The fossil record shows that life began early and became more complex and diverse over geological time; major milestones include the first simple cells, the Cambrian appearance of abundant shelly animals, the move of life onto land, and the rise and fall of major groups; evolution by natural selection explains the changes, fossils provide the evidence, and mass extinctions repeatedly reset the course of life (for example the end-Permian and end-Cretaceous events).

What this dot point is asking

Eduqas wants you to use the fossil record to describe the origin and development of life: that life began early and became more complex and diverse over geological time, the major milestones (first cells, the Cambrian explosion of shelly animals, life moving onto land, the rise and fall of major groups), the explanation (evolution by natural selection, with fossils as the evidence), and the role of mass extinctions in resetting life. This connects the fossils you met in the sedimentary module to the geological time framework, and questions reward reading the record rather than just listing organisms.

The answer

Life through geological time

The fossil record, ordered by the age of the rocks that contain the fossils, shows a clear trend: life has become more complex and more diverse through geological time. The oldest rocks hold only simple single-celled life; complexity is added stage by stage in younger rocks.

This trend is visible because fossils occur in a definite order. Younger rocks lie on older ones (superposition), so the fossils in them form a sequence from oldest at the base to youngest at the top. Particular fossils are found only in rocks of a particular age, which is what makes fossils useful for dating and correlating rocks.

The major milestones

A few landmarks summarise the development of life:

• The first simple cells. Life began early in Earth's history as simple single-celled organisms (bacteria-like cells). For a very long time life stayed microscopic.
• The Cambrian explosion. Relatively suddenly, the rock record fills with abundant shelly animals (trilobites, brachiopods and many others). Hard parts both made the animals more successful and made them far more likely to fossilise.
• Life moves onto land. Plants, then animals, colonised the land, opening vast new habitats. Land plants produced the swamps that later became coal.
• The rise and fall of major groups. Groups such as fish, amphibians, reptiles (including the dinosaurs) and mammals each rose to dominance and, in some cases, declined or died out. The appearance and disappearance of a group in the record marks its origin and its extinction.

Evolution explains the changes

The mechanism behind these changes is evolution by natural selection: individuals vary, those whose features suit their environment are more likely to survive and reproduce, and so favourable features become more common over generations. Over long spans of geological time this produces new species and the gradual change of life. The fossils are the evidence: they show the actual sequence of forms through time, including transitional forms that link earlier and later groups.

Mass extinctions reset the course of life

The development of life has not been smooth. Several times a large fraction of species died out worldwide in a relatively short interval, a mass extinction:

• The end-Permian event was the largest, wiping out a huge proportion of marine species.
• The end-Cretaceous event removed the dinosaurs (and many other groups), and is linked to a large meteorite impact (recorded by an iridium-rich clay) together with massive volcanism.

Extinctions matter because they reset life: removing dominant groups frees up ecological niches, and the survivors then diversify to fill them. The dominance of mammals today, for example, followed the extinction of the dinosaurs.

Examples in context

Example 1. Trilobites as time markers. Trilobites appear in Cambrian rocks, flourish, and vanish at the end-Permian extinction. Their presence dates a rock to the Palaeozoic, and their absence above the Permian marks that extinction.

Example 2. Mammals after the dinosaurs. Small mammals lived alongside the dinosaurs but stayed minor. Once the end-Cretaceous extinction removed the dinosaurs, mammals diversified rapidly into the niches left empty, eventually including the large mammals of today.

Try this

Q1. State what the order of fossils in undisturbed strata tells you. [1 mark]

• Cue. It shows the sequence of life through time, oldest fossils at the base and youngest at the top (by superposition).

Q2. Explain why animals with hard parts are more common in the fossil record than soft-bodied animals. [2 marks]

• Cue. Hard parts (shells, bones) resist decay and are quickly buried and preserved, while soft parts usually rot away before they can fossilise.

Q3. Name one mass extinction event and one group it affected. [2 marks]

• Cue. End-Cretaceous extinction, which removed the dinosaurs (or end-Permian, which removed many marine groups including trilobites).