The explosion of animal life
Cambrian
Paleozoic Era · 538.8 – 485.4 million years ago
The Cambrian Period opened the Paleozoic Era about 538.8 million years ago and lasted roughly 53 million years. Within it, nearly all the major animal body plans alive today appear in the fossil record for the first time, in an episode known as the Cambrian explosion.
Before the Cambrian, life was mostly microbial, with a scattering of soft-bodied Ediacaran organisms. By its end the seas held eyes, jointed legs, shells, grasping limbs and the first large predators, and the basic architecture of the animal kingdom had been laid down.
Key facts
- Began
- 538.8 MaMillion years ago
- Ended
- 485.4 MaMillion years ago
- Duration
- 53.4 million years
- Era
- Paleozoic“Ancient life”
- Global temperature
- ~22°CRoughly 21–22°C · today 14.5°C
- Plants on land
- None on land
- Atmospheric CO₂
- Very high, perhaps ~4,000 ppm (≈ 14× pre-industrial)
- Oxygen
- Lower than today; estimates vary widely
- Sea level
- Rising sharply, flooding continental interiors
- Continents
- Gondwana in the south; Laurentia, Baltica, Siberia apart
- Signature life
- Trilobites, Anomalocaris, Opabinia, early chordates
- Began with
- First complex burrows (Treptichnus pedum)
- Ended with
- Trilobite turnovers and a gradual faunal shift
Ages follow the International Chronostratigraphic Chart (ICS). Global temperatures are rough reconstructions after Scotese et al. (2021); ancient CO₂, oxygen and sea-level values carry large uncertainties.
Epochs of the Cambrian
- Terreneuvian538.8 Ma – 521 Ma
- Series 2521 Ma – 509 Ma
- Miaolingian509 Ma – 497 Ma
- Furongian497 Ma – 485.4 Ma
Continents and oceans
The Cambrian world looked nothing like today's. Most land was gathered into Gondwana, a vast continent assembled shortly before the period began, which stretched from the South Pole towards the equator and included what would become Africa, South America, Antarctica, Australia and India.
Smaller continents drifted separately: Laurentia (the core of North America) straddled the equator, while Baltica and Siberia lay further south. The young Iapetus Ocean was widening between Laurentia, Baltica and Gondwana, and the immense Panthalassic Ocean covered much of the rest of the planet.
Throughout the period sea level rose dramatically, drowning low-lying continental margins and interiors. These broad, sunlit shallow seas created enormous areas of habitat for the new animal life, and their sediments now form the rock layers that record it.
Climate and atmosphere
The Cambrian was generally a greenhouse world with no large permanent ice sheets. Carbon dioxide levels were probably many times higher than today, although estimates from this distant time span a wide range and should be treated as rough.
Oxygen in the atmosphere and oceans seems to have been lower than modern levels and may have fluctuated, with episodes of low-oxygen deep water. Many scientists think a rise in oxygen, combined with ecological arms races between predators and prey, helped make large, active animals possible.
Life in the Cambrian
Life in the Cambrian was almost entirely marine. Over roughly 20–25 million years, animals evolved hard parts, complex eyes and new ways of moving, burrowing and hunting, transforming the seafloor from a microbial mat into a busy ecosystem.
Trilobites
Trilobites appear around 521 million years ago and quickly became the most recognisable Cambrian animals. Their mineralised exoskeletons and compound eyes made of calcite crystals fossilised superbly, and thousands of species are known.
Strange predators
Anomalocaris, a swimming radiodont with grasping frontal appendages and a circular mouth, was one of the largest animals of its day. Opabinia, with five eyes and a long clawed nozzle, remains one of the oddest creatures ever found.
First chordates
Our own lineage was already present. Pikaia from the Burgess Shale is an early chordate, and fish-like animals such as Haikouichthys and Myllokunmingia from China are among the earliest known vertebrates.
Sponge and archaeocyath reefs
In the early Cambrian, cup-shaped sponge-like archaeocyaths built the first animal reefs alongside microbes. They declined sharply by the middle of the period, leaving reef building mainly to microbes for millions of years.
Timeline of key events
- 538.8 Ma
Cambrian begins
The base of the period is marked by the appearance of complex burrows of Treptichnus pedum at Fortune Head, Newfoundland.
- 521 Ma
First trilobites appear
Trilobites enter the fossil record and rapidly diversify across the world's shallow seas.
- 518 Ma
Chengjiang biota
Soft-bodied animals, including early vertebrates, are buried and exquisitely preserved in what is now Yunnan, China.
- 513 Ma
Archaeocyath reefs collapse
An extinction episode, sometimes called the Sinsk or Botomian event, wipes out most reef-building archaeocyaths.
- 508 Ma
Burgess Shale fauna
Mudflows at the foot of an undersea cliff entomb a rich community including Opabinia, Hallucigenia and Pikaia.
- 497 Ma
SPICE event begins
A large carbon-isotope excursion coincides with ocean changes and trilobite extinctions in the late Cambrian.
- 485.4 Ma
Transition to the Ordovician
The first appearance of a key conodont at Green Point, Newfoundland, marks the start of the Ordovician.
How the Cambrian ended
The Cambrian did not close with a single catastrophe. Instead, its later part saw a series of extinction pulses that repeatedly reshaped trilobite faunas, linked to changes in ocean temperature, oxygen and sea level, including a major carbon-isotope shift known as the SPICE event around 497–494 million years ago.
The boundary with the Ordovician, about 485.4 million years ago, is defined at Green Point in Newfoundland by the first appearance of a particular conodont, a tiny eel-like animal with tooth-like elements.
Where to see Cambrian fossils
The most famous site is the Burgess Shale in British Columbia, Canada, about 508 million years old and discovered by Charles Walcott in 1909. The older Chengjiang biota of Yunnan, China, a UNESCO World Heritage Site, preserves soft-bodied animals in astonishing detail.
Other key localities include Sirius Passet in northern Greenland, the Emu Bay Shale on Kangaroo Island in Australia, and the Wheeler Shale of Utah, famous for its abundant trilobites. Natural history museums in Toronto, Washington and London display outstanding Cambrian collections.
Surprising facts about the Cambrian
- Hallucigenia was first reconstructed upside down and back to front: its paired spines were mistaken for legs, and its head was initially identified at the wrong end.
- The land was essentially bare during the Cambrian. Apart from microbial crusts, there were no plants or animals living on the continents, though some animals left trackways on tidal flats.
- Trilobite eyes had lenses made of the mineral calcite, making them among the oldest known complex eyes, built from rock-like crystals rather than living tissue.
- The name Cambrian comes from Cambria, the Latin name for Wales, where the geologist Adam Sedgwick studied these rocks in the 1830s.
- Anomalocaris may have reached around half a metre long, a true giant at a time when most animals were only a few centimetres in size.
- Many of the animal groups that appear in the Cambrian, including arthropods, molluscs, echinoderms and chordates, still dominate the oceans more than 500 million years later.
Where it sits in deep time
The globe beside this article shows Earth 510 million years ago, reconstructed from the PALEOMAP paleo-elevation models. Step to another period and watch the continents drift into place.