The Cambrian Explosion, known informally as Biology's Big Bang, refers to the event that greatly increased the variety of animal species and created the major types of animals that exist today. Scientists refer to this event as an "explosion" not because it was a period of violent activity, but because an incredible amount of evolutionary activity occurred in a relatively short length of time. This burst of evolution occurred during the early years of the Cambrian period (approximately 570 million to 495 million years ago). The Cambrian Explosion ran its course over several tens of millions of years, but this time period is quite brief considering the length of time that multicellular life has existed on Earth (about 600 million years).
In addition to the phyla that are still present today (phyla Porifera [sponges], Cnidaria [jellyfish], Platyhelminthes [flatworms], Nematoda [nematode worms], Molluska [clams], Annelida [roundworms], Arthropoda [insects, spiders], Echinodermata [sea urchins], and Chordata [vertebrates]), a number of other phyla were created and have since become extinct (including phyla Priapulida, Onychophora, and some unclassifiables).
The Burgess Shale (in British Columbia, Canada) and the Chingjiang (in Yunnan Province, China) provide fossil evidence for the outcome of the Cambrian Explosion. The fossils of both localities are impressive for the details they provide of early, soft-bodied marine organisms, and for the diversity of animal forms they display. Prior to Charles Walcott's 1909 discovery of the Burgess Shale in the western Canadian Rockies, life was thought to have evolved at a gradual, constant rate. The Burgess Shale fossils, formed 530 million years ago, shortly after the Cambrian Explosion, showed that the diversity of animal forms arose abruptly relative to the age of animal life as a whole.
No one knows exactly how the Cambrian Explosion started, but a number of theories have been offered. Many scientists believe that violent and abrupt changes in climate forced animal life to diversify and adapt to new and harsh conditions. Prior to the Cambrian Explosion, the present-day continents of Africa, South America, India, Antarctica, and Australia formed a giant landmass called Gondwana. Shortly before the Explosion, the abrupt shifting of Gondwana and Laurasia (ancestral North America) across Earth's surface caused volcanic eruptions and earthquakes, creating a hostile environment in which marine animals had to compete for survival.
These dramatic geological events mixed more oxygen dissolved into the water, which helped make possible processes such as respiration, cell division, and the synthesis of proteins that are important in body structure and support. In this respect, the movement of tectonic plates that resulted in an upheaval of Earth's geology contributed to the creation of complex body plans. The evolution of organs such as brains, digestive guts, and shells opened the door for the new creatures to increase in complexity. These animals were then able to forage more effectively, move more efficiently, and protect themselves.
The tectonic shift created a new ecological battleground for these animals to compete. The breakup of Gondwana and its subsequent reorganization created a larger area of marine habitat in which the new creatures would compete. This competition catalyzed the adaptive radiation (evolution of a large variety of specialized forms) of the lineages into their respective niches.
Another mystery surrounding the Cambrian Explosion is why new phyla, or major lineages, have not evolved since then. Some scientists assert that after successful body plans arose during the Cambrian, genetic limitations dictated that change occur only within the set lineages. Rather than create dramatically new body forms, these existing forms were modified into more complex animals. Because modern animals have been evolving and adapting for millions of years since their ancestors first appeared, they would have an advantage over any new "hopeful monsters" that may come about through mutation. Hopeful monsters are forms that are so different from the "tried and true," existing with low chances of survival.
Late twentieth-century efforts to solve the mysteries of the Cambrian Explosion focused on molecular genetic techniques. New technology allowed scientists to extract DNA from fossils of the first Cambrian animals and to study certain aspects of these animals' biology. In this manner, scientists can learn about the development and evolutionary relationships of these animals.
see also Burgess Shale and Ediacaran Faunas; Cambrian Period; Geological Time Scale.
Judy P. Sheen
Erwin, Douglas, Jim Valentine, and David Jablonski. "The Origin of Animal Body Plans." American Scientist 85 (1997):126-137.
Gould, Stephen Jay. Wonderful Life: The Burgess Shale and the Nature of History. New York: W. W. Norton and Co., 1989.
Knoll, Andrew H., and Sean B. Carroll. "Early Animal Evolution: Emerging Views from Comparative Biology and Geology." Science 284 (1997):2129-2137.
Scientists agree that the Cambrian explosion is one of the most significant events in the history of life. It is marked by a series of biological changes that took place over a relatively short period of geologic time during the early Cambrian, 543 to 520 million years ago. (The entire Cambrian period ranged from 543 to approximately 490 million years ago.) First and foremost, the Cambrian explosion is marked by the global appearance of organisms with skeletal hard parts in the fossil record in contrast to the strictly soft-bodied creatures prior to this. Initially, these skeletal structures were simple in design, such as minute cylindrical tubes, tiny cones, and rudimentary jawlike appendages . However, they evolved rapidly into larger and more elaborate structures comparable to the exoskeletons of many living invertebrate groups. These early skeletons were constructed from a diverse array of materials that form the building blocks of skeletons to this day, including calcium carbonate, calcium phosphate, and silica.
Coincident with the appearance of skeletons was the phenomenal diversification of metazoan life. Paleontologists Stephen Jay Gould in Wonderful Life (1989) and Simon Conway Morris in Crucible of Creation (1998) provide detailed, popular accounts of the amazing evolution of Cambrian animals, though they reach somewhat different conclusions with regard to the implications for the subsequent history of life.
However, all paleontologists agree that virtually all of the modern invertebrate groups made their first definitive appearance in the early Cambrian, including phylum Annelida (worms), phylum Mollusca (clams, snails, cephalopods), phylum Echinodermata (starfish, urchins, sea lilies), phylum Arthropoda (trilobites, crabs, lobsters, insects), and phylum Brachiopoda (lamp shells). Along with these familiar groups came more obscure animals such as the Archaeocyatha, which are an interesting assemblage of sponge-like fossils that presumably led a quiet existence on the Cambrian seafloor filtering food particles from the water column. The Archaeocyatha became extinct by the middle Cambrian. A significant geological phenomenon associated with this great diversification of metazoan life is the enhanced record of bioturbation (the mixing of sediments by organisms seeking food and/or shelter) in rocks of Cambrian age. Prior to the Cambrian, most marine sediments were relatively undisturbed by animal activity.
Many scientists correlate the abrupt appearance of skeletons and the burst of biotic evolution in the early Cambrian with chemical changes in the world ocean, specifically an increase in the concentration of oxygen. Many scientists also point to the evolutionary first appearance of predatory lifestyles, with organisms adapting to this new ecological pressure with the construction of protective skeletons and the selection of burrowing habits. (An animal residing beneath the sediment surface is far less likely to be preyed upon.) Still others suggest that the relatively wide-open Cambrian oceans were an ideal setting for large-scale evolutionary experimentation and the origin of Phyla. Regardless of the driving mechanisms, the Cambrian explosion will forever remain one of the defining episodes in the history of life on Earth.
see also Evolution
Raymond R. Rogers
Conway Morris, Simon. The Crucible of Creation. Oxford: Oxford University Press, 1998.
Cowan, Richard. History of Life. Boston: Blackwell Science Inc., 2000.
Gould, Stephen J. Wonderful Life. New York: W. W. Norton and Company, 1989.