Describe and evaluate the lines of evidence that support evolution.

What this dot point is asking

Fossil evidence

Fossils are the preserved remains or traces of past organisms. Because fossils form in layers of sedimentary rock, deeper layers are generally older, allowing scientists to order them in time. The fossil record shows that life has changed over hundreds of millions of years, that many extinct species existed, and that simpler forms generally appear before more complex ones.

Transitional fossils are especially important because they show intermediate features between older and newer groups. Dating methods, including radiometric dating using the decay of isotopes, allow fossils and rock layers to be assigned absolute ages.

Comparative anatomy

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

• Homologous structures have the same basic underlying structure but different functions, such as the pentadactyl (five-fingered) limb seen in humans, whales, bats, and cats. The shared plan indicates descent from a common ancestor, with divergent evolution adapting the limb to different uses.
• Analogous structures have similar functions but different underlying structure, such as the wings of insects and birds. These arise through convergent evolution, where unrelated species independently evolve similar solutions, and they do not indicate close relationship.
• Vestigial structures are reduced features that had a function in ancestors but little or none now, such as the human appendix or the pelvic bones of whales.

Biogeography

The geographic distribution of species supports evolution. Species on isolated land masses, such as Australia's marsupials, are often unique because they evolved separately after isolation. Islands frequently have closely related species derived from a single colonising ancestor, as Darwin observed with the finches of the Galapagos. Distribution patterns match the history of continental movement and isolation rather than random placement.

Embryology and development

Many vertebrate embryos show striking similarities early in development, such as pharyngeal (gill) arches and tails, even in species that look very different as adults. These shared developmental features reflect inherited genetic instructions from a common ancestor.

Molecular and biochemical evidence

Modern biology provides some of the strongest evidence. Comparing DNA sequences, amino acid sequences of proteins, and shared biochemical pathways shows that more closely related species have more similar molecules. The near-universal genetic code and shared molecules like cytochrome c indicate that all life shares a common origin.

Evaluating the evidence

No single line of evidence proves evolution alone, but the strength of the case comes from many independent sources all pointing to the same conclusion. Fossils, anatomy, biogeography, embryology, and molecular data are consistent with each other, which is why evolution is accepted as the unifying theory of biology.