7.6 Evidence of Evolution

Evolution is supported by data from geography, geology, physics (radioactive decay), biochemistry and mathematics. What makes the case strong is that these independent kinds of evidence agree with each other. A family tree built from fossils matches one built from anatomy, which matches one built from DNA.

Fossils are preserved remains or traces of organisms. They show that species living in the past were different from today's, and they include transitional forms. Tiktaalik, about 375 million years old, had features of both fish and early four-limbed animals, and fossil early whales had hind legs.

Scientists date fossils in several ways:

• Age of the rock layers: sedimentary rock forms in layers, with younger layers on top of older ones. A fossil's layer gives its relative age.
• Radiometric dating: radioactive isotopes decay at a steady rate, measured as a half-life (the time for half of the isotope to decay). Carbon-14 has a half-life of about 5,730 years and works for remains up to roughly 50,000 years old. Older rocks are dated with isotopes that have much longer half-lives, often in volcanic layers above and below a fossil.
• Geographic data: where fossils are found, including on continents that have since drifted apart, helps place them in time and history.

Homologous structures share the same underlying structure because they were inherited from a common ancestor, even if they now do different jobs. The forelimbs of humans, cats, whales and bats all have the same set of bones (one upper arm bone, two forearm bones, wrist bones and finger bones), used for grasping, walking, swimming and flying.

Vestigial structures are reduced remnants of structures that were functional in ancestors. Whales have small pelvic bones not attached to legs, some snakes have tiny hip and leg bones, and humans have a tailbone. These make sense only as inherited leftovers.

Be careful not to confuse these with analogous structures, like the wings of birds and insects, which do similar jobs but evolved independently (convergent evolution, 7.10). Analogous structures don't show common ancestry.

Comparing DNA nucleotide sequences or protein amino acid sequences gives some of the strongest evidence. Mutations accumulate over time, so species that split from a common ancestor more recently have fewer differences. Humans and chimpanzees, for example, have nearly identical sequences for many proteins.

Biogeography, the distribution of species across the planet, also fits. Species on islands usually resemble species on the nearest mainland (Galápagos species resemble South American ones), and Australia's many marsupials reflect its long isolation.