Geological Time
Before the development of radiometric dating in the twentieth century, scientists knew about the existence of fossils, and could see that some fossils were older than others by the sequence of rock layers. However, they had no way of attaching numbers to any of the changes they observed in the fossil record. Several landmarks that stand out in the process of evolution were used as boundaries in the delineation of past times.
In the nineteenth century, scientists were not aware of bacterial fossils, or even fossils of soft–bodied organism. To them, fossils seemed to indicate that life suddenly appeared at the beginning of the Cambrian (when fossils of hard–bodies organism first appeared). The era from the beginning of Earth's existence to 570 million years ago, is the Proterozic “before–life”. Next is the Paleozoic “old–life” era from about 570 million years to 245 million years ago. This ear saw a marvelous diversification of life, including the development of fish, amphibians, lands plants, animals, and rudimentary forms of reptiles. The third great era 245 million years ago to 65 million years ago was the Mesozioc “middle–life”, also known as the age of dinosaurs, when the major vertebrate life forms on Earth were large reptiles. Finally, the Cenozoic “new–life” era began with the extinction of the dinosaurs some 65 million years ago and continues to the present day. This is the time when mammals proliferated and began to dominate the Earth. The human species arose at the very end of the Cenozoic era.
Mass Extinction's and the Rate of Evolution:
Under normal circumstances, the rate of extinction seems to be such that roughly 10% to 20% of the species at any given time will be extinct in a matter of 5 to 6 million years. The fossil record, however, shows that not all extinction's are “normal.” Mass extinction's, rare catastrophic events in the past caused large numbers of species to become extinct suddenly.
By “large numbers of species” scientists mean anywhere from 30% to 90% of the species alive at the time. The term “Suddenly” means that scientists believe a time too short to be resolved by standard geological techniques. The extinction took place over a period of a few tens of thousands of years, or a couple of days.
The best known of these mass extinction's is the one in which the dinosaurs perished some 65 million years ago, at the end of the Cretaceous period, which was the end of the Mesozoic era. In that extinction, about two–thirds of all living species disappeared. In some cases, as with ocean plankton, the numbers climbed as high as 98%. However, the extinction at the end of the Mesozoic era was neither the larger nor the most recent mass extinction. About 250 million years ago, near the end of the Paleozoic era, about 80% of existing species disappeared in a single extinction event. A some what milder extinction, which wiped out 30% of the existing species, appears about 11 million years ago. In fact, geologist who study the past history of life in detail distinguish about a dozen mass extinction's.
One of the most interesting explanations for how these mass extinction's could occur began in 1980 by a father–and–son team. Luis Alvarez (Noble laureate in physics) and Walter Alvarez (a geologist). They suggested on evidence they accumulated, that the impact of a large asteroid killed off the dinosaurs and other life forms. This impact raised a cloud that blocked out the sunlight for several months. This catastrophe disturbed the planet's ecosystem and it is amazing that anything survived.
STOP AND THINK!
WHAT WOULD HAPPEN IF AN ASTEROID LIKE THE ONE HYPOTHESIZED BY LUIS AND WALTER ALVAREZ HIT THE EARTH TODAY?
Most scientists accept than an asteroid hit the Earth at the end of the Cretaceous, and agree that it was least partly responsible for the mass extinction. This conclusion gained credence in 1992. A large creator over 100 miles across under the seafloor near the Yucatán Peninsula. (607) Less certain is the role that other factors played in these events. The world ecosystem was under a great deal of stress at that time because of rapid changes in climate and the recent creation of mountain chains, both of which altered habitats.
Mass extinction's illustrated an important role about the history of life on our planet. Evolution is not smooth, gradual progress through time. There are times when sudden changes (such as those in the mass extinction's) follow rapid evolution as new species develop to take the place of those that disappeared. After the extinction of the dinosaurs, for example, the number of species of mammals increased dramatically. Scientists continue to debate about the rate evolution. There are two extremes views in the debate.
- The gradualism hypothesis, which holds that most change occurs as a result of the accumulation of small adaptations.
- The punctuated equilibrium, which holds that changes usually occur in short burst, separated by long periods of stability.
It now appears that both of these extremes, and probably any rate of evolution in between, occurred at some time in the Earth's past.
SCIENCE NEWS
Death Swept Earth at End of Permian
(Science News, Vol. 153—May 16, 1998, p. 308)
Geologists have recently discovered evidence that shows that the largest mass extinction in history, occurred at the end of the Permian period, lasted between 10,000 and 165,000 years. Researchers previously assumed that the mass extinction lasted for many millions of years. During this mass extinction, 70% of land vertebrates and 85% of ocean species were extinguished.
Previously, scientists speculated that the extinction's occurred by the movement of tectonic plates, but such an explanation is incongruent with a time span short as 10,000 years. Other possible explanations for the mass extinction include Siberian volcanic volcanic eruptions, the elimination of photosynthesis in surface waters and the strike of carbon–rich comet.
The researchers dated samples of volcanic ash layers in Southern China and in Texas. The samples contained zircon, whose crystalline lattice structure includes uranium. As time passes, the uranium turns into lead. However, measuring the ratio uranium to lead in the zircon, the researches, successfully dated volcanic ash that was scattered before and after the mass extinction. They found that the extinction occurred somewhere 252.3 and 251.4 million years ago. The result needs confirmation by studies of other land and oceans sites.
