Fossils and embryos as evidence

Fossils are remains of the hard parts of life forms, found in rocks. A cross-section of the crust shows sediments laid one over another through earth's history.

Rock layers of different ages hold fossils of different life forms, those that probably died while that layer was forming. Some resemble modern organisms; others are extinct, such as dinosaurs.

Fossils in successive layers show the geological period each form lived in: life forms changed over time, and some are confined to certain time spans. So new forms arose at different times. This is paleontological evidence.

The age of a fossil is worked out by radioactive dating.

The dinosaur family tree links extinct dinosaurs to their living counterparts, crocodiles and birds.

Embryological support was proposed by Ernst Haeckel: some features appear in the embryos of all vertebrates but not in adults. For example, every vertebrate embryo, human included, develops a row of vestigial gill slits just behind the head, but they work as gills only in adult fish.

Karl Ernst von Baer studied embryos closely and rejected the idea: an embryo never goes through the grown-up stage of some other animal.