extreme winter climates, early humans found a way to【C14】______without the warmth of a fire.【C15】______the researchers mention, early humans are【C16】______and persistent(坚持的). But some researchers【C17】______that it is still a puzzling explanation and it still【C18】______some serious thinking about how【C19】______the human being could have survived on a seasonally【C20】______food supply despite having small teeth and small guts. Up to now, no one has solved this problem yet.