Fossils: Clues to Earth's Past

But here's the mystery: whales live in the ocean. How could whale fossils end up in the middle of a desert, hundreds of miles from any sea? Even more surprising, these ancient whales had something modern whales don't have — small back legs. Today's whales have no legs at all.

Scientists realized these fossils were about 37 million years old. The fossil evidence told two big stories: the Sahara Desert environment was very different long ago, and the whales themselves had changed over time.

• How could whale fossils end up in the middle of a desert?
• What does the presence of whale bones tell us about what the Sahara used to be like?
• Why would ancient whales have small back legs when modern whales don't?

The whale bones in the Sahara are fossils — the preserved remains or traces of organisms that lived long ago. Fossils are found in layers of rock called sedimentary rock, which forms slowly over millions of years as mud, sand, and other materials pile up and harden. Fossils give scientists powerful evidence about two big things: how organisms have changed over time and how environments have changed over time.

Investigation question: How can we use fossils in different rock layers to figure out how an environment changed over time?

What to do: Imagine you're a paleontologist studying a cliff face. You carefully chip away at the rock and record which fossils you find in each layer. The bottom layers are the oldest, and the top layers are the newest. By comparing the types of fossils in each layer, you can infer what the environment was like at different times in the past.

Tools paleontologists use: rock hammers, chisels, brushes, magnifying lenses, field notebooks, and reference guides to identify fossil species.

Look at the diagram above. As we move from the bottom layer (oldest) to the top layer (newest), the fossils change dramatically. Ocean animals give way to swamp creatures, then forest animals, and finally grassland animals. The fossils are like a timeline — they show that this one location changed from an ocean to a swamp to a forest to a grassland over millions of years.

When scientists study the rock layers and their fossils carefully, two important stories emerge. First, the environment at any one place can change dramatically over time. What is a desert today might have been an ocean millions of years ago. Second, the organisms that live in a place can change too — not just because different creatures move in, but because the organisms themselves can change over many generations.

Story 1: Changing Environments

Scientists use fossils to infer — that means to figure out using clues — what an ancient environment was like. If they find fossils of sea creatures, they infer that area was once underwater. If they find fossils of tropical plants, they infer that area was once warm and rainy. The data table below shows real examples of how fossil evidence helps scientists infer past environments.

Story 2: Changing Organisms

The second story is about how the organisms themselves have changed. When scientists compare fossils of ancient animals to their modern relatives, they often find important differences. The ancient whales from the Sahara had small back legs — but today's whales have no back legs at all. Ancient horses were about the size of a dog and had multiple toes, but modern horses are much larger and have a single hoof. These differences in fossils, recorded across different layers of rock, provide evidence that organisms change over very long periods of time.

The horse fossil evidence is one of the clearest examples in science. Scientists found fossils from many different rock layers and pieced together how horses changed over millions of years. The oldest horse fossils show small, multi-toed animals that lived in forests. Over time, as environments shifted to open grasslands, horse fossils show them getting bigger with fewer toes — until they had the single strong hoof we see today. The changes in the organisms match the changes in the environment!

One of the most important ideas in science is cause and effect — the idea that when something happens (a cause), it leads to a result (an effect). When scientists look at fossil evidence, they see powerful cause-and-effect relationships. When environments change, organisms must either adapt, move, or they may not survive. Scientists look for patterns across many different examples to build confidence in their explanations.

The crosscutting concept of cause and effect shows up everywhere in science — not just with fossils. Let's look at how this pattern appears in different fields:

Understanding fossils isn't just about the past — it helps us solve real problems today. Scientists, engineers, and other professionals use fossil evidence in many important ways.

Scientists and engineers are constantly working together to use fossil evidence in new ways. Every time a new fossil is discovered, it adds another piece to the giant puzzle of Earth's history.

• Fossil — The preserved remains or traces of an organism that lived long ago, usually found in rock. Examples include bones, teeth, shells, leaf imprints, and footprints.
• Paleontologist — A scientist who studies fossils to learn about ancient life and environments.
• Sedimentary rock — Rock that forms in layers when materials like sand, mud, and tiny shells pile up and harden over millions of years. Most fossils are found in this type of rock.
• Infer — To figure something out using evidence and reasoning, even when you can't observe it directly. Scientists infer what ancient environments were like by studying fossil clues.
• Trace fossil — A fossil that shows evidence of an organism's activity, such as footprints, burrows, or nests, rather than the organism's actual body.
• Environment — The natural surroundings of an organism, including the climate, land features, water, and other living things in that area.
• Organism — Any living thing, including plants, animals, fungi, and bacteria.
• Evidence — Information, data, or observations used to support or explain a scientific idea. Fossils are evidence about the past.