Fossil evidence for evolution

A fossil is a preserved remain or trace of an organism that lived in the past. Fossils can include preserved parts of an organism or traces of its activity. They provide evidence of organisms that are no longer alive today.

Fossils found in rocks of different ages show which organisms lived at different times. Comparing fossils from older and younger rocks shows that the forms of life present on Earth have changed over time. Some organisms found as fossils are no longer found alive, while living organisms may differ from organisms preserved in older rocks.

Comparisons between fossils and living organisms can also reveal similarities in their structures. Such similarities are consistent with common ancestry: organisms share features because they descended from an earlier ancestor. Fossils therefore provide evidence that life has changed over time and that groups of organisms can be related through ancestry.

Limits of the fossil record

The fossil record is the collection of fossils that provides evidence of past life. It is incomplete because fossilisation occurs only under particular conditions, so most organisms do not leave fossils. Geological processes can also destroy fossils after they form. As a result, the absence of a fossil does not necessarily mean that an organism never existed.

Despite these gaps, fossils provide evidence of change over time. Alongside other evidence, they support the widely accepted theory of evolution by natural selection.

Fossil formation

A fossil is the preserved remains or trace of an organism found in rock. Fossils can form when parts of an organism are preserved, when minerals replace its parts, or when evidence of its activity is preserved.

How fossils form

Parts of an organism may remain preserved if the conditions needed for decay are absent. For example, a lack of oxygen or moisture can prevent or slow the activity of microorganisms that cause decay. The preserved parts may then become fossils within rock.

Fossils can also form as an organism’s parts decay and are gradually replaced by minerals. The minerals leave a record of the parts that were present.

An organism does not have to leave part of its body to produce a fossil. Traces of its activity, such as footprints, burrows and rootlet traces, can be left in sediment. If the sediment later becomes rock, the traces can be preserved as fossils.

Fossil evidence of early life

Many early organisms were soft-bodied. Their bodies decayed readily, so they left few fossil traces. Geological activity has also destroyed much of the evidence that did form.

Because so little evidence remains, scientists cannot be certain how life began on Earth.

Interpreting evolutionary trees

An evolutionary tree shows lineages and how they have branched from common ancestors. A lineage is a line of descent through generations. Each branch point represents a common ancestor from which two or more lineages diverged.

Comparing relationships on a tree

To compare organisms, trace their lineages back to the branch point they share. Organisms whose lineages share a more recent common ancestor are more closely related than organisms whose common ancestor is further back in the tree.

For example, if the lineages of organisms A and B meet at a branch point before joining the lineage of organism C, A and B share a more recent common ancestor. They are therefore more closely related to each other than either is to C.

A tree does not imply that one living organism is the ancestor of another living organism. Instead, their lineages can be traced back to a shared ancestor. A tree shows the order of branching, but it supports conclusions about when branching occurred only if a time scale is provided.

Reading evolutionary charts, graphs and tables

Evolutionary information can also be presented in charts, graphs and tables. Before comparing the information, check the headings, axes, units and key to identify what is being measured, which organisms or periods are being compared, and what the symbols or colours represent.

Then extract values or identify patterns to compare organisms or describe changes over time. A graph supports conclusions about the timing of a change only when it provides a time scale.