Cambrian Explosion: The Sudden Rise of Animal Life on Earth

The Cambrian Explosion was the rapid appearance of most major groups of animals in the fossil record at the start of the Cambrian Period, beginning about 538.8 million years ago. Within roughly 20 to 30 million years, the seas filled with arthropods, molluscs, worms, sponges, echinoderms and the first relatives of vertebrates, many of them with shells, eyes and limbs. The event laid down the basic body plans of animal life that still exist today, and explaining it remains one of the central puzzles of evolutionary biology.

Background: life before the Cambrian

For about three billion years before the Cambrian, life on Earth was mostly microscopic. Bacteria and other single-celled organisms built mats and mound-like structures called stromatolites in shallow water. Oxygen built up in the atmosphere in stages, first during the Great Oxidation Event more than two billion years ago and again late in the Precambrian.

Between about 720 and 635 million years ago, the Earth went through severe global glaciations, sometimes called Snowball Earth. After they ended, the Ediacaran Period saw the first large, complex organisms. The Ediacaran biota included soft-bodied fronds, discs and quilted forms, among them early animals such as Dickinsonia and the possible mollusc relative Kimberella.

Most Ediacaran organisms, however, looked very different from later animals. They had no shells, few signs of active movement and no clear legs, guts or eyes. Only a handful of simple burrows suggest that some could move through the sediment.

What happened

The start of the Cambrian

The base of the Cambrian is defined by the first appearance of a distinctive trace fossil, Treptichnus pedum, a branching burrow left by a worm-like animal. The reference section is at Fortune Head in Newfoundland, Canada. The current international date for this boundary is about 538.8 million years ago.

In the earliest Cambrian, often called the Terreneuvian, the most common fossils are the “small shelly fauna”: tiny tubes, cones, plates and spines, usually only a few millimetres long. They show that animals were beginning to build hard skeletons from calcium carbonate and calcium phosphate. Burrows also became deeper and more complex, showing that animals were mixing the sea floor in new ways.

The burst of new body plans

From about 521 million years ago, the record changes dramatically. Trilobites, the armoured arthropods that would dominate Paleozoic seas, appear and quickly diversify. Brachiopods, molluscs, echinoderms, sponges and reef-building archaeocyath sponges spread through shallow seas.

Sites with exceptional preservation show that soft-bodied animals were even more diverse. The Chengjiang biota of Yunnan, China, about 518 million years old, preserves worms, arthropods with legs and antennae, comb jellies and some of the earliest known fishes. The Sirius Passet site in northern Greenland is of similar age.

By about 508 million years ago, the Burgess Shale of British Columbia records a complex marine community with predators such as Anomalocaris, filter feeders, scavengers and burrowers. Many of the animals found there belong to the early branches of groups that survive today, such as arthropods and chordates.

A new kind of ecosystem

The Cambrian Explosion was not only about new shapes. It also created the first food webs that look broadly modern. Predators with grasping limbs and powerful mouths appear, along with prey that had shells, spines and burrowing habits for protection. Some trilobites bear healed bite marks.

Eyes are a key feature of the period. Trilobites had compound eyes with mineral lenses, and some radiodonts, the group that includes Anomalocaris, had large compound eyes with thousands of lenses. Vision probably made hunting and escape far more effective.

Proposed causes

Scientists have proposed many explanations, and most now think several factors worked together.

Oxygen. Animals that are large and active need oxygen. Geochemical evidence suggests oxygen in the oceans and atmosphere rose during the late Ediacaran and Cambrian, though the timing and size of the increase are still debated.

Ocean chemistry. A worldwide erosion surface called the Great Unconformity lies beneath many Cambrian rocks. One hypothesis, proposed by Shanan Peters and Robert Gaines in 2012, is that erosion released large amounts of calcium and other ions into the sea, making it easier for animals to build skeletons.

Genetic toolkits. Groups of genes that control body layout, including the Hox genes, are shared across most animals. Their evolution may have made it possible to build bodies with heads, segments and limbs, which could then be varied in many ways.

Ecology. Once predators appeared, prey animals were under pressure to develop armour, speed or burrowing habits, and predators in turn evolved better weapons and senses. This “arms race” may have accelerated diversification. The zoologist Andrew Parker has argued that the evolution of eyes was particularly important.

Preservation. Part of the “explosion” may reflect the appearance of hard parts, which fossilize much more easily than soft tissue. Molecular clock studies, which estimate when lineages split by comparing DNA, generally place the origin of animals well before the Cambrian, perhaps 650 to 800 million years ago. Many lineages may therefore have existed earlier as small, soft-bodied forms that left few fossils.

Darwin’s dilemma and later debates

Darwin’s problem

Charles Darwin was aware of the problem. In On the Origin of Species, published in 1859, he discussed the sudden appearance of animal fossils in the oldest fossil-bearing rocks then known, which he called the Silurian, following usage of the time. He acknowledged it as a serious difficulty for his theory of gradual evolution and suggested that the earlier record had not been preserved or not yet discovered.

Later discoveries have largely confirmed his suggestion. Microfossils of Precambrian life were recognized in the 1950s, the Ediacaran biota became widely accepted as Precambrian at around the same time, and research on early Cambrian rocks has shown that diversification happened in steps over millions of years rather than all at once.

Wonderful Life and its critics

The Burgess Shale fossils, restudied from the late 1960s by Harry Whittington, Derek Briggs and Simon Conway Morris, sparked a famous debate. In his 1989 book Wonderful Life, Stephen Jay Gould argued that many Cambrian animals represented completely separate body plans that later died out, and that the survival of particular groups owed much to chance.

Conway Morris challenged this view in The Crucible of Creation (1998). Since then, most researchers have concluded that the strange Cambrian animals are best understood as early “stem-group” members of living phyla: early branches that lack some features of their modern relatives. Anomalocaris, for example, is now seen as an early relative of arthropods.

Myths and facts

Myth: all animal groups appeared at once. The Cambrian Explosion unfolded over tens of millions of years in several steps, from small shelly fossils to trilobites and then to the rich communities of Chengjiang and the Burgess Shale. Sponges and other simple animals almost certainly existed earlier, and some groups, such as bryozoans, are not securely known until the Ordovician.

Myth: there was no life before the Cambrian. Microbial life is known from rocks more than 3.5 billion years old, and complex organisms lived during the Ediacaran. What changed in the Cambrian was the abundance of large, mobile animals with hard parts.

Myth: the explosion contradicts evolution. Darwin saw the pattern as a challenge, but later discoveries of Precambrian fossils, transitional forms and molecular evidence fit an evolutionary explanation. Scientists still debate the speed and causes, not whether animals evolved.

Fact: it was fast in geological terms. Even so, 20 to 30 million years is a very long time, enough for many millions of generations of small marine animals.

Timeline

• c. 635 million years ago: The Ediacaran Period begins after the end of the Marinoan glaciation.
• c. 575–539 million years ago: Large, complex Ediacaran organisms live on the sea floor.
• c. 538.8 million years ago: The Cambrian Period begins, marked by the trace fossil Treptichnus pedum.
• c. 538–521 million years ago: Small shelly fossils and increasingly complex burrows spread.
• c. 521 million years ago: The first trilobites appear in the fossil record.
• c. 518 million years ago: The Chengjiang biota is buried in southern China.
• c. 508 million years ago: The Burgess Shale community is preserved in western Canada.
• c. 485.4 million years ago: The Cambrian ends and the Ordovician Period begins.
• 1859: Darwin discusses the sudden appearance of animal fossils in On the Origin of Species.
• 1909: Charles Doolittle Walcott discovers the Burgess Shale.

Legacy

Almost every major animal phylum with a good fossil record first appears in Cambrian rocks or shortly afterwards. The body plans established then, such as the segmented, jointed bodies of arthropods or the notochord of chordates, still underlie animal life, including human beings.

The explosion was followed in the Ordovician by the Great Ordovician Biodiversification Event, when the number of species and families rose further but few fundamentally new body plans appeared. Later mass extinctions removed many groups, yet the basic architecture of animal life set in the Cambrian has persisted for more than half a billion years.

Why it matters

The Cambrian Explosion marks the moment when complex animal life became abundant, visible and ecologically interconnected. Studying it helps scientists understand how new body plans evolve, how environment and biology interact, and how the deep ancestry of humans traces back to small chordates swimming in Cambrian seas.