Layers of mud and sand pile up and slowly turn into rock. In a stack of rock layers, the layers at the bottom are older than the layers on top. A principle called faunal succession says each layer of rock holds its own record of life, and the layers follow each other in a known order. Fossil dating comes in two steps. Relative dating puts fossils in order, older and younger. Radiometric dating of the rocks around them then gives an age in years. The radioactive atoms in some rocks break down at a steady, constant rate, so they work like a clock that starts when the rock forms. Counting what is left tells scientists how long ago the rock formed.

William Smith saw that fossils in rock layers appeared in the same order in many places, even across England, and this gave geology a way to match up rock layers far apart. Index fossils are used to work out the age of sedimentary rocks, and when the same index fossil turns up in rocks in different places, the rocks formed around the same time. Where a paleontologist digs depends on the age and kind of rock: rock from an ancient shallow sea may hold fossils of corals and other sea animals. Sea fossils sit high in some mountains because slow movements of Earth's crust pushed old sea floor up. The rocks of the Guadalupe Mountains in Texas were lifted skyward in the past 20 million years.

Tiktaalik had fins with thin ray bones, scales and gills like a fish, but also a neck, shoulders and sturdy wrist bones like a four-legged animal. Tiktaalik fossils are about 375 million years old, and Tiktaalik was found in the Arctic, in Nunavut, Canada. Tiktaalik had lobed fins that it may have used to move around on the shore. Archaeopteryx had feathers, wings and a wishbone like a modern bird, but also teeth and a long bony tail. Many scientists call it a mix of dinosaur and bird features. Archaeopteryx lived in the Late Jurassic Period, and its fossils were found in limestone in Germany.

Georges Cuvier, a French anatomist, firmly established that extinction is real. As of November 2026, more than 99 percent of all species that have ever lived are thought to be extinct. This is an estimate, not a count. The fossil record is skewed toward animals with hard parts, because soft tissues and organs do not usually fossilise, and many soft-bodied species are missing from it. Fossils tell us which plants and animals lived long ago. Fossils help show how living things changed over time. Fossils can show what the climate was like long ago. Fossils are our best evidence for the history of life on Earth.

Words to know

• faunal: about animals
• succession: coming in order
• radiometric: using radioactive clocks
• anatomist: body expert
• limestone: kind of rock