Cambrian Explosion

views updated

Cambrian Explosion

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