The New Catastrophism in the History of Life
by Peter M. Sheehan, Curator of Geology
EDITOR'S NOTE: This article is reprinted from LORE magazine, an MPM publication that is a benefit of museum membership. ©1996 Milwaukee Public Museum, Inc.
Many catastrophic events, such as asteroid impacts and glaciations, are being documented in the geologic record. The effects of these catastrophic events are prompting paleontologists to revise our understanding of the history of life, but resistance by some members of the paleontologic community to accepting catastrophic explanations continues. This resistance is due in part to a paleontologic controversy that was settled more than 100 years ago.
In the early 19th century paleontologists were beginning to understand the history of life on Earth. Enough of the fossil record had been uncovered to establish the fact that plants and animals had changed through time. Two distinct explanations of the changes in the fossil record were put forth.
Long before Charles Darwin, Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (1744-1829) proposed an evolutionary theory. Examining invertebrate fossils from the Paris Basin, he found evidence that today's plants and animals had formed by gradual modification from those that had existed previously.
Georges Lopold Chrtien Frdric Dagobert, Baron Cuvier (1769-1832) proposed a catastrophic theory. Based on his studies of vertebrate fossils from the Paris Basin, he believed that the extinct life forms in the fossil record had not evolved into modern ones. Rather, his interpretation was that the Earth had occasionally been devastated, and all plants and animals killed. After each extinction event, new and different life forms were created through divine intervention.
During the 19th century paleontologists tested these theories by collecting detailed sequences of fossils. It soon became clear that there were no times when all organisms became extinct. Rather, organisms living in today's world had a long series of ancestors in the fossils record. Life seemed to have evolved continuously and gradually through time. With the acceptance of Charles Darwin's description of natural selection, the mechanism of gradual evolution became clear. In the early twentieth century evolution was viewed as a process governed by the competition of organisms with each other. The "survival of the fittest" became the central theme of paleontology. Evolution through natural selection was a slow process requiring long intervals of geologic time. During the last 25 years paleontologists have shown that most change occurs during the relatively brief intervals when new species evolve, but evolutionary change was still viewed as a gradual process as species replaced each other over long periods of geologic time.
The discovery that an asteroid impact 65 million years ago coincided with a mass extinction that killed large numbers of animals both in the oceans and on land (including the dinosaurs) is forcing a reevaluation of our understanding of the history of life. Surviving the environmental calamity at a mass extinction event was more a matter of good luck than being well adapted. Groups of animals that were very widely distributed or that had relatively simple morphology tended to survive.
In the revised view of the history of life, gradual evolution was the dominant process during most of Earth's history. However, at several times in the past, calamitous events such as asteroid impacts and glaciations wiped out the dominant life forms and allowed survivors to radiate into the habitats that were vacated by the extinction. For example, mammals radiated from small, seed- and insect-eating animals into large-bodied browsers and carnivores, but only after browsing and carnivorous dinosaurs had been eliminated from Earth.
This new realization that gradual evolution was interrupted by several catastrophic events is forcing paleontologists to reconsider events in the history of life that had seemed well understood. Older explanations, such as the decline of the dinosaurs being caused by competition with more advanced mammals, are being abandoned. But paleontology has such deep roots in gradualism, stemming from the controversy between Cuvier and Lamarck, that many paleontologists are still unwilling to acknowledge a new role for catastrophism.
Perhaps the most telling evidence that gradualism alone can not explain sudden mass extinction events, is that the sudden events effected organisms both in the oceans and on land. Only global events that effect both terrestrial and oceanic ecosystems are capable of causing these extinctions. Whatever killed the dinosaurs also devastated marine habitats, because the extinction event was just as severe in the oceans as on land.
In the last 600 million years there were five mass extinctions. During these events more than half of the species on Earth died.
Asteroid impacts are associated with the two most recent events. The largest event, about 245 million years ago, occurred when continental drift brought all the continents together into a super continent, probably causing climatic deterioration. No clear cause has yet been found for the event 365 million years ago. The oldest mass extinction, at the end of the Ordovician Period, about 430 million years ago, was caused by an ice age when glaciers covered Africa, which was then at the south pole.
The record of the Ordovician extinction is preserved in rocks in Wisconsin. So much ice was trapped in the glaciers that sea level dropped more than 100 feet. Shallow seas covering the continents disappeared, and animals in these seaways became extinct. Rocks of this age in Wisconsin are found in cliffs near Mayville, along the east side of Lake Winnebago, and on the west side of the Door Peninsula.
Smaller extinction events are present also, and some of them have been associated with extraterrestrial events such as asteroid impacts and orbital changes which effected the climate. Regional extinction events may have had local causes such as volcanic eruptions.
Lamarck and Darwin were not wrong; life evolved continuously on Earth. But Cuvier also was partially correct--there were catastrophic events that redirected the history of life. Cuvier was mistaken only in his belief that all life was not eliminated by the catastrophes.