You might have heard stories about cavemen or seen them on television. Some people use the word “caveman” to describe primitive men and women who lived on Earth long ago, used weapons like spears and clubs, and found food by hunting and gathering.

There were primitive people who lived in caves thousands of years ago. But scientists don’t usually use the word “caveman” to describe ancient people. They use more precise terms depending on what kind of early humans they’re talking about. Primitive humans started appearing on Earth long ago — two hundred thousand years ago — and we call these humans homo sapiens. You and I are homo sapiens, too. There are older kinds of human ancestors like the Neanderthals, a kind of early human that lived in Europe thousands of years before homo sapiens appeared.

Neanderthals have since died out. As with dinosaurs, scientists know about them because of fossils: bones and other objects that we dig out of the ground. Scientists who study old things buried in the ground are called paleontologists.

In cartoons and stories you sometimes see cavemen and dinosaurs together. But scientists know that people and dinosaurs never lived on Earth at the same time. The dinosaurs died out long before humans appeared on Earth.

Years ago, dinosaurs roamed the Earth. But while there are no dinosaurs alive today, there are creatures that are related to dinosaurs — reptiles such as snakes and alligators, as well as some fish, plants and other creatures.

We know that dinosaurs died out millions of years ago, but where did today’s reptiles come from?

The world was very different when the dinosaurs were alive — it was much hotter and wetter. Over time, the world changed and it became harder for dinosaurs to survive. Some scientists believe that an asteroid (a large rock from space) may have hit the earth in the time of the dinosaurs, causing the temperature to drop suddenly.

When big changes like this happen, some creatures are well suited to survive, and others aren’t. Creatures that are not well suited to survive in the world will die out. A creature that is well suited to survive will have lots of babies that will often be even better suited to survive.

After the earth cooled during the time of the dinosaurs, most dinosaurs died out. Those few who survived had babies who were smaller, smarter, and quicker. As time went on, those babies had babies of their own, and over the course of thousands of years, these creatures didn’t look like dinosaurs at all anymore. They had changed into new kinds of animals, and these animals were well suited to survive in the changed world.

All types of creatures change slowly over time. Scientists call this process evolution.