What Was the Cambrian Explosion?

The main phase spans roughly twenty to twenty five million years starting around 538 million years ago. That sounds enormous, but against a backdrop of three billion years of largely microbial life it is a very sharp change.

In that window the fossil record goes from soft bodied simple organisms to animals with eyes, legs, shells, guts and mouths. Almost every major group of animals alive today, including our own chordate lineage, has its first known representatives from this period.

Most of what we know comes from a handful of exceptional fossil sites, above all the Burgess Shale in the Canadian Rockies. Conditions there preserved soft tissue, so we see whole animals rather than just hard parts.

The creatures found are genuinely strange. Opabinia had five eyes and a grasping nozzle. Hallucigenia was so odd it was originally reconstructed upside down. Anomalocaris was a metre long predator with grasping appendages, the largest animal of its time.

No single explanation is agreed on, and most researchers now favour a combination of factors:

• •Rising oxygen levels, allowing larger and more active bodies
• •The evolution of eyes, which made predation and escape possible
• •An evolutionary arms race between predators and prey driving armour and speed
• •The development of genetic toolkits that made complex body plans possible
• •Chemical changes in seawater allowing shells and skeletons to form

The word is doing a lot of work. Molecular evidence suggests several animal lineages had already split well before the Cambrian, meaning the divergence began earlier than the fossils show.

What may have exploded is not animal life itself but hard parts. Shells and skeletons fossilise easily, soft bodies almost never do, so the sudden appearance in the record could partly reflect animals becoming preservable rather than animals suddenly existing.

The Cambrian explosion is where the basic body plans of animals were established, and remarkably few new ones have appeared since. Evolution has produced enormous variety within those plans but has rarely invented a genuinely new one.

That makes this period a kind of foundation for everything that followed. The arrangement of a body with a front and back, a gut running through it and paired limbs along the sides was settled half a billion years ago and has held ever since.

The Cambrian did not begin from nothing. The preceding Ediacaran period, from about 635 to 538 million years ago, has its own strange fossil record of soft bodied organisms with frond, disc and quilted shapes.

Researchers still argue about what these were. Some may be early animals, some may belong to groups with no living relatives at all, and some may not be animals in any sense. Most of them vanish at the start of the Cambrian, so the explosion may have involved a replacement of one experiment in multicellular life by another.

Geologists need a precise starting line for the Cambrian, and they chose an unexpected marker. The official boundary is defined at a reference rock section at Fortune Head in Newfoundland, Canada, based on the first appearance of a particular trace fossil called Treptichnus pedum. This is not a body fossil but a pattern of burrows. It records animals moving through sediment in more complex ways than before, which suggests that active, burrowing creatures had appeared even before hard shells became common.

The age of about 538 million years comes from radiometric dating. Volcanic ash layers sandwiched between sedimentary rocks contain tiny zircon crystals, and the decay of uranium into lead inside them works as a natural clock. Dating ash beds above and below fossil layers brackets their age. These methods also show that the Burgess Shale is not the oldest window onto Cambrian life. The Chengjiang fossils of Yunnan, in southern China, are roughly ten million years older and also preserve soft tissues, giving an earlier view of the same burst of animal diversity.

The Burgess Shale was discovered in 1909 by Charles Walcott, who collected tens of thousands of specimens and fitted most of them into familiar groups such as arthropods and worms. Decades later, researchers in Cambridge restudied the fossils in detail and concluded that many were far stranger than Walcott had thought. In his 1989 book Wonderful Life, Stephen Jay Gould popularised their work, arguing that several animals represented entirely separate body plans that left no descendants, and that the survival of some groups over others was largely a matter of chance.

Later research has softened that conclusion. As more fossils were found, especially in China, and as methods for building family trees improved, many of the weird wonders were placed as early relatives of living groups. These are known as stem groups: branches that split off before all the features of the modern group had evolved. Anomalocaris and Opabinia, for instance, are now widely seen as early offshoots on the arthropod lineage. The debate changed how palaeontologists think about classification, since an odd-looking fossil can still belong near the base of a familiar group.

The sudden appearance of animal fossils troubled Charles Darwin. In On the Origin of Species, published in 1859, he admitted that the abrupt appearance of many animal groups in the lowest fossil-bearing rocks then known was a serious difficulty for his theory, which predicted long, gradual change. If animals had evolved from simpler ancestors, there should have been a long earlier history, yet the rocks below seemed almost empty. Darwin argued that the fossil record was very incomplete and hoped older remains would eventually be found, but he acknowledged that he could not yet explain the gap.

The gap began to close in the twentieth century. In 1946 the geologist Reg Sprigg found impressions of soft-bodied organisms in the Ediacara Hills of South Australia, though their age was not clear at first. In 1957 a schoolboy, Roger Mason, reported a frond-shaped fossil in Charnwood Forest in Leicestershire, in rocks known to be older than the Cambrian. Named Charnia, it became one of the first complex fossils widely accepted as Precambrian. Microscopic fossils described in the 1950s from the Gunflint Chert in Ontario, nearly two billion years old, confirmed that microbial life had existed for an immense time before animals.

Ordinary fossilisation needs hard parts, so sites like the Burgess Shale are exceptions that need explaining. The Burgess animals lived on a muddy sea floor at the foot of a tall underwater cliff of reef limestone. Flows of very fine mud repeatedly buried them, quickly and completely. The buried sediment was low in oxygen, which slowed the bacteria that normally break down soft tissue and kept burrowing scavengers away. The organisms were flattened into thin films of carbon, often coated with clay minerals, which preserved the outlines of guts, gills, muscles and, in rare specimens, even traces of the brain.

Other sites preserve life in completely different ways. In the Orsten deposits of Sweden, from late in the Cambrian, tiny arthropods only a millimetre or two long were replaced by calcium phosphate in three dimensions, keeping legs and bristles that can be studied under an electron microscope. Each style of preservation has its own biases. Burgess-type sites favour animals that lived on or near the sea floor where the mud settled, while phosphate replacement works best on very small creatures. Comparing several windows like these, rather than trusting any single one, is how palaeontologists check whether an apparent absence is real or just a gap in preservation.

The Cambrian explosion was the rapid appearance of complex animals with eyes, shells and skeletons around 538 million years ago, and it set the basic body plans that almost all animals still use today.