Evidence of Evolution

Fossils form when organisms (or traces of them) are preserved, typically in sedimentary rock, over long time spans. Because sediment layers form sequentially, fossils found in deeper layers are generally older than those in shallower layers, letting scientists reconstruct a chronological sequence of life. Radiometric dating (measuring the decay of isotopes like carbon-14 or potassium-40) provides absolute ages, calibrating the relative fossil sequence to actual timescales.

Transitional fossils capture intermediate stages between major groups, providing direct physical evidence for common ancestry and gradual anatomical change. Tiktaalik roseae shows a mosaic of fish and tetrapod traits (fins with wrist-like bones, a mobile neck) bridging aquatic and land vertebrates; Archaeopteryx shows both reptilian features (teeth, a bony tail) and avian features (feathers, wings), bridging theropod dinosaurs and modern birds.

The geographic distribution of species often only makes sense in light of evolutionary history and continental drift. Closely related species tend to be found near each other geographically (e.g., the many distinct finch species of the Galápagos Islands resemble a South American mainland finch ancestor more than they resemble unrelated birds elsewhere with similar diets). Similarly, marsupials are heavily concentrated in Australia, consistent with their evolutionary isolation there after the breakup of the supercontinent Gondwana.

Island biogeography in particular illustrates adaptive radiation: isolated islands, colonized by few ancestral species, often show rapid diversification into many ecological niches unoccupied by competitors.