The Cambrian Period: How animals exploded onto the scene

How and why the Cambrian explosion occurred has been the subject of debate ever since fossils of this remarkable event were first discovered.

What was the Cambrian explosion?

The Cambrian Period, 539 million to 485 million years ago, is considered so significant that the four billion years before it are often lumped together as simply the Precambrian. During the Cambrian’s early ‘explosion’ phase, animals diversified into full-bodied creatures with a range of body plans and lifestyles.

These body plans define what scientists call phyla – the major groups that we categorise all animal life into. There are nearly 40 animal phyla alive today. It’s long been thought that the majority of these appeared during the Cambrian Period – although fossils from China now indicate that some key groups may actually have appeared before the Cambrian began.

The largest phylum – the one that contains the most species – is Arthropoda. Arthropods have segmented bodies, tough exoskeletons and jointed appendages, such as antennae, mouthparts and legs.

Trilobites are the characteristic arthropods of the Cambrian. They evolved rapidly and over time displayed a great variety of eyes, armour and sizes. Over their 270-million-year existence, around 20,000 species of trilobites evolved, but they’re all extinct today. Living arthropods include all modern-day insects, arachnids, such as spiders and ticks, and crustaceans, such as crabs, shrimp and lobsters.

Other phyla that arose during the Cambrian include Mollusca, which today includes snails, squid and limpets, and Echinodermata, which includes starfish, sea urchins and sea lilies. Segmented worms in the phylum Annelida, mud-burrowing worms in the phylum Priapulida and dozens of others, many of which are also worm-like, arose too.

Our own phylum, Chordata, also first appeared in the Cambrian. Chordates at this time included small jawless fishes, which were the first animals with backbones.

Not all animals rode the wave of the Cambrian explosion. For example, colonies of Bryozoa, also known as ‘moss animals’, don’t appear to have evolved their unique body plan and lifestyle until the following Ordovician Period, 485 million to 443 million years ago. Bivalves, a group of shelled animals within Mollusca that include modern clams and oysters are known from some Cambrian fossils, but also didn’t diversify until the Ordovician.

How long did the Cambrian explosion last?

The Cambrian Period lasted from around 539 million to 485 million years ago. However, the ‘explosive’ phase, where animals rapidly evolved, is thought to have only lasted for the first 20 million years of the period.

Researchers discovered this with the help of trilobites. Despite being abundant in the Cambrian, their fossils pose a puzzle. They seem to appear suddenly in the fossil record, with great diversity, around 521 million years ago.

Scientists know that trilobites were around and evolving before this, as they’ve found traces of them walking on and digging into sandy and muddy seabeds. However, they didn’t yet have their hard exoskeletons, meaning their bodies didn’t fossilise. This makes it hard to know how they – and all the other Cambrian animals – were evolving over time.

When trilobites did begin to fossilise, we can see that they were abundant and diverse. This makes them useful for tracking evolutionary changes. Dr Greg Edgecombe, one of our merit researchers, has studied the different branches of the trilobite evolutionary tree to estimate how rapidly their features changed over time.

This has helped us to figure out how quickly animals were evolving during the Cambrian, while also giving clues as to when the last common ancestor to all trilobites lived.

Greg and his team discovered that this ancestor likely lived not long before the sudden diversity seen in the fossil record. Yet the rate of evolution of trilobites throughout most of the Cambrian was surprisingly stable.

“There was a very short burst of accelerated evolution at the start, then it flat-lined for the rest of the Cambrian,” says Greg. It means that rather than there being a long and slow evolutionary process throughout the Cambrian, it was more of a quick spurt at the start, during which all the major animal body plans came into being.

What was the world like before the Cambrian?

Understanding life before the Cambrian is tricky, as animals only had ‘soft’ bodies, which were less likely to be fossilised. Single-celled organisms had existed for billions of years and were sometimes found in colonies, such as the famous ‘stromatolites’, which still live in Australian lagoons.

In the period immediately before the Cambrian – known as the Ediacaran – animals first appeared, some of them making meandering trails as they grazed on the seafloor. Others are known from fossils with wide, soft bodies, resembling pillows or bags, that still baffle palaeontologists. They don’t look like any animal types in the rest of the fossil record or any alive today.

Did complex life appear before the Cambrian?

The Cambrian Period has typically been thought of as the defining moment in the diversification of large, multicellular lifeforms, including animals. However, new discoveries are shifting our understanding of how and when animals first appeared, pushing their roots further back in time.

A 2026 study of hundreds of fossils from Yunnan Province, China, suggests that an explosion of animals may have occurred some four million years earlier than previously thought. This was near the end of the preceding Ediacaran Period, meaning some key groups may actually have evolved before the Cambrian began.

Among the new Ediacaran fossils are early bilaterians – worm-like animals with bilateral symmetry – with some showing signs of sophisticated feeding adaptations. There also appears to be early relatives of echinoderms. Some of the forms are completely new while others look like species we see in Cambrian rocks, which builds on our understanding of the transition between these periods of time.

What animals lived in the Cambrian seas?

Although the Cambrian was the origin of most of the animal body plans we see today, it doesn’t mean they’re immediately recognisable. Here are five of the weirdest animals found in the Cambrian oceans.

Anomalocaris

At a time when most marine animals were small, the early arthropod Anomalocaris grew up to 56 centimetres in length. It had large eyes on stalks, each containing more than 24,000 lenses, giving it sharp vision for hunting prey.

This top predator’s features earned it the nickname the ‘great white shark’ of the Cambrian oceans. However, new evidence shows it wasn’t the terror of trilobites it was once rumoured to be, and instead snacked on soft-bodied prey.

Kylinxia

Today, most animals have two eyes while insects tend to have five, however, the unusual Cambrian arthropod Kylinxia had just three. It’s an arrangement not seen in any living arthropods today.

Scientists think Kylinxia represents an early branch of the arthropod family tree, before the last common ancestor of all living members of the group evolved. This suggests that many shared characteristics of these animals, such as their head shield covering several segments, evolved during the Cambrian explosion.

Hallucigenia

The name says it all with this one! Although discovered in the early 1900s, the first 3D reconstruction of Hallucigenia wasn’t put together until 1977. It showed a creature standing on stilts on the muddy seafloor, reaching into the water with tentacles to catch food.

When better fossils were discovered in China, experts realised they had things upside-down. The stilts were actually spikes and the tentacles were paired rows of legs, like on their closest living relatives, velvet worms, which belong to the phylum Onychophora.

Balcoracania

It’s hard to pick just one trilobite species out of the thousands that once thrived in Earth’s oceans, but Greg says his favourite is probably Balcoracania dailyi.

“It’s an early Cambrian species from a place where I’ve been working for many years – Kangaroo Island, off the South Australian coast,” he says.

“It holds the world record for the most segmented body of any trilobite known – we counted 103 segments in its thorax, the part of the body between the head and the abdomen. Nothing else comes close!”

Wiwaxia

Wiwaxia was a spiky customer. It had a soft body armoured with scales and spines to protect it from predators. While these parts are sometimes found on their own, the Burgess Shale deposit in Canada contains hundreds of complete specimens. Even with this abundant evidence, scientists still debate whether Wiwaxia is a member of the annelid worm group or the muscular-footed phylum Mollusca.

What caused the Cambrian explosion?

Greg explains that “the Cambrian is a time of body plan innovation and of diversification, but it’s also an ecological phenomenon as life responded to changing environmental conditions.”

There were many such changes during this period, combined with new genes that regulated body patterns and segmentation. Here are several of the key environmental factors that likely led to the great explosion of life.

Oxygen

One key factor may have been the increase in free oxygen in the oceans. Oxygen had been rising for a couple of billion years since the evolution of photosynthesis but was at first quickly bound to minerals such as iron.

Once all the iron was bound and used up, oxygen began to exist as free molecules in the atmosphere. Atmospheric levels continued to rise till it began to mix into the oceans, eventually perhaps reaching a threshold that could sustain complex animal life.

Minerals

Another factor could be the increase in certain mineral-forming elements in the oceans – particularly calcium and phosphorus, which help animals build hard parts such as shells and exoskeletons. These minerals may have been released from volcanic vents in the seafloor or from the erosion of great mountains on land.

The predator-prey arms race

Animals evolving at this time moved into new areas – sometimes literally, with mobile organisms more able to take advantage of better conditions than their fixed cousins. Many other innovations during this time are thought to have upped the ‘arms race’ between predators and prey. For example, animals employed a range of spines, armour and other defences to avoid capture by animals that evolved complex eyes, shell-crushing mechanisms and other new ways to bite.

Food webs

Animals respond to changing environmental conditions, but they also change the environment themselves. For example, animals started burrowing at this time, stirring up sediments on and oxygen within the seafloor.

Plankton – microscopic animals and plants that form the base of the food web – also grew larger, providing a food source for more animals throughout the water column. These changes opened up new ecological niches – new areas where it was possible to live.

Where are Cambrian fossils found?

For the Cambrian Period, the most famous fossil site is the Burgess Shale, which was discovered in the mountains of British Colombia, Canada, in 1909. Initially, the animal forms preserved within were described as close relatives of living groups, but decades later were thought to include “weird wonders”, some of them considered to be extinct phyla.

The collection of fossils here is known as a Konservat-Lagerstätte – a German phrase referring to exceptional preservation of both hard and soft animal parts. The Burgess Shale was likely formed when a huge mudslide rapidly buried a large area of the seafloor. It preserves a snapshot of life 508 million years ago, in the middle Cambrian, and revealed many of the period’s characteristic animals for the first time.

The Chengjiang Fossil Site in China was discovered later, but preserves an earlier phase of the Cambrian, 518 million years ago. This is within the Cambrian explosion era and records one of the earliest complex ecosystems we know about.

Several other important Cambrian fossil sites are known across Australia, the USA and Greenland. In the UK, fossils in Shropshire’s Comley Quarry date back 550 million years – this is where Britain’s earliest trilobites were discovered in the 1880s.

China is still yielding exceptional discoveries, with a new site in Huayuan announced in 2026 revealing 91 new species of a variety of animals. Here, preservation is so exceptional scientists can see down to the cellular level, revealing legs, gills, guts, eyes and even nerves. It’s even possible to reconstruct fossil larvae in 3D, helping unravel the story of early animal evolution.