What is the fine-grained topic of the following text: Historically speaking sugar wasn't a terribly easy thing to get, but it is a good signal to your body that a food is high in calories, which it needs in order to survive, so sugar tastes good, because the people who ate a lot of sweet things when they were available were better at surviving the winter and passing on their genes. Only return the topic.
The historical significance of sugar and its impact on human survival.