Science

Uniqueness and Timing

The "Cambrian Explosion" refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains. It may represent the most important evolutionary event in the history of life on Earth.

The beginning of the explosion is generally placed about 542 million years ago, during the Cambrian Period at the start of the Palaeozoic Era (the same time the Ediacarans disappear from the fossil record). While the explosion was rapid in geological terms, it took place over millions of years - the Burgess Shale, at 505 million years old, records the tail end of the event. The explosion is particularly remarkable because all major animal body plans (each more or less corresponding to a distinctive Phylum - Mollusca and Chordata, for example) appeared during this time, changing the biosphere forever.

Graphic showing when different animal groups arose

The origin and diversification of animals during the Cambrian Explosion. Dotted lines represent the probable range of particular groups of animals. Solid lines represent fossil evidence. Extinct groups are represented with a circled-cross. Cones represent the approximate origin and diversification of the modern phyla (the crown groups). The basic body plan of major groups of animals (today's phyla) had already evolved by the time of the Burgess Shale. (modified after Xiao and Laflamme, Peterson et al and Dunn et al.).

The Cambrian Explosion and the Origin of Modern Marine Ecosystems

The rapid appearance of a wide variety of animals - particularly bilaterians - led to the development of radical new ecological interactions such as predation. Consequently, ecosystems became much more complex than those of the Ediacaran. As the number and variety of organisms increased, they occupied a variety of new marine environments and habitats. Cambrian seas teemed with animals of various sizes, shapes, and ecologies; some lived on or in the sea floor (a benthic lifestyle), while others actively swam in the water column (nektonic).

The fundamental ecological structure of modern marine communities was firmly established during the Cambrian. By the end of the Period, some animals had also made the first temporary forays onto land, soon to be followed by plants.

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Fossil Evidence

Mineralized Skeletons

The early record of the Cambrian Explosion is based on fossils - principally the appearance of mineralized skeletons and complex trace fossils. The typically tiny skeletal elements from this time are called "small shelly fossils." These constitute a highly varied assortment of sclerites, spicules, tubes, and shells, suggestive of several different types of animals. Unfortunately, many of the fossils remain poorly understood and are difficult to classify within known taxonomic groups.

A selection of 13 small, shelly fossils

Early Cambrian sclerite-bearing animals. 1, Siphogonuchites. 2, Hippopharangites. 3, Lapworthella. 4, Eccentrotheca. 5, 6, Microdictyon. 7, Tumulduria. 8, Scoponodus. 9, Jaw-like elements of Cyrtochites. 10, Porcauricula, 11, Dermal element of Hadimopanella. 12, Cambroclavus. 13, Paracarinachites. Scale bars = 0.1 mm.

© Swedish Museum of Natural History. Photos: Stefan Bengtson.

 Five fossils of animals living in tubes

Early tube-dwelling animals. 1, Cloudina, one of the earliest animals with a mineralized skeleton reinforced with calcite (late Neoproterozoic). 2, Aculeochrea, with an aragonite-reinforced tube (Precambrian-Cambrian boundary beds). 3, Hyolithellus, an animal reinforcing its tube with calcium phosphate (early Cambrian). 4, Olivooides, possibly a thecate scyphozoan polyp. 5, Pre-hatching embryo of Olivooides. Scale bars = 0.1 mm.

© Swedish Museum of Natural History. Photos: Stefan Bengtson.

Seven fossilized shells from the Early Cambrian

Early Cambrian shell-bearing animals. 1, Archaeospira, a possible gastropod. 2, Watsonella, a possible mollusc. 3, Cupitheca. 4, 5, Aroonia, a probable stem-group brachiopod. 6, 7, Conch and operculum of Parkula, a hyolith. Scale bars = 0.1 mm.

© Swedish Museum of Natural History. Photos: Stefan Bengtson.

About 521 million years ago, trilobites made their first appearance in the Cambrian fossil record. These armoured animals had of three dorsal shelly parts - a cephalon (head), a segmented thorax (body), and a pygidium (tail section). Trilobites eventually became one of the most ubiquitous groups of invertebrate organisms in the Palaeozoic seas. They survived for almost 300 million years, and their fossils can be found from the Cambrian to the Permian periods.

Colour photograph of trilobite fossil

An early (Lower Cambrian) trilobite species, Eoredlichia takooensis from Emu Bay Shale, Kangaroo Island, Australia. Specimen length = 6 cm.

© Royal Ontario Museum. Photo: David Rudkin.

Trace Fossils and the Cambrian Substrate Revolution

Trace fossils also become considerably more complex and diverse in Early Cambrian rocks. During the late Ediacaran, metazoans produced only simple horizontal traces on the surface of the sea floor. Starting in the Cambrian, animals began to tunnel vertically through the sediments and exhibit more varied behaviours, providing indirect evidence that mobile bilaterians with differentiated tissues and organs had already evolved.

Large slab of rock showing worm-like protuberances

Early Cambrian trace fossils. Treptichnus pedum from the Lower Cambrian Mickwitzia Sandstone, Sweden (Swedish Geological Survey, Uppsala). Scale bar = 1 cm.

© University Lyon 1. Photo: Jean Vannier.

The rise of these bilaterians permanently altered the nature of the sea floor, an event commonly referred to as the Cambrian Substrate Revolution.

During the Precambrian, the upper layers of mud, sand or silt on the sea floor remained relatively firm, thanks to bacterial mats that covered and stabilized the surface. These mats also served as a primary food source for Ediacaran organisms capable of grazing along the sea floor (see Kimberella). The burrowing animals of the Cambrian were able to tunnel down through the microbial mats, churning the sediment beneath and making it soupier. The burrowers may have started tunneling to access new sources of food (such as the sunken carcasses of planktonic organisms buried on the sea floor) or to escape predation by digging deep into the substrate.

Left, graphic showing undisturbed layers; Right, graphic showing disturbed layers with animals

Changes in substrate types during the Cambrian substrate revolution. Left, Precambrian Period; right, Cambrian Period.

The burrows opened up new ecological niches beneath the sea floor as water and oxygen could now get into the sediment layers. At the same time, bacterial mats were progressively destroyed and forced into more restricted habitats (i.e., in environments unfavourable for animals). This change in the substrate is thought to be partly responsible for the demise of the Ediacaran biota. Other factors (such as a change in water chemistry or an increase of predators) may also have played important roles in their extinction (see above).

The revolution turned the once-uniform sea floor into a heterogeneous patchwork, opening up a variety of new niches for animals - including those of the Burgess Shale - to exploit.

Burgess Shale-Type Deposits and the Burgess Shale Biota

Exceptionally well-preserved soft-bodied fossils of Cambrian age were first described from the Burgess Shale over 100 years ago. Today, dozens of Burgess Shale-type deposits with comparable assemblages of fossils have been found around the world. These deposits are usually found in Lower and Middle Cambrian rock layers, but may extend as far as the early Ordovician. These deposits are characterized by a similar mode of preservation called "Burgess Shale-type preservation".

The most notable sites are those located around the original Burgess Shale locality in Canada (the Walcott Quarry in Yoho National Park, British Columbia), along with the Maotianshan Shales of China (of which the Chengjiang in Yunnan Province is the most famous).

Other significant occurrences include the Kaili deposit in China and sites in the western United States of America (Spence Shale and Marjum and Wheeler Formations in Utah, Pioche Formation in Nevada), Greenland (Sirius Passet), and Australia (Emu Bay Shale).

Graphic showing locations of Burgess Shale-type deposits around the world

Utah deposits

Scrub landscape in Utah

General outcrop of the Middle Cambrian Marjum and Wheeler Formations, Utah, USA.

© Pomona College. Photo: Robert Gaines.

Close-up view of fossil with spines

Acinocricus cricus, part of a lobopod from the Spence Shale.

© Royal Ontario Museum. Photo: Jean-Bernard Caron.


Images of landscapes and fossils from different Burgess Shale-type deposits in Utah.

© Pomona College. Photos: Robert Gaines (landscapes) and Royal Ontario Museum. Photos: Jean-Bernard Caron (fossil specimens).

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Sirius Passet

A desolate landscape in Greenland

Aerial view of Sirius Passet, Greenland.

© Natural History Museum of Denmark (Geological Museum). Photo: David Harper.

A rock containing a fossil, posed alongside a notebook for scale

A typical Lower Cambrian naraoid-like arthropod (book = 20x12 cm).

© Natural History Museum of Denmark (Geological Museum). Photo: David Harper.


Fossils from the Lower Cambrian Sirius Passet locality in Greenland.

© John Peel

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Chengjiang

A hill in China

Maotianshan Hill, China where the first Lower Cambrian Chengjiang fossils were discovered, including Naraoia spinosa (see below). The name Chengjiang comes from a nearby village in Yunnan Province.

© Nanjing Institute of Geology and Palaeontology Chinese Academy of Science. Photo: Fangchen Zhao

Left, a red-stained fossil trilobite; Right, a Chinese postage stamp showing a fossil.

Naraoia spinosa (left), Microdictyon sinicum (top right), and Hallucigenia fortis (bottom right) from the Chengjiang locality (scale bars = 1 cm).

© Nanjing Institute of Geology and Palaeontology Chinese Academy of Science. Photos: Maoyan Zhu


Fossils from the Lower Cambrian Chengjiang locality in China.

© Nanjing Institute of Geology and Palaeontology Chinese Academy of Science. Photos: Maoyan Zhu

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Kaili

View of a Chinese village.

View taken along the trail leading to the Middle Cambrian Kaili locality, Guizhou Province, China.

© Royal Ontario Museum. Photo: Jean-Bernard Caron.

Left, seven shrimp-like fossils; Right, a faint fossil

Several specimens of the primitive echinoderm Sinoeocrinus (left) (size = 12 cm) and a single specimen of the arthropod Marrella (right) (size = 6.4 cm). This is the only occurrence of Marrella outside the Burgess Shale localities in Yoho National Park.

© Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Photos: Jih-Pai Lin.

Emu Bay

Left, workers excavating a fossil bed; Right, image of a striated fossil worm.

Left: General view of excavation of the Lower Cambrian Emu Bay Shale on Kangaroo Island, Australia (October 2010); right, Palaeoscolex, a common fossil worm from the site (size = 9 cm).

© University of New England. Photos: John Paterson.

Fossil of a cup-shaped organism

The nektonic arthropod Isoxys (size = 7 cm).

© University of New England. Photo: John Paterson.

Compared to conventional fossil deposits, in which only the remains of more durable body parts are typically preserved, Burgess Shale-type deposits provide a much more complete picture of a normal Cambrian marine community. In modern marine settings, animals with mineralized body parts (shells, carapaces, etc.) account for only a minor component of the total diversity. This is also the case in most Burgess Shale-type deposits where the shelly assemblage usually represents a small percentage of specimens collected. Thus, without the fossilized remains of soft-bodied organisms, especially from the Burgess Shale, our knowledge of Cambrian ecosystems would be extremely limited.

Similarities among various Burgess Shale-type deposits around the world suggest the deep marine ecosystem was geographically uniform and evolutionarily conservative from the Lower to at least the Middle Cambrian (i.e., similar types of animal fossils are recovered through this whole interval, spanning at least 15 million years). The characteristic assemblage of organisms is often referred as the Burgess Shale-type biota.

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Explaining the Explosion

Why did the Cambrian explosion happen when it did, and why was it such a unique event? While there is no current consensus among scientists, most researchers agree the explosion cannot be ascribed to a single, simple causal mechanism. The potential triggers can be classified in three main categories: environmental, genetic, and ecological. Deciphering the impact of each of these factors remains one of the most important challenges faced by palaeontologists today.

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Was there an Explosion at all?

Very few organisms ever enter the fossil record; after death, their remains are usually completely destroyed and recycled. Animals with hard body coverings, such as trilobites, are much more likely to be preserved than those with only soft body parts. So the evolutionary development of mineralized shelly parts by different groups would be marked in the rock record by a sudden jump in fossil numbers. Thus, preservation bias alone could create the appearance of an "explosion" of new life forms at the beginning of the Cambrian.

When he published On the Origin of Species in 1859, Charles Darwin puzzled over the apparently sudden appearance of complicated organisms in the fossil record at the beginning of the Cambrian Period. He noted this could be used as an argument against his controversial new theory, which predicted a more gradual appearance of simpler organisms. At the time, Darwin pointed to the imperfection of the fossil record as his only defence, arguing complex animal life must have lived long before the Cambrian, but traces of that life had not yet been found.

The presence of large, soft-bodied, putative animals (problematic as they may be) in Ediacaran seas does indeed make the "explosion" appear less abrupt. But the fact remains that the Early Cambrian was a time of major change in marine animal communities and environments, with the rapid and unprecedented advent of disparate new body plans and novel ecological niches. By the end of the period, every major animal phylum was firmly established, and life after the Cambrian was radically different from what had gone before. So it is safe to call this event an "explosion" - it was crucial to the evolution of life on Earth as we know it.

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Triggers of the Cambrian Explosion

Environmental Explanations

Before complex animals could evolve on Earth, there had to be an environment favourable for their survival. Researchers have examined a number of environmental factors that might have been instrumental in the evolution of new body plans, but the two strongest contenders are a rise in oxygen levels and the end of extreme glacial conditions.

Oxygen

Multicellular animals use oxygen to fuel their metabolism. At low oxygen levels, they don't function well … without it, they cannot survive. Photosynthesis could have caused a rise in the amount of oxygen in the seas and atmosphere near the beginning of the Cambrian, allowing the evolution of larger, more complex animals with respiratory and circulatory systems.

However, there does not seem to be much variation in oxygen levels across the Ediacaran-Cambrian boundary. Earlier increases might have triggered the evolution of large Ediacaran metazoans prior to the explosion, and a subsequent post-explosion rise in oxygen levels may have allowed animals to adopt more active, energy-intensive lifestyles such as swimming and hunting.

Glaciation

Another possible environmental explanation for why the explosion occurred when it did involves glaciers. Some researchers have suggested the entire Earth was covered with ice before the Cambrian explosion. (This is known as the "Snowball Earth" hypothesis.)

The ice would have limited the number of evolutionary niches for life in the sea, and blocked most of the sunlight on which cyanobacterial mats and algae depend. But once the glaciers receded, huge expanses would suddenly be opened for life: an ideal situation for experiments on different body plans. Unfortunately for the hypothesis, the last worldwide glaciation seems to have ended around 635 million years ago - nearly 90 million years before the first signs of the Cambrian explosion in the fossil record (which was followed by another major regional glaciation around 580 million years ago). Even if there is no direct triggering link between Precambrian glaciations and the Cambrian explosion, the post-glacial period was a crucial time in evolution. The appearance of the first large and complex multicellular organisms shortly after the return to a warmer global climate suggests that environmental conditions had become ripe for them to evolve.

Developmental Explanations

Some scientists have argued there was nothing in the environment during the Precambrian-Cambrian transition that was particularly unique. For these researchers, the answer to the question "why did the Cambrian explosion take place when it did?" can be found within the organisms living at the time. According to this approach, life first had to evolve the ability to develop new and diverse body plans.

Developmental genes in animals regulate how and when other genes operate to "build" the organism through its earliest life stages. Many important developmental genes are shared between widely-divergent animal groups. They are so closely shared that control genes from a lab mouse work perfectly well in a fruit fly. This conservation means those control genes must also have been present in the last common ancestor to both the fruit fly and the mouse. Very small changes in developmental genes can have a surprisingly large impact on the resulting organism. For instance, changes to the so-called hox genes in fruit flies can cause a fly to sprout an extra set of wings, or to grow legs where the antennae should be. From this, it could be argued that the fuse setting off the Cambrian explosion may have been ignited when the genome in the ancestor of all modern animals reached a level of complexity (including the evolution of hox-like developmental genes) sufficient to create radically new body plans. This would have provided more raw material for natural selection to act upon.

Such genomic changes might have been in the making long before the Cambrian, perhaps giving rise to the Ediacarans through a novel developmental pathway that was never repeated after their extinction. Following the Cambrian explosion and the evolution of the body plans that we know today, large-scale developmental change seems to have been "locked" into place, and no new body plans have appeared since.

Ecological Explanations

The explanation of the Cambrian explosion may lie not in the wider environment, or in the controlling genes, but in the complex ecological interactions between animals. The explosion may be a result of co-evolution, with different inhabitants of the Cambrian ecosystem being "pushed" to evolve by changes within that ecosystem.

For example, the emergence of predators might have stimulated the evolution of skeletalization (including mineralized plates) for protection, or swimming as a means of escape. Before predation became widespread, early "experiments" in different body plans could have briefly thrived because species interactions were probably more limited.

Moving into previously-unexploited environments would allow even a poorly-adapted animal to survive, perhaps with one of the "exotic" body plans seen in the Cambrian. New ecological niches - particular spaces in the ecosystem occupied by species - would have been created by organisms interacting with the environment.

Examples might include the evolution of zooplankton making organic material available for bottom dwellers, or the evolution of burrowing making new interactions with the sediment possible. But over tens of millions of years, as more organisms moved into these niches, the resulting competition would remove the less-fit body plans, leaving only the ancestors of today's phyla.

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