Planet Earth would be a scary place for humans if dinosaurs still ruled the world. Though there are still some traces of life from the Jurassic Period, the Age of Dinosaurs more than 65 million years ago. This mass extinction is believed to be the result of an asteriod hitting Earth. A new report by the journal Biological Reviews called " The Extinction of Dinosaurs" concludes that this impact was, indeed, the cause of the dinosaurs' _ . But the space blast  wasn't the only reason these creatures aren't still around today. Dinosaurs need food to survive. Meat-eating monsters like the Tyrannosaurus Rex  were at the top of the food chain,  and fed off plant eaters like the horned Triceratops  .  These herbivores    were decreasing in population after the asteroid hit Earth, which left the meat-eating species less food to survive on. "In any ecosystem where you remove links to key species, that community has problems," Richard Butler, one of the review's authors, told National Ceographic. While the herbivore population was going down, Earth's temperatures were rising when volcanoes erupted. Hot vapors and gases began wiping out some of the dinosaur population and weakening the survivors. These changes made the asteroid's impact especially powerful. It caused more volcanoes to erupt, heated up Earth's atmosphere, and led to a sharp drop in the level of oxygen in the oceans. With the dinosaurs gone, mammals  began to evolve into bigger and more diverse species. Many animals we see today, like birds, sharks, and even some cats and dogs, appeared after the asteroid hit Earth. But none rule Earth quite like the dinosaurs did.. The text is mainly about  _  .
the evolvement of mammals on Earth the gradual process of climate change the importance of the balance of ecosystem the cause of the extinction of the dinosaurs