Cambrian Period
The Cambrian period (570 million years ago) marks an extraordinary shift in the evolution of life. It ushers in the beginning of the Paleozoic Era (the age of ancient life).
Cambrian period and surrounding time periods.
| Era | Period | Epoch | Million Years Before Present |
| Paleozoic | Permian | | 286 |
| Pennsylvanian | | 320 |
| Missipian | | 360 |
| Devonian | | 408 |
| Silurian | | 438 |
| Ordovician | | 505 |
| Cambrian | | 570 |
In the Precambrian, a three-billion-year period of evolutionary stasis, the dominant life-forms were prokaryotes (tiny one-celled bacteria) and blue-green algae, both of which thrived in the steaming waters and nitrogen-and sulfur-rich air of a geologically turbulent Earth. Prokaryotes are the simplest forms of life, undifferentiated cells with no nucleus that reproduce by fission, the splitting of the parent cell into two. Prokaryotes live off hydrogen, sulfur, and nitrogen and they release free oxygen as a waste product. The prokaryotes' leisurely existence continued for five-sixths of recorded time, during which their massive colonies of cyanobacteria, fossilized as stromatilites, bubbled out enough oxygen to form eventually an atmosphere and a corresponding ozone shield against sterilizing ultraviolet radiation. This development appears to have set the stage for what has been described as the Big Bang of Biology, the Cambrian Explosion.
This trilobite fossil is one of 120 different fossilized species found in the Burgess Shale in Yoho National Park, British Columbia, Canada.Cambrian rock is named after the Latin "Cambria," meaning Wales. It was there that Cambrian rock was first studied for fossils in the late 1800s. Since then it has been found on every continent, with a particularly fertile deposit having been discovered in British Columbia, Canada. The latter is known as the Burgess Shale, a fine-grained, mudstone siltstone rock unit only about 200 feet long and 8 feet thick. Stephen Jay Gould has described it as the most important fossil deposit ever found. Dating to the mid-Cambrian of about 520 million years ago, the Burgess Shale has more than 120 animal species represented in it. The Burgess fossils demonstrate that the Cambrian period was a riot of experimentation in size, shape, and abilities. Animals that swam, that burrowed, and that foraged appeared at this time. A huge diversity of forms emerged. Some would succeed and continue to exist, while many others would disappear forever. The beginnings of every existing major phyla of animals can be found in the Burgess Shale and in other layers of Cambrian rock in Greenland and China. Over 900 species of marine life have been discovered at these locations, including sponges, jellyfish, annelids, mollusks, arthropods, and chordates with rudimentary backbones. One of the most interesting innovations found in Cambrian period animals was their ability to secrete a mineralized skeleton.
What could have caused this remarkable outburst of evolutionary life? The single most galvanizing event of the late Precambrian was the appearance of eukaryota, life-forms that stored DNA in a nucleus and were capable of organizing bodies consisting of more than one cell. Eukaryotes allowed for the possibility of specialization, since the individual cells did not each have to perform every task as long as they could communicate chemically with one another. This cooperation between cells set life-forms free to explore every design and variable in size and shape imaginable. Eukaryotes also developed the capability for sexual reproduction, which increases genetic diversity. Rather than duplicating the genetic material exactly as simple fission does, sexual reproduction ensures that a constant shuffling of genetic material will maximize the number of mutations and variations possible. This again allows for radical divergences in the exploration of the environment. Theseadvances in eukaryote organisms, combined with the new, oxygenated atmosphere of the planet, would appear to have allowed for the outburst of metazoans—multicelled animals—in the Cambrian rocks.
Geological Time Scale.
Bibliography
Fortey, Richard. Fossils: The Key to the Past. Cambridge, MA: Harvard University Press, 1991.
Friday, Adrian, and David S. Ingram, eds. The Cambridge Encyclopedia of Life Sciences.London: Cambridge University Press, 1985.
Lambert, David. The Field Guide to Prehistoric Life. New York: Facts on File, 1985.
McLoughlan, John C. Synapsida: A New Look into the Origin of Mammals. New York: Viking Press, 1980.
Steele, Rodney, and Anthony Harvey, eds. The Encyclopedia of Prehistoric Life. New
York: McGraw Hill, 1979.
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