Evidence for evolution

What evidence shows that evolution happened?

Fossils (palaeontological evidence) are the remains or traces of past life, preserved in rocks. Rock layers form one on top of another, so deeper layers are older. Each layer holds different kinds of life, showing that life forms have changed over time, and that many have died out (such as the dinosaurs).

Embryos. Ernst Heckel noticed that embryos of all vertebrates, including humans, pass through a stage with features such as rows of gill slits, and argued this showed common ancestry. Karl Ernst von Baer later showed that embryos never pass through the adult stages of other animals, so this argument is not accepted.

Comparing body structures.

• Homologous organs have the same basic structure (and the same origin) but are used for different jobs. The forelimbs of a whale, bat, cheetah and human all have the same bones (humerus, radius, ulna, carpals, metacarpals, phalanges), though they swim, fly, run and grasp. Among plants, the thorn of Bougainvillea and the tendril of Cucurbita are both modified stems. Homology shows common ancestry, with descendants changing in different directions: divergent evolution.
• Analogous organs do the same job but have different structures and origins. The wings of a butterfly and of a bird; the eyes of an octopus and of a mammal; the flippers of penguins and dolphins; and the sweet potato (a root) and potato (a stem), both of which store food. They show that similar ways of life can shape unrelated organisms alike: convergent evolution.

Evolution seen in action. In England, before industrialisation, most peppered moths were white, well hidden against pale tree trunks covered with whitish lichens. After industrialisation (by 1920), smoke and soot darkened the tree trunks and killed the lichens. White moths now stood out and were eaten by birds, while dark moths were hidden. Soon dark moths outnumbered white ones. Nature had selected the better-hidden form: industrial melanism. (Lichens do not grow in polluted air, so they are used as indicators of pollution.)

Faster examples happen around us: insects resistant to pesticides, and bacteria resistant to antibiotics, appear within years, because the few resistant individuals survive and multiply.