Tree of Life

Tree of Life 🌳

There are millions of species on Earth. To make sense of them, biologists **classify** — sorting living things into groups based on what they have in common. This module runs from the classic system built 250 years ago to the modern three-domain tree, and how to read the branching tree of life.

The Linnaean system 🔬

In the 18th century, **Carl Linnaeus** classified living things by their **structure and characteristics** into a nested hierarchy of groups: **Kingdom → Phylum → Class → Order → Family → Genus → Species** Each level is more specific than the one before, so a **species** is the smallest, most precise group.

Order the ranks

An interactive activity.

Two names for every species 🏷️

Linnaeus also gave us the **binomial** ('two-name') system. Every species has a scientific name made of its **genus** then its **species**, e.g. *Homo sapiens* (humans) or *Panthera leo* (the lion). It is written with a **capital** genus and a **lower-case** species, in *italics*. Because the name is universal, scientists anywhere in the world know exactly which organism is meant.

Reading the name

In the binomial name *Panthera leo*, what does the first word, "Panthera", tell you?

• The genus the organism belongs to
• The species
• The kingdom
• The domain

Better tools, better groups 🧫

Classification is not fixed — it changes as our tools improve. As **microscopes** developed and scientists could study cells' **internal structures**, and as **chemical analysis** of organisms' **biochemistry** advanced, we learned far more about how organisms are really related. New evidence meant the old systems had to be **updated**.

The three-domain system 🌐

In 1990, **Carl Woese** used evidence from **chemical analysis** (comparing the RNA of organisms) to propose a new top level: three **domains**. • **Archaea** — primitive bacteria, often living in **extreme** environments (hot springs, salty lakes). • **Bacteria** — true bacteria. • **Eukaryota** — organisms whose cells have a **nucleus**: protists, fungi, plants and animals.

Match the domain

Branches of the tree 🌲

An **evolutionary tree** is a model showing how scientists think organisms are **related**. It is built from the **classification data** of living species plus **fossil** data for extinct ones. The golden rule for reading one: the **more recent** two organisms' **common ancestor**, the **more closely related** they are — follow the branches back to where they join, not how near the tips sit on the page.

Read the tree

On an evolutionary tree, species A and B share a common ancestor 2 million years ago, while A and C share a common ancestor 10 million years ago. Which pair is more closely related?

• A and B — they share a more recent common ancestor
• A and C — their ancestor is older
• They are all equally related
• Whichever two branch tips are drawn closest together

In the exam 🎓

Tree climbed. Grade-9 habits for classification: • Know the hierarchy **Kingdom → Phylum → Class → Order → Family → Genus → Species**, and the **binomial** name (genus + species). • Classification changed because **microscopes** and **chemical analysis** revealed more — leading to **Woese's** three domains (**Archaea, Bacteria, Eukaryota**). • On an evolutionary tree, a **more recent common ancestor = a closer relationship** — never judge from branch-tip position alone.