Evidence for Evolution

In short: Several kinds of evidence show that species have evolved from common ancestors. Fossils show life changing over time, homologous structures and similar embryos show shared ancestry, and the more similar two species' DNA and proteins are, the more closely related they are.

Starting point: what you already know

• Natural selection changes populations over many generations (see the previous chapter).
• Rocks form in layers, with younger layers on top of older ones (Year 9 and 10 Earth science).
• DNA codes for proteins.

Fossils

A fossil is the preserved remains or trace of a living thing from long ago. Fossils usually form when a dead organism is buried quickly in sediment, and minerals slowly replace its hard parts.

The fossil record shows

• simple life appearing first and more complex life later,
• groups appearing in an order (fish, then amphibians, then reptiles, then mammals and birds),
• transitional fossils that share features of two groups, like Tiktaalik (fish to land animal) and Archaeopteryx (dinosaur to bird).

Dating fossils

Relative dating puts fossils in order. Fossils in lower rock layers are usually older. Index fossils (species that lived for a short time but were widespread) help match layers in different places.

Absolute dating gives an age in years using radioactive isotopes that decay at a fixed rate. The half-life is the time for half the isotope to decay.

Comparative anatomy

Homologous structures have the same basic bone pattern but different uses. A human arm, a bat wing, a whale flipper and a cat leg all have one upper bone, two forearm bones, wrist bones and fingers. This points to a common ancestor. It is evidence of divergent evolution.

Analogous structures do the same job but are built differently, like a bird wing and an insect wing. They show convergent evolution, not close relationship.

Vestigial structures are reduced parts with little or no use now, but which were useful in ancestors: the human tailbone, whale hip bones, and the tiny wings of an emu.

Comparative embryology

Early embryos of fish, birds and mammals look very alike. All have tail-like and gill-slit-like structures at some stage. Shared development suggests shared ancestry.

Biogeography

Biogeography is the study of where species live. Australia has most of the world's marsupials and all of its living monotremes (except a few in New Guinea). This makes sense because Australia split from the supercontinent Gondwana and drifted alone for about 45 million years. Its animals evolved in isolation. Related plants such as southern beeches are found in Australia, South America and New Zealand, which were all joined in Gondwana.

Biochemical evidence

All living things use DNA, the same genetic code and the same 20 amino acids. That itself points to a single origin of life.

The more closely two species are related, the more similar their DNA sequences and proteins (like haemoglobin or cytochrome c). Humans and chimpanzees share about 98.8% of their DNA.

Fewer differences mean a more recent common ancestor.

Quick summary

• Fossils show life changing over time. Transitional fossils link groups.
• Relative dating uses rock layers. Absolute dating uses radioactive half-life.
• Homologous structures show common ancestry. Analogous structures show convergent evolution. Vestigial structures are leftovers.
• Similar embryos, the spread of species (Gondwana) and DNA and protein similarity all support evolution.
• Fewer differences in DNA or protein means more closely related.

Quiz