Evidence for human evolution: WACE Year 12 Human Biology Unit 4

What this dot point is asking

WACE wants you to know the main types of evidence for evolution and to evaluate their strengths and limitations. No single source proves evolution alone; the strength of the case is that independent lines of evidence agree.

Fossil evidence

Fossils are the preserved remains or traces of organisms from the past. They provide direct physical evidence of organisms that lived long ago and show that life has changed over time. In sedimentary rock, deeper layers (strata) are generally older, so the sequence of fossils through the layers records change over time. Transitional fossils show intermediate features between older and more recent forms.

Fossils are dated by two main methods. Relative dating uses the position of strata to put fossils in order from oldest to youngest. Absolute (radiometric) dating uses the known decay rate of radioactive isotopes, such as carbon-14 for recent remains and potassium-argon for older volcanic rock, to give an estimated age in years.

The fossil record has limitations: fossilisation is rare and requires specific conditions, so the record is incomplete and biased toward hard parts like bone and teeth and toward organisms living where sediment forms.

Comparative anatomy

Comparing the body structures of different species reveals patterns that point to shared ancestry.

Homologous structures have the same basic structure and origin but may perform different functions, such as the pentadactyl (five-fingered) limb shared by humans, whales, bats and other mammals. The shared underlying plan indicates descent from a common ancestor, with the limb modified for different uses (divergent evolution).

Analogous structures perform the same function but have different origins, such as the wings of insects and birds. They result from similar selection pressures (convergent evolution) and do not indicate close relationship.

Vestigial structures are reduced remnants of organs that were functional in ancestors, such as the human appendix or coccyx, and are evidence of descent with modification.

Biochemical and DNA evidence

The most powerful modern evidence is molecular. All living things use the same DNA code and similar metabolic molecules, which itself suggests common ancestry.

More precisely, the more closely two species are related, the more similar their DNA base sequences and the more similar the amino acid sequences of shared proteins such as haemoglobin and cytochrome c. By counting the differences, biologists estimate how recently two species shared a common ancestor. Human and chimpanzee DNA, for example, is extremely similar, which places chimpanzees as our closest living relatives.

A molecular clock uses the roughly constant rate at which mutations accumulate to estimate the time since two species diverged: more differences mean a longer time since they shared an ancestor.

How this maps to the exam

Expect questions that ask you to describe a type of evidence, to interpret data such as a stratigraphic column or a protein or DNA comparison table, or to distinguish homologous from analogous structures. The strongest answers note that independent lines of evidence (fossil, anatomical, molecular) agree, which is what makes the case for evolution robust.