Evidence for Evolution
Use fossils, homologous structures, embryology, DNA comparisons, and biogeography as evidence for common ancestry and evolutionary change.
A lesson of Biology, in Evolution and Diversity of Life, about 10 minutes.
By the end of this lesson, you will be able to describe and evaluate multiple lines of evidence supporting evolution, explain how each type of evidence provides independent confirmation, and connect fossil records to molecular evidence.
The Big Idea
Evolution — the change in heritable characteristics of populations over successive generations — is one of the most strongly supported theories in all of science. Multiple independent lines of evidence from completely different fields all point to the same conclusion. This convergence is what makes evolution a scientific theory, not a hypothesis.
Lines of Evidence
1. Fossil Record
Fossils show a history of life with older, simpler forms in deeper (older) rock layers and progressively more complex forms in newer layers.
Transitional fossils: organisms showing intermediate characteristics between ancestral and descendant groups.
Examples: Tiktaalik (fish-to-tetrapod transition), Archaeopteryx (dinosaur-to-bird transition), Australopithecus (ape-to-human transition).
Radiometric dating: uses decay rates of radioactive isotopes ($^{14}$C, $^{40}$K) to determine fossil age with precision.
2. Comparative Anatomy
Homologous structures: same origin (common ancestor), different function. The bones in a human arm, whale flipper, bat wing, and horse forelimb are structurally identical.
Analogous structures: different origin, similar function (convergent evolution). Butterfly wings and bird wings are similar in function but not structurally related.
Vestigial structures: reduced remnants of structures that were functional in ancestors. Human tailbone (coccyx), appendix, whale pelvic bones.
3. Comparative Embryology
Early embryos of different vertebrates (fish, reptiles, birds, mammals) look remarkably similar — all have gill slits and tails at early stages.
Structures that differ greatly in adults are nearly identical in early embryonic development — suggests common ancestry.
4. Molecular Evidence (Strongest Modern Evidence)
DNA sequence comparisons: organisms that share more recent common ancestors have more similar DNA sequences.
Example: Human and chimpanzee DNA is ~98.7% identical. Human and yeast share ~40% of genes.
Cytochrome c amino acid sequence: nearly identical across vertebrates; more differences in more distantly related organisms — exactly the pattern predicted by evolution.
Endogenous retroviruses (ERVs): viral DNA inserted into the genome at the same locations in both humans and chimps — impossible to explain unless both inherited them from a common ancestor.