In Earth Science you learned how fossils form and how superposition puts rock layers in order. Put the two together and the rocks become a record of life over time.
Key idea
In undisturbed rock layers, the oldest layers are at the bottom. The fossils in them, from bottom to top, show that life has changed over Earth's history: many species lived for a time and went extinct, simpler forms appear before more complex ones, and new groups appear in the same order all over the world.
Why it works
Sediment settles on top of what is already there, so each layer is younger than the one below it. If life had always been the same, we would find the same fossils in every layer. Instead, trilobites appear in old layers and are missing from all the young ones: they died out about 252 million years ago. Mammal fossils appear only in younger layers. And fossils with mixed features show up between the groups they link.
Worked example: Which is older?
Fossil X is in a layer 30 m deep. Fossil Y is in a layer 5 m deep in the same undisturbed cliff. Which lived first?
- Deeper layers formed first (superposition).
- X is deeper.
Answer: X lived first.
Worked example: A fossil in between
Tiktaalik, found in 2004 in Arctic Canada in rocks about 375 million years old, has fish scales and fins, but also a neck and sturdy fin bones like simple limbs. Why does it matter?
- Older layers have fish but no four-limbed animals.
- Younger layers have four-limbed animals.
- Tiktaalik, from between them in time, has features of both.
Answer: It is a transitional fossil linking fish and four-limbed animals, found where the record predicted.
Common mistake
Students think the fossil record is a complete list of every species, or that all life appeared at once. Fossils form only in special conditions (quick burial, hard parts), so most living things never fossilized and the record has gaps. But the order it shows is the same worldwide, and new finds keep fitting it.