Phylogenetic Tree of Life

Every organism alive today — from the bacterium in your yogurt vat to the blue whale — shares a common ancestor. The tree of life maps those relationships across 4 billion years. You are a twig on one of its youngest branches.

👆 Click any branch or node

Explore the tree of life — click any labeled group to learn when it evolved, what organisms belong to it, and why it matters.

Click any labeled group on the tree — branches, domains, kingdoms, or phyla

A phylogenetic tree is a hypothesis — not a fact. It represents our best current understanding of evolutionary relationships based on genetic, anatomical, and fossil evidence. Trees get revised as new data comes in.

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The biggest mistake people make reading trees

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The most common error is reading tips left-to-right and assuming the one on the far right is the "most evolved" or "most advanced." This is completely wrong. Every tip on a phylogenetic tree represents a lineage that has been evolving for the same amount of time — they're just evolving in different directions. Bacteria are not "primitive" — they are extraordinarily well-adapted organisms that have been evolving for 3.5 billion years. Humans are not the pinnacle of evolution — we are one recent twig on one branch of the Animalia kingdom.

💡 Also remember: you can freely rotate any branch at any node without changing the meaning of the tree. The positions of tips relative to the page don't matter — what matters is which node they share.

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Monophyletic, paraphyletic, polyphyletic — know the difference

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Monophyletic (clade): An ancestor + ALL of its descendants. The "correct" grouping in modern taxonomy. Example: Mammals = all descendants of the first mammal ancestor. ✓

Paraphyletic: An ancestor + SOME (not all) descendants. Example: "Reptiles" traditionally excludes birds — but birds are descended from reptiles, so traditional "Reptiles" is paraphyletic. ⚠️

Polyphyletic: A group assembled from multiple unrelated ancestors. Example: "Warm-blooded animals" includes birds + mammals, which evolved warm-bloodedness independently. ✗

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How phylogenies are built

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Modern phylogenies are built primarily from DNA and RNA sequence data — comparing gene sequences between organisms and calculating how many mutations separate them. The more similar the sequences, the more recently the organisms shared a common ancestor. Scientists also use morphological traits (body structure), fossil records, and biochemical data. Multiple lines of evidence are combined to build the most accurate tree possible. The field is called molecular phylogenetics, and it has completely revolutionized our understanding of how life is related — including revealing that Archaea are more closely related to Eukarya than to Bacteria.

4.6 billion years of Earth history — life appeared surprisingly early and stayed microbial for most of it. Everything you can see with the naked eye evolved in the last 600 million years — the final 13% of Earth's history.

4.6 bya Earth forms ▼

Earth coalesces from solar nebula material. The young planet is molten, bombarded by asteroids, and has no oxygen in its atmosphere — mostly nitrogen, CO₂, methane, and ammonia. The conditions are nothing like today, yet life will emerge within 600 million years.

~4.0 bya LUCA — Last Universal Common Ancestor ▼

The single ancestral cell from which ALL life on Earth descends. LUCA was not the first living thing — just the common ancestor of everything alive today. It had DNA, ribosomes, and a cell membrane. We can infer its characteristics by looking at genes shared across all three domains of life. LUCA likely lived in hydrothermal vents on the ocean floor.

~3.5 bya First prokaryotes — Bacteria and Archaea diverge ▼

The earliest fossil evidence of life — stromatolites (layered mats built by microbial communities). Bacteria and Archaea diverge into separate domains. These are prokaryotes: no nucleus, no membrane-bound organelles. They are the dominant form of life on Earth for the next 2 billion years. The world belongs to microbes.

~2.7 bya Cyanobacteria begin producing oxygen ▼

Cyanobacteria evolve oxygenic photosynthesis — using sunlight, water, and CO₂ to produce glucose and O₂. For millions of years oxygen builds up in the ocean, where it rusts iron (producing the banded iron formations we mine today). Then it starts entering the atmosphere — an event so catastrophic for anaerobic life it's called the Great Oxidation Event. For most organisms alive at the time, oxygen was poison. For us, it's everything.

~2.4 bya Great Oxidation Event — atmosphere transforms ▼

Oxygen concentrations in the atmosphere rise dramatically, triggering the largest mass extinction in Earth's history (for anaerobic organisms). But it also opens the door to aerobic respiration — a far more efficient way to extract energy from food. Organisms that can use oxygen gain an enormous metabolic advantage. This single event shaped the trajectory of all complex life.

~1.5–2 bya First eukaryotes — endosymbiosis ▼

A momentous event: an Archaean cell engulfs a bacterium — but instead of digesting it, keeps it alive. That bacterium becomes the mitochondrion. Later, some cells engulf cyanobacteria — those become chloroplasts. This is the endosymbiotic theory, supported by the fact that both organelles still have their own circular DNA, divide by binary fission, and have double membranes. The first eukaryotes have nuclei, membrane-bound organelles, and dramatically more complexity than anything that came before.

~1.2 bya First multicellular organisms ▼

Cells begin cooperating rather than competing. Early multicellular organisms appear — algae, then simple animals. Cell differentiation evolves: different cells in one organism specialize for different jobs. This division of labor allows far greater complexity than any single cell could achieve. Once multicellularity evolves, the door opens to every animal, plant, and fungus that has ever lived.

~541 mya Cambrian Explosion — animal body plans appear ▼

In a geological eyeblink (~20 million years), most major animal body plans appear in the fossil record for the first time. Eyes, limbs, shells, exoskeletons, nervous systems — the basic blueprints for virtually every animal alive today. The trigger is debated: rising oxygen? The evolution of eyes? Ecological arms races? Whatever caused it, the Cambrian Explosion is the most dramatic diversification event in the history of animal life.

~252 mya Permian-Triassic extinction — 96% of species lost ▼

The "Great Dying" — Earth's most severe mass extinction event. Volcanic eruptions in what is now Siberia released massive amounts of CO₂, triggering rapid climate change, ocean acidification, and oxygen depletion. An estimated 96% of marine species and 70% of terrestrial vertebrate species went extinct. It took 10 million years for biodiversity to recover. The few survivors went on to diversify into the ecosystems we know today — including the ancestors of dinosaurs and mammals.

~66 mya K-Pg extinction — dinosaurs gone, mammals rise ▼

An asteroid ~10 km wide strikes the Yucatán Peninsula (now Mexico), triggering global firestorms, a "nuclear winter" from debris blocking sunlight, and the collapse of food chains worldwide. Non-avian dinosaurs go extinct (birds survive — they are dinosaurs). About 75% of all species are lost. The small, warm-blooded, opportunistic mammals that survived diversify explosively into every ecological niche left vacant. 66 million years later, one of their descendants is studying this event in BIO 111.

~300,000 ya Homo sapiens evolve in Africa ▼

Modern humans appear in East Africa. If Earth's entire 4.6-billion-year history were compressed into a single year, Homo sapiens would appear at 11:58 PM on December 31st — with less than 2 minutes to go. All of recorded human history — agriculture, writing, science, medicine, the internet — fits into the last few seconds of that year. We are extraordinarily new. And we have already altered the planet in ways that will be visible in the fossil record millions of years from now.

Click any event to expand · mya = million years ago · bya = billion years ago

Taxonomic classification — the system for organizing all life. Each rank is more specific than the one above. Click any organism below to see its full classification.

The hierarchy — broadest to most specific

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Choose an organism to classify

Select an organism above to see its full classification and how it fits into the tree of life.

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The mnemonic — Dear King Philip Came Over For Good Soup

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Dear → Domain

King → Kingdom

Philip → Phylum

Came → Class

Over → Order

For → Family

Good → Genus

Soup → Species

Binomial nomenclature (the scientific name) uses Genus + species — always italicized. Homo sapiens, Streptococcus thermophilus, Escherichia coli. Genus is capitalized, species is lowercase. This system was invented by Carl Linnaeus in 1758 and is still used worldwide in every language.

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Why classification keeps changing

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Traditional classification was based on morphology (body shape) — which grouped organisms by what they look like. Modern classification uses phylogenetics — grouping by evolutionary relationships. These often disagree. Fungi look more like plants but are actually more closely related to animals. Birds are technically reptiles by phylogenetic standards. Whales are mammals that look like fish. As DNA sequencing gets cheaper and more comprehensive, classifications are updated regularly. What your textbook says may already be outdated — that's science working correctly.

Key vocabulary

Phylogeny

The evolutionary history and relationships among organisms or groups. A phylogenetic tree visualizes these relationships.

LUCA

Last Universal Common Ancestor — the single ancestral cell from which all life on Earth descends. ~4 billion years ago.

Clade

A group consisting of an ancestor and ALL of its descendants. The fundamental unit of modern classification.

Sister taxa

Two groups that share the most recent common ancestor — the closest relatives on a phylogenetic tree.

Endosymbiosis

The theory that mitochondria and chloroplasts were once free-living prokaryotes engulfed by early eukaryotic cells. Explains their own DNA, double membranes, and binary fission.

Binomial nomenclature

The two-part scientific naming system: Genus species. Always italicized. Invented by Linnaeus in 1758.

Molecular phylogenetics

Building evolutionary trees using DNA/RNA sequence comparisons rather than physical traits alone.

Exam reminders

The three domains are Bacteria, Archaea, Eukarya. Archaea are more closely related to Eukarya than to Bacteria — this surprises everyone.

Endosymbiotic theory evidence: mitochondria and chloroplasts have their own circular DNA, divide by binary fission, and have double membranes.

Reading trees: position doesn't imply advancement. Every tip has been evolving the same amount of time. Bacteria are not primitive.

A clade = ancestor + ALL descendants. If you exclude any descendant, it's paraphyletic (not a valid clade in modern taxonomy).

Scientific names: Genus capitalized, species lowercase, always italicized. Homo sapiens, E. coli.

The Cambrian Explosion (~541 mya) produced most major animal body plans in a geologically short time. Know this date.

The 3 domains at a glance

Bacteria — prokaryotes, peptidoglycan cell walls, 70S ribosomes, circular DNA. E. coli, S. thermophilus, Cyanobacteria.

Archaea — prokaryotes, unique membrane lipids, no peptidoglycan, extremophiles. Methanogens, halophiles, thermophiles.

Eukarya — nucleus, membrane-bound organelles, 80S ribosomes. Protists, Fungi, Plantae, Animalia.