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 where each level is more specific than the one before.

Order the ranks

Put the levels of Linnaean classification in order, from the largest, broadest group down to the smallest.

• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

Reading the name

Two big cats are named `Panthera leo` and `Panthera tigris`. What does that tell you about them?

• They belong to the same genus but are different species
• They are the same species, known by two different common names
• They belong to the same species but to two different genera
• They are unrelated to one another, because the second word of each name differs

Written correctly?

Only ONE of these follows the binomial convention. Tap it.

• Four students write the same species name four different ways:
• Panthera Leo
• ,
• Panthera leo
• ,
• panthera leo
• ,
• leo Panthera

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**.

Why it changed

Classification is not fixed. As _____ improved, scientists could see the internal structures of cells. As the _____ of organisms advanced, they learned far more about how species are really related. That new evidence meant the old groupings had to be _____.

The three-domain system

In 1990 **Carl Woese** used evidence from **chemical analysis**, comparing the RNA of organisms, to propose a new top level sitting above kingdom: three **domains**.

Match the domain

Which domain?

A newly found single-celled organism has no nucleus and lives in water at 90°C beside a deep-sea vent. Which domain does it most likely belong to?

• Archaea, because it is a primitive bacterium living in an extreme environment
• Bacteria, because every single-celled organism without a nucleus is a true bacterium
• Eukaryota, because only organisms with a nucleus survive extreme temperatures
• None, because a new domain has to be created for each newly discovered organism

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

Classify a new find

A survey team brings back an organism nobody has recorded before. Work out how it gets classified and named.

• Under the microscope, its cells have a clear nucleus. Which domain does that place it in?
• The team wants to know which known species it is most closely related to. Which evidence settles that most reliably?
• It turns out to belong inside an existing genus, Rana. How should its scientific name be written?
• A colleague says the classification is now settled for good. Are they right?

Explain the change

Explain why the classification of living organisms has changed since Linnaeus, and describe the three-domain system that resulted.

• Say what Linnaeus originally classified organisms by.
• Name the two developments that gave scientists better evidence.
• Say what that evidence revealed which structure alone could not.
• Name Woese, and name all three domains.
• Say what sets Eukaryota apart from the other two.