Modern humans have existed for about 200,000 years, but dinosaurs dominated life on Earth for nearly 200 million years. This vast span of time is called the Mesozoic Era and is divided into three distinct periods. The reign of the dinosaurs and other giant reptiles came to an abrupt end in a mass extinction 66 million years ago, but not every kind of dinosaur was wiped out.
The first dinosaurs appear in the middle of the Triassic Period. They are small, nimble animals that scamper on powerful hind legs, using their stiff tails to balance and their small arms to handle food. This successful formula soon leads to variations. Some dinosaurs evolve into plant-eaters, growing longer necks that help them reach leaves or armored skin for protection. Others specialize in hunting. While dinosaurs rule the land, other prehistoric reptiles adapt to life in the ocean and air.
In the Jurassic Period, plant-eating dinosaurs reach gigantic sizes, making them the largest animals ever to walk on Earth. Exactly why this happens isn’t clear, but one theory is that predators target smaller animals, driving a process of natural selection that makes both prey and predator become larger and larger. Meanwhile, the smallest dinosaurs evade predators by taking flight - they evolve into the first birds.
During the Cretaceous Period, Earth’s continents slowly drift toward their current configuration, moving about as fast as human toenails grow. There are now more kinds of dinosaurs than ever, including flightless, feathered giants and small but ferocious carnivores with hooklike foot claws that might be used to disembowel prey. At the end of the Cretaceous, all types of giant prehistoric reptiles disappear in a mass extinction, perhaps victims of a catastrophic asteroid strike, but birds survive.
End of the Dinosaurs
Almost 66 million years ago, a catastrophic event occurred that wiped out more than half of life on Earth, including the dinosaurs. Most experts believe this mass extinction was caused by an enormous meteorite crashing into Earth. Such a huge impact would have created a worldwide cloud of dust and fumes, choking animals and blocking out the Sun’s light and warmth. The planet’s climate would have changed dramatically, making life impossible for many species.