By 555 million years ago, animals emerge. The comb jellyfish is one of the earliest known true animals. Originally, we thought simple sponges evolved first, but genetic analyses changed that.

From simple early animals arose one of the deepest splits in animal history. One branch led to cnidarians, including jellyfish, corals, and sea anemones. The other led to bilaterians, the great line of animals with left and right sides. From that bilaterian line came the major aquatic branches, including fish, cephalopods, and arthropods, as well as the major land animals, including insects, amphibians, reptiles, mammals, and birds. Today, jellyfish remain as living heirs of one ancient branch, while we and most familiar animals belong to the other.

Fungi and animal ancestors split 950 mya (single-celled)
Multicell animal ancestors start 640 mya
True Animal Evolution Starts 555 mya

Prokaryotic Life

Archaea look like bacteria at first glance — small, simple, and lacking a nucleus. But they are fundamentally different.

3.73 Billion Years Ago (after LUCA)

Membrane and metabolic diversity.

Archaea Diverge

Both archaea and bacteria are prokaryotes — cells without nuclei.

3.73 Billion Years Ago (shortly after LUCA)

Ether-linked membranes and distinct genetic machinery

Touch: Life Learns to Feel Force

About 3.72 billion years ago, right after LUCA, when cells emerged, touch became the most ancient form of biological sensing: required to physically navigate reality.

~3.72 Billion Years Ago (after prokaryotes)

Mechanical sensitivity to pressure and membrane stretch

The First True Eukaryotes

Eukaryotes divided labor within single-celled life, featuring a true nucleus and membrane-bound organelles. From them branched plants, fungi, and animals.

2.4 Billion Years Ago (+/- 300 million years)

Great Oxidation Event: Third Atmosphere

planet-blue-atmosphere

~2.4 Billion Years Ago

Cause: Cyanobacteria Produce Oxygen

Bacterial Endosymbiosis: Origin of Eukaryotes

About 2 billion years ago, bacteria are added to cells and that group leads to eukaryotes. You are a walking chimera ecosystem made of an Archaea host and trillions of Bacterial power-plants.

~2.4 Billion years ago (+/- 100 million)

Bacteria are added to eukaryote ancestor cells

Protozoa Evolve

About 2 billion years ago, eukaryotic cells are defined by a membrane-bound nucleus and internal organelles. Their emergence created the structural foundation for complex life.

2 Billion Years Ago (+/- 100 million years)

Nucleus and internal organelles

LECA: Likely Sexual Reproduction

LECA is the Last Eukaryotic Common Ancestor. LECA reproduced sexually pushing the mixing of DNA back before 1.75 billion years ago.

~1.75 billion years ago (+/- 50 million)

Last Eukaryote Common Ancestor

Unikonts: Single-Motor flagella Reform

By 1.3 billion years ago, our animal-fungi ancestor, the Unikonts, stopped using two pulling flagella and narrowed it down to one pushing one.

~1.3 Billion Years Ago (+/- 100 million)

Transition to a Single Flagellum

Intracellular Flow and Nutrient Exchange

Eukaryote cells evolve cytoplasmic streaming and cytoskeleton-guided transport systems to circulate nutrients, organelles, and waste internally.

~1.2 Billion Years Ago (+/- 300 million)

Cytoplasmic streaming and vesicle transport

Opisthokonts: True Posterior Flagellum

By 1.15 billion years ago, our animial-fungi ancestor evolve a true posterior flagellum. Single-celled animal sperm has a lineage back to this ancestor.

~1.15 Billion Years Ago (+/- 50 million)

12 unique amino acids + glycogen energy storage + True Posterior Flagellum

Oogamy: Early Gamete Specialization Before Animals

Sperm and egg cell on microscope. Scientific background.

Sexual reproduction predates animals, and differentiated sperm–egg systems evolved in single-celled eukaryotic lineages long before animals emerged.

~1.1 Billion Years Ago (inferred, +/- 100 million)

Small motile gamete and larger nutrient-rich gamete

Animal Ancestors Split Off: Cadherin Cell Glue (Holozoa)

This image shows three plausible body plans for early holozoans, ancestors of animals. A loose spherical cluster suggests early cadherin-based adhesion. The pear-shaped flagellated cell reflects choanoflagellate-like forms. The amoeboid shape represents flexible, crawling types.

~750 Million Years Ago (+/- 50 million)

Snowball Earth: When Ice Reached the Equator

For tens of millions of years, Earth plunged into its deepest known freeze. Ice sheets reached sea level at low latitudes, perhaps even the equator, turning the planet into a near-global ice world and reshaping the path toward complex life.

From 717 million years ago through 635.

Cause: Continental Drift, Falling CO₂

First Multicellular Animals

640 million years ago, the first multicell animals were almost certainly just collections of types of cells and reproduced.

640 Million Years Ago (+/- 20 million)

Stable cell adhesion and tissue specialization

Presentient Animals Emerge: The Ediacaran Prelude

Before the Cambrian Explosion, early animal body shapes were already experimenting. Some evolved into larger, more complex forms, from frond-like patterns to disks and tubes. In this strange world, nerve nets and proto-nervous systems were beginning to appear.

635 to 590 Million Years Ago

Proto-brain; Pre-brain memory; Presentient.

First Animal Egg Layers

~620 Million Years Ago (+/- 20 million)

tissue-level reproduction

Animal Chemoreception: Proto-Taste and Smell Emerge

Chemoreception is older than animals. Found across bacteria, fungi, and plants, it later became the foundation for animal taste and smell as early nerve nets and proto-nervous systems gave rise to brains.

600 Mya

Bilaterian Split: The Origin of Agency

The bilaterian branch gave rise to today's arthropods, mollusks, fish, amphibians, reptiles, birds, and mammals. The significant idea is directionality. From a radial (circle) to a bilateral (line) symmetry, life transitioned from a passive "being" to an active "doing."

590 Million Years Ago (± 10 million)

Agency and directional action with intent.

First True Animals – Comb Jellyfish

Jellyfish moving through water

555 Million Years Ago (+/- 5 million)

Nerve nets and muscle cells

Internal Fluid Transport in Early Animals

Before true circulatory systems evolved, early animals began moving fluids through internal body spaces, helping nutrients reach deeper tissues and waste move out.

~550 Million Years Ago (+/- 25 million)

Diffusion and body-cavity circulation

True Circulatory Systems: Blood Veins

545 Million Years Ago (after through-gut digestion: mouth/anus)

Circulatory fluid transport (veins)

Vision Emerges: The Pre-fish Chordates

540 Mya

Vision Emerges; Proto-Simple Brains; Pre-vertebrate Cord.

Paleozoic Era: The Age of Synapsids

The Paleozoic era is marked by the rise of complex animal life 538.8 million years ago. It ends with the end-Permian mass extinction 252 million years ago. A volcanic cascade global warming event.

From 538.8 to 251.902 million years ago.

287 Million years: From burrowing to extinction.

First Vertebrates

530 Million BCE

530 to 520 Million Years Ago

Earliest Known Hunter

520 Million Years Ago

First Simple Brains; Proto-Short-Term Memory; Simple Sentience.

Simple Sentience Settles: Haikouichthys

520 Million BCE

Simple Brains; Proto-Short-Term Memory; Simple Sentience.

Simple Cephalopod Sentience Evolves

510 Million BCE

Not a fish ancestor, not our ancestor.

Asaphid trilobite

An Ordovician asaphid-like trilobite from an ancient shallow sea, broad-bodied and well armored, representing one of the many early arthropod forms that flourished long before life moved onto land.

Lived from 470 to 445 million years ago.

Ordovician–Silurian Extinction: Ice Strikes the Seas

The Ordovician–Silurian extinction shows how climate change can reshape evolution by collapsing old ecosystems and opening space for new life.

~444 Million Years Ago

Cause: Global Cooling and Falling Seas

Keratin Genes & the Rise of Scales in Fish

425 Million Years Ago

425 MYA (+/- 15 Million Years)

Oldest Known Air Breather

414 Million BCE

Lungs Evolve: Lobe-Finned Fish and the Lungfish Ancestor

Living fossil fish, Coelacanth.

400 Million BCE

The Senegal Bichir: A Living Fossil

An early-diverging ray-finned fish whose lobed pectoral fins and paired lungs reflect an ancient branch of bony fish evolution.

~380 million years ago (± 15 million)

Air breathing lungs and lobed pectoral fins

Long-Term Memory Evolves: Tiktaalik

375 Mya

Complex Brains; Long-Term Memory; Simple Sentience.

Oceans Lose Their Breath

The Devonian extinction shows that evolution can be reshaped not by one sudden blow, but by a long collapse in ocean health.

~372–359 Million Years Ago

Cause: Ocean Anoxia

Land Hearing Emerges: Amphibians

370 Mya

Land Hearing Emerges; Yet Larger Brains.

Amniotes Emerge: Amniotic Eggs

Corn snake hatching, Pantherophis guttatus guttatus, also know as red rat snake

The amniotic egg evolved in the first amniotes, which evolved into today's reptiles, birds, and mammals.

340 Million years ago (+/- 10 million)

Ancestor or reptiles, birds, and mammals.

First Land Herbivore: Tyrannoroter heberti

Tyrannoroter heberti (≈307 million years ago). One of the earliest known plant-experimenting tetrapods, Tyrannoroter heberti hints that land herbivory began not with giants, but with small, evolutionary pioneers over 300 million years ago.

307 million years ago

2026 Discovery Pushing Back Herbivores

Stem Selachians: Modern Sharks LCA

Note the torpedo-shaped body and sweeping tail — a hydrodynamic design that would define sharks for hundreds of millions of years: tapered head, dorsal fin, and powerful tail.

~300 million years ago (± 10 million years)

Proto-Play

Proto-play emerged in animals as brains got more complex about 300 million years ago. Something like enjoyment or satisfaction evolved as animals mimicked survival-like skills.

300 million years ago

±20 million years

Early Complex Sentience Emerges: Dimetrodon

By about 280 million years ago, Dimetrodon was one of the best-known predators of the Early Permian. It stalked rivers and floodplains alongside caseid synapsids, large amphibians like Eryops, and a landscape of Calamites, Sigillaria, ferns, and early seed plants.

295 Million BCE

Complex Brains; Long-Term Memory; Early Complex Sentience.

The Synapsid World of the Late Permian

A Late Permian river world about 255 million years ago, where synapsids still ruled the land. A gorgonopsid stalks near the water while dicynodonts gather at the river’s edge and pareiasaurs move through the floodplain, alongside amphibians, large insects, and hardy pre-flowering plants.

255 Million years ago.

The P-T Extinction

The Permian-Triassic extinction was not just the end of many species. It was a planetary reset that destroyed the old synapsid-dominated world and opened the door for the archosaur line that would later give rise to dinosaurs.

251,902,000 years ago (+/- 900 years).

Cause: Massive Volcanic Eruptions in Siberia

Mesozoic Era: Age of Dinosaurs

The Mesozoic era starts with the end-Permian mass extinction 252 million years ago. It ends the reign of dinosaurs with the K–Pg extinction 66 million years ago.

From 251.902 to 66.0 million years ago.

186 Million years: Dinosauria reigned from extinction to extinction.

Archosauria Diverge Within Reptiles

LCA of crocodiles and birds — the larger archosaur branch that later gave rise to crocodilians, pterosaurs, dinosaurs, and birds.

~250 million years ago (+/- 10 million)

LCA of crocodilians and birds (pterosaurs & dinosauria)

Bird-line Archosaurs: Asilisaurus kongwe (within Ornithodira)

Asilisaurus shows that the bird-line archosaurs were already evolving dinosaur-like bodies before true dinosaurs appeared.

245 Million Years Ago

Dinosauromorphs Emerge: Erect hind-limb posture leads to birds and dinosaurs.

Bird-line Archosaur: Nyasasaurus parringtoni

Nyasasaurus is a late bird-line archosaur from just before Dinosauria clearly emerge. It sits on the dinosaur side of Ornithodira, but its exact placement remains uncertain: some analyses place it within Dinosauria, while others place it just outside the group, near other bird-line archosaurs.

243 Million Years Ago

Strengthened hip and shoulder architecture

XX/XY Sex System Emerges: A Tale of Mammalian Evolution

Around 240 million years ago, during the late Triassic period, a crucial evolutionary development unfolded within the lineage that would give rise to mammals. It is believed that the XX/XY sex-determination system emerged in a common ancestor of mammals, possibly within the genus Therapsida, a group of synapsids that exhibited both reptilian and mammalian traits.

240 Million Years Ago

Marasuchus lilloensis

"<a href="https://commons.wikimedia.org/w/index.php?curid=50251453" target="_blank" rel="noopener noreferrer">Marasuchus NT small</a>" by <a href="https://commons.wikimedia.org/wiki/User:NobuTamura" target="_blank" rel="noopener noreferrer">Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/</a> is licensed under <a href="https://creativecommons.org/licenses/by-sa/4.0/" target="_blank" rel="noopener noreferrer">CC BY-SA 4.0</a>

240 Million Years Ago

Elongated hind limbs, better running, reduced forelimbs

Dinosauria Emerge: True Dinosaurs!

Dinosauria emerge from a single population of a species about 238 million years ago. This population will lead to all dinosaurs and birds including T.Rex, Brontosaurus, and Triceratops.

238 Million years ago (+/- 5 million)

Fully open hip socket (perforated acetabulum)

Pterosaurs Diverge From Dinosaur Ancestors (within Ornithodira)

Pterosaurs were the first vertebrates to conquer the air, evolving a unique "finger-wing" anatomy that allowed them to dominate the skies for 160 million years.

~237 million years ago (+/- 2 million)

Pterosauria line: Not in dinosauria (split first).

Theropod Line Diverges Within Dinosaria (from Saurischia)

Theropods were the agile, sharp-toothed dinosaur branch that refined the classic predator body plan. They stood fully upright on two legs, balanced with long tails, used grasping hands, and carried specialized skulls and recurved teeth built for active hunting. Over time, this branch produced everything from small early predators to giant hunters—and eventually birds.

~233 million years ago (±2 million years)

Ancestor of T.Rex and bird-line.

Sauropodomorph Line Diverges Within Dinosaria (from Saurischia)

Sauropodomorphs, in their early forms, were lightly built, often partly bipedal, with long necks, small heads, leaf-shaped teeth, and grasping hands.

~232 million years ago (±2 million years)

Ancestor to the sauropods like brontosaurus.

Ornithischians Diverge Within Dinosauria “Bird Hipped”

Ornithodira is the broader branch that includes dinosaurs, birds, and pterosaurs.

~229 million years ago (±4 million years)

LCA of Pterosaurs and Birds (pterosaurs & dinosauria).

Eoraptor lunensis.

A speculative reconstruction of Eoraptor lunensis. Eoraptor reminds us that classification is not always neat at the beginning of a lineage. Early dinosaurs can be hard to classify because of a mix of traits.

229 Million Years Ago (± 1.5 million)

Platypus–Ape Common Ancestor

The last common ancestor of platypus and ape lived around 225 million years ago, during the Late Triassic. It wasn’t a modern mammal yet, but a very late synapsid—or early mammaliaform—already carrying the essentials: fur for insulation, a warm-blooded metabolism, and mammal-style teeth. From this small, generalized creature, every living mammal would diverge—both the strange egg-laying platypus and the most self-aware ape.

225 million years ago (±5 million years)

When dinosaurs rose, our line quietly began.

Pterosaurs Emerge

Pterosaurs were not dinosaurs but do share a common ancestor. They are a distinct group of flying reptiles that emerged in the Late Triassic.

215 Million years ago (+/- 5 million)

Morganucodon: An Early Mammalian

Morganucodon is an example of a plant eater likely similar to our direct-line ancestors around this time. It is not a direct human ancestor but is among the early mammaliaforms, close to the lineage leading to true mammals.

203 Million BCE (+/- 3 million)

Differentiated teeth and true mammalian jaw

Triassic–Jurassic Extinction: Volcanoes Open the Age of Dinosaurs

As Pangea cracked apart, massive volcanic eruptions poisoned air and oceans. This image includes early dinosaurs as foreshadowing: survivors waiting in the smoke before their Jurassic rise.

~201 Million Years Ago

Cause: Massive Volcanic Eruptions

Early Play Evolves in Mammals

Play evolved as one of the group survival traits. Lower play abilities evolved in mammals like rodents about 190 million years ago. Higer play abilities evolved in mammals like cats about 80 million years ago.

190 Million Years Ago (+/- 10 million years)

Parental care, brain plasticity, extended juvenile period

Mammals: First Live Births

An example of early live birth is the protomammal Kayentatherium, Jurassic period. This cynodont is related to early mammals and its clutch size suggested egg-laying, providing clues about the transition to live birth. The switch to live birth in mammals, including marsupials and placentals, likely evolved once at their common ancestor, suggesting live birth in mammals has a deep evolutionary history.

185 Million BCE

Placental nutrient transfer (in placentals)

Diplodocid LCA: The Age of Giant Necked Sauropods

The common ancestor of the diplodocids is still unknown, but it gave rise to several distinct giant-necked forms. In Diplodocus, notice the long, narrow skull. In Apatosaurus, note the deeper, more robust skull and heavier build. Finally, in the slimmer Brontosaurus, notice the similar shape but somewhat lighter, less massive form.

~178 Million Years Ago (+/- 5 million)

Proceratosaurus (T.Rex ancestor)

Proceratosaurus had the same general tyrannosaur-style look: a big head, long tail, strong hind legs, short forelimbs, and a built-for-biting predator shape: D-shaped front teeth and a crest on top of the skull.

Lived from 169 to 164 million years ago.

Not a bird ancestor, but part of the theropod mix.

Bashanosaurus primitivus

Bashanosaurus primitivus is one of the earliest known stegosaurs and a strong candidate for representing an early form close to the ancestry of later plated dinosaurs like Stegosaurus.

~168 million years ago.

Stegosaurus ancestor

Diplodocus

Diplodocus was the longer, slimmer, more stretched-out sauropod, famous for its especially long neck and whiplike tail. It is one of the longest land animals and generally more slender in build.

Lived ~161 to 146 million years ago.

28–33.5 m long: longer, whiplike, slimmer.

Brontosaurus

Brontosaurus, in the revived interpretation, looks broadly similar to Apatosaurus but is argued to be less massive and less robust. A bit lighter-built overall.

Lived ~156 to 145 million years ago.

20 to 22 meters (65 to 72 feet).

Apatosaurus

Apatosaurus was the heavier, more robust sauropod — more muscular-looking, with a thicker, lower-set neck and a bulkier frame.

Lived ~156 to 151 million years ago.

21 to 23 meters (69 to 75 feet): Heavier, more muscular.

Avialae: The Bird Line Diverges (Theropoda)

Around 155 million years ago, an early avialan was probably already broadly Archaeopteryx-like, yet still unmistakably dinosaurian: small feathered theropod with teeth, claws, and a long bony tail.

155 Million years ago (+/- 5 million)

Stegosaurus

Stegosaurus is the classic plated dinosaur most people picture: large back plates, a small head, and a spiked tail used for defense. It lived late in the Jurassic

Lived 152 to 145 million years ago.

First True Bird: Archaeopteryx

Archaeopteryx reminds us that major transformations often happen gradually, with old traits and new traits living side by side for a long time. The story of birds began with their dinosaur past.

149 Million years ago (+/- 1 million)

Confuciusornithiformes Birds Emerge (Now Extinct)

Weird carryovers and side experiments: clawed wings, elaborate ribbon-like tail feathers, and a mix of advanced beak features with a still primitive dinosaurian body.

~131 Million years ago.

Extinct bird line (clawed wings, elaborate ribbon-like tail feathers)

Enantiornithes Birds Emerge (Now Extinct)

Enantiornithes were one of the most successful early bird branches of the Cretaceous, but unlike Confuciusornithiformes, they generally kept their teeth and often looked a bit more like small, sharp-faced bird-dinosaurs than beaked proto-birds.

~131 Million years ago.

Extinct bird line (clawed wings, teeth)

Complex Sentience Settles: Eomaia scansoria

The rise of Eomaia scansoria, an early placental mammal, marks a definitive leap towards "Complex Sentience" in the evolutionary saga leading to humans. It's also plausible that it possessed a foundational level of self-awareness, or what can be termed as Proto Self-awareness. A rudimentary sense of self.

circa 125 Million BCE

Complex Brains; Long-Term Memory; Complex Sentience; Likely Proto Self-aware.

Animal Vocabulary: Dozens of Words

Eomaia scansoria in their natural environment from about 125 million years ago. These early mammals likely lived in a lush, prehistoric forest setting and had a vocabulary, or signaling, range into the dozens of words, well, gestures.

125 Million Years Ago

Expanded Limbic System, reward circuitry

The Last Stegosaurus: Wuerhosaurus

Wuerhosaurus was one of the last known stegosaurs, carrying the classic plated-and-spiked body plan into the Early Cretaceous of what is now China.

110 Million years ago (+/- 10 million)

Argentinosaurus

Argentinosaurus shows how far the sauropod body plan could go. By the Late Cretaceous, some titanosaurs had become the largest land animals known, turning the long-necked dinosaur design into one of evolution’s most extreme achievements.

Lived from 97 to 93.5 million years ago.

Zuniceratops

A smaller, earlier horned dinosaur that helps show the transition toward the larger, more elaborate ceratopsids.

Lived from about 90 to 89 million years ago.

Triceratops ancestor

Hesperornithiformes Birds Emerge (Now Extinct)

Hesperornithiformes were extinct Cretaceous diving birds that lived outside modern bird crown-group Neornithes. They show that highly specialized aquatic birds had already evolved by about 90 million years ago.

~90 Million years ago.

Neornithes (Crown Birds) emerge

Three branches of modern birds evolved from within neornithes: Struthio camelus (the ostrich), Gallus gallus (chickens), and Passer domesticus (the house sparrow is a good one).

~90 Million years ago (+/- 10 million).

Pteranodons Emerge

Pteranodon was one of the great soaring pterosaurs of the Late Cretaceous, but it was only one branch in a much larger pterosaur story.

~88 million years ago (+/- 4 million)

Palaeognathae Birds Emerge

Palaeognathae is the living bird branch that includes ostriches, emus, cassowaries, rheas, kiwis, and tinamous.

~85 Million years ago (+/- 10 million).

Ground birds: ostriches, emus, tinamous, etc.

Appendix

The appendix is an example of a Phenotype Variation -- a trait that varies among individuals. In fact, something like 1 in 100,000 people are born without an appendix.

80 Million BCE

Higher Play Evolves in Social Mammals

80 Million Years Ago (+/- 10 million years)

Enlarged neocortex

Galloanserae Birds Emerge (from Neognathae)

Galloanserae is the living bird branch that includes landfowl and waterfowl: chickens, turkeys, pheasants, ducks, geese, and swans.

~80 Million years ago (+/- 8 million).

Fowl: chickens, turkeys, pheasants, ducks, etc.

Neoaves Birds Emerge (from Neognathae)

Neoaves is the enormous living bird branch that includes all birds that are not part of the ostrich-tinamou branch and not part of the duck-chicken branch.

~78 Million years ago (+/- 4 million).

Led to common birds: crows, sparrows, robins, hawks, owls, hummingbirds, etc.

Styracosaurus

Styracosaurus was a striking horned ancient cousin of Triceratops, showing that ceratopsids branched into different styles long before the dinosaurs came to an end.

Lived ~76 to 75 million years ago.

Tyrannosaurus Rex

T. rex lived in western North America about 69 to 66 million years ago. All dinosaurs, except potentially three lines of bires, went extinct 66 million years ago, when the Chicxulub asteroid hit.

Lived from ~69 to 66.04 million years ago.

Triceratops

Three facial horns, broad frill, and powerful four-legged body. It was one of the last great non-avian dinosaurs and is the classic fully developed ceratopsid most people picture when they think of horned dinosaurs. Lived from about 68 to 66 million years ago.

Lived from ~68 to 66.04 million years ago.

The Last Pterosaurs

By the end of the Cretaceous, the surviving pterosaurs were mostly advanced, toothless pterodactyloids.