Have you ever WONDERed how scientists can know so much about things that happened thousands of years ago? For example, how do they know what certain dinosaurs looked like? After all, those dinosaurs have been extinct for thousands of years, right?

But what about the parts of those dinosaurs that were left behind? Occasionally, bones, teeth, and other parts may have left an imprint in a rock that survives to this day. When scientists find these imprints — like an ancient x-ray — they can learn a lot about the animals that left them. What are we talking about? Fossils, of course!

Fossils are the actual remains or impressions left by plants or animals that were once alive hundreds or thousands of years ago. Over time, the organic (living) material left behind is replaced with minerals, leaving a fossil that is like stone but looks like the original plant or animal.

Not all plant and animal remains become fossilized over time. Certain conditions have to exist for fossilization to take place. For example, many fossils form when plant and animal remains are buried — and thereby preserved — by mud, sand, or soil.

Fossilization also takes a lot of time. How long? How about 10,000 years or more. So if you go and bury a plant leaf under a pile of mud in the backyard, don't expect a fossil to form in your lifetime!

Despite the requirements of time and preservation, fossils can be found just about anywhere. From the tops of mountains to the depths of the seas, fossils can be found all over Earth. Some sit on top of sandy beaches while others stay hidden deep underground.

Fossils are often unearthed during construction or new mining projects. As the ground is dug up and moved about, fossils once hidden deep underground suddenly come to light. In a similar way, you can often find fossils in shallow stream beds, as the constantly flowing water cuts through old earth to reveal what's hidden below.

From time to time, you might hear the term “living fossil." Knowing what you already know about fossils, this probably seems like an odd term. However, it's used to refer to species alive today that are the exact same as species for which fossils have been found. This means that these species have been alive for tens of thousands or even millions of years and remain unchanged.

One example is the non-flowering plant species called Ginkgo, of which there is one species considered a living fossil: Ginkgo biloba. Scientists have found fossils recognizable as relatives of modern Ginkgo biloba from 270 million years ago!

Wonder What's Next?

You can see right through tomorrow’s Wonder of the Day!