Brainlag PlusWhat the fossil record shows and how rock layers date it, how fast resistant bacteria spread, how extinctions are measured, and Edexcel's human fossils, stone tools and the pentadactyl limb.
Fossils and rock layers
Sedimentary rock forms in layers, one on top of another, so deeper layers are usually older.
Comparing fossils from layer to layer shows how groups of organisms have changed, and when some became extinct: their fossils stop appearing in younger layers.
- Rock formed at 4 cm every 1000 years; a fossil is 240 cm down
- 240 ÷ 4 = 60 lots of 1000 years
- 60 × 1000 = 60 000 years old (an estimate: the rate may have varied)
The oldest rocks hold only simple organisms; more complex ones appear later. Scientists cannot be sure how life began, because early forms were soft-bodied and left few traces, and many traces were destroyed by geological activity.
Deeper usually means older. A fossil with features of two groups (such as feathers and teeth) suggests that one group evolved from the other.
The rest of this lesson and its 13 practice questions are in Plus.
Screen 1 of 6, free to read.
- 13 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards