We can learn from fossils how much or how little different organisms have…

The idea

Fossils are preserved remains or traces of organisms from the past, and comparing them with living species reveals the extent of change over geological time. Some lineages show little change: horsehoe crabs in rocks hundreds of millions of years old closely resemble modern forms, so they are often called living fossils. Others show major change: comparing ancient whale fossils with modern whales reveals the gradual loss of hind limbs and the shift to flippers. Fossils also document extinction, showing that many organisms no longer exist. By arranging fossils by rock age, we can see whether a body plan stayed stable or was modified, and how life on Earth developed.

What earns the marks

• Fossils are remains or traces of organisms preserved in rocks, ice or amber, and their age can be estimated from the rock layer in which they are found.
• Comparing the structure of a fossil with a living organism shows whether the body plan has changed a lot or only a little over time.
• Organisms such as horseshoe crabs have changed very little over hundreds of millions of years, whereas whale ancestors show major changes in limb structure.
• Fossils provide evidence that some species became extinct, because their fossils appear in older rocks but no living examples exist.
• Sequencing fossils from oldest to youngest allows trends in body structure to be identified as life developed on Earth.

Common mistakes and how to put them right

• Thinking every fossil is a complete skeleton.

  Put it right: Most fossils are fragments, impressions or traces, so conclusions are based on the features preserved.

• Assuming all organisms change at the same rate.

  Put it right: Some lineages remain similar for long periods while others change substantially.

• Believing fossils show the exact age of an organism without evidence.

  Put it right: Age is inferred from the rock layer or dating methods, not from the fossil alone.

Exam tips

• When describing change, name the specific feature compared, such as limb bones or shell shape, rather than saying the organism 'changed'.
• Use the command word: 'describe' needs features and trends, while 'explain' needs a reason linking the fossil evidence to change over time.
• Quote data from any chart or table, such as 'the fossil is 300 million years old', to support your comparison.