A fossil is formed when an organism is left in an oxygen-free state, where its structure does not decompose. Instead, the organism's building materials are either retained or replaced by undegradable minerals.

Fossils are stored in the layers of Earth's soil and rocky crust, as well as in swamps and ice. Prehistoric insects have also been found inside amber.

Fossils: On the left, a petrification fossil (ammonite, ancient mollusk). In the middle, a fossil of a dragonfly. On the right, a footprint of a dinosaur.

Fossils provide evidence for the gradual development of organisms. The deeper the remains of animals and plants are found in the soil, the more rudimentary their structures are. Correspondingly, the fossils closer to the surface and thus in the newer strata, are more similar in structure to current organisms.

Comparing the similarity of structures such as limbs, it can be observed that two species have developed from the same ancestors. For example, in the forelegs of vertebrates, a certain common basic structure can be observed, from which they have formed into a shape typical of each species.

The biotechnological processes of animal and plant breeding are examples of accelerated evolution. Breeding is based on differences in characteristics between individuals. The breeder selects the desired characteristics from certain animal or plant species, and by fusing them changes the species in the desired direction. Most modern cultivated plant species have been created this way.

Molecular similarities provide evidence for the shared ancestry of life. DNA sequence comparisons can show us how closely different species are related. The genetic similarity of species is also evidence for evolution: the more similarities there are between the genomes of two species, the closer their developmental histories have been. For example, the genes of humans and chimpanzees are 99.4% similar in terms of their functional genes, while the genesof humans and mice are 80% identical. 

Today, there are a few species that are said to be living fossils, as they have remained virtually unchanged for millions of years. These include the hedgehog, the ginkgo and the platypus. Thus, the development of the species and changes in their appearance are not always necessary for the survival of the species.