Evolution
When it comes to evolution, scientists tell us that we mammals started out small and only got to be human (and human-sized) after the dinosaurs died out. And diseases have their own evolution: How did HIV (and the AIDS it leads to) migrate from monkeys to men? The reason that mammals stayed small (most of them were the size of today’s rats) during the dinosaur days was because most of the dinosaurs were herbivores (vegetarians), so they ate most of the plants that existed in that time.
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