What the fossil record tells us
The fossil record can be read like the pages of a book. In rock layers that have not been disturbed, the deeper layers are older, so fossils in deeper layers belong to living things that appeared earlier. Scientists use this order to follow when groups of life appeared. The record also shows mass extinctions, when a huge number of species disappear in a short time. One of them, about 66 million years ago, ended the age of the non-bird dinosaurs; roughly three quarters of all species (about 75 %, an approximate figure) died out. Scientists study several hypotheses: a large meteorite impact, severe climate change and changes in the air, huge volcanic eruptions, and changes in sea level and in the shapes of the continents. The record also shows family links: the Asian elephant is the closest living relative of the mammoth. And it shows old environments: whale skeletons at Wadi Al-Hitan in Fayoum show that a sea once covered that area, and the Petrified Forest east of Maadi in Cairo shows that a humid forest grew where it is dry today. Careful: the record is incomplete, so we say “scientists conclude from the evidence”, and a gap is not proof of anything. Humans appeared tens of millions of years after the non-bird dinosaurs died out.
| Clue | What it tells us |
|---|---|
| fossils in deeper layers | older kinds of life (if the layers are undisturbed) |
| many species vanish from the layers | a mass extinction |
| whale skeletons in a desert | a sea once covered the area |
| mammoth and Asian elephant | a close family link |
Layer stack, with a grown-up. Stack five coloured sheets of paper as five rock layers, and slip a small card with a drawing of a “fossil” between two of the sheets. Which sheets are older, and which are younger? Now fold the whole stack in the middle: does the rule “deeper is older” still work? Scientists must always check whether layers have been folded or flipped.