The Tree of Life
- Biology
- Ages 14–18 · Grades 9–12
What you’ll find out
- Fossils, comparative anatomy, embryology and DNA independently support common ancestry.
- Phylogenetic trees show relationships: branching points are common ancestors, and shared derived traits mark the branches.
- Classification has moved from Linnaeus’s ranks to evolutionary history, including Woese’s three domains.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Many lines of evidence
Common ancestry, the idea that all living things are related through descent from shared ancestors, is supported by several independent lines of evidence. The fossil record contains transitional forms. Tiktaalik, a 375-million-year-old fish with sturdy fins and a neck, links fish to four-legged animals. Fossil whales with hind legs show how whales evolved from land mammals. Comparative anatomy reveals homologous structures. A human arm, a bat wing and a whale flipper share the same bones in the same arrangement. Molecular evidence is strongest of all. Every organism uses almost the same genetic code, and DNA similarities match the patterns found in fossils and anatomy.
Reading the tree of life
Scientists show evolutionary relationships using branching diagrams called phylogenetic trees or cladograms. Each tip represents a species or group. Each branching point, or node, represents a common ancestor. Two groups are more closely related if they share a more recent common ancestor. In the 1700s, Carl Linnaeus created a system for naming and grouping species. He gave each species a two-part name, such as Homo sapiens for humans. Today, classification is based on evolutionary relationships. Genetic studies divide all life into three domains: Bacteria, Archaea and Eukarya. Trees built from DNA can surprise us. They show that birds are living dinosaurs and that hippos are the closest living relatives of whales.
Curriculum links: NGSS HS-LS4-1 · England KS4: Biology · Cambridge IGCSE: Biology · NCERT Biology, Class 11