THE MYSTERY OF MASS EXTINCTIONS
(Dr. Girish Chandra)
One of the most puzzling phenomena in the evolutionary history of animals is the periodic mass extinctions. From time to time our planet has experienced holocausts of gigantic magnitude which wiped out from the face of earth majority of life forms. Life then had to start and grow afresh. Often the waves of extinctions engulfed the most successful evolutionary lines, thus defying the Darwinian concept of natural selection and survival of the fittest. The shattered earth took millions of years to recover and attain the richness of life again, till the mysterious hand of nature again removed majority of animals, leaving a trail of destruction all around. What causes the mass destructions on earth and why? The answer is wrapped in deep mystery, as by now time has washed off most of the evidences.
A study of 600 million years’ history of animal evolution reveals five major extinctions of animals with a periodicity of about 200 million years. Minor extinctions have occurred in between showing a periodicity of 50 million years; Marine animals exhibit a periodicity of 26 million years. The extinction waves have been so intense that out of 1800 families of marine animals that had evolved during 280 million years since Permian, 970 have become extinct by now. While not much is known about the Cambrian extinction 600 million years ago, when only primitive life inhabited the earth, the Silurian-Devonian extinction 200 million years later wiped out from the face of earth the most abundant animals of that time–the Trilobites. Earth was quiet for the next 150 million years when the Permian extinction wave removed 71 % of the tetrapod families that lived at that time. This was followed by the Triassic extinction 50 million years later which killed 80% of the tetropod (4-legged animals) families.
End-Cretaceous extinction 70 million years ago is the best known of all. Although it caused extinction of only 61% of tetrapods, it removed from earth the mightiest of all creatures that ever lived–the dinosaurs. Life of all forms was at its zenith at that time.
Another significant extinction occurred in the late Pleistocene (about 50,000 BP) when over 76% of large terrestrial animals vanished from America, including horse which had evolved there. In Australia 86% of land animals suffered extinction. Surprisingly tropical Africa suffered only 19% loss. The most significant animal to have succumbed to this holocaust was woolly mammoth, a cousin of modem elephant sporting gigantic curved tusks and one foot long hairs all over the body. The frozen corpses of these giants have been discovered in Siberia. The animals were so abundant before their sudden death that tusks of about 20,000 animals have been taken out in 20 years. Sir Henry Howarth in his book, The Mammoth and the flood, writes in detail about their sudden death and freezing under piles of ice. Three frozen mummies were found standing erect and facing north. Some of them still had grasses in their mouth and stomach contents revealed buttercups, sedge-grasses, beans, shoots of fir and pine.
It is a strange coincidence that following every major extinction, there has been sudden emergence of larger groups of animals and plants. For example, after the Cretaceous extinction, mammals, flowering plants, fruit trees and grasses evolved rather suddenly. Similarly, Permian extinction led to the emergence of reptiles and dinosaurs, which ruled the earth for the next 200 million years. The Triassic extinction, which killed 80% of tetrapod families, caused the origin of birds.
CAUSES OF EXTINCTIONS
It is a general belief that species become extinct because they are eliminated by natural selection when the environment changes and new ones emerge to take their place. But these extinctions were apparently not the result of atmospheric changes or ice ages. Fossils point to the sudden mass destructions. Woolly mammoth seems to have died out by freezing in a sudden cold wave. But then why animals living in tropics like ground sloth, giant lemurs of Madagascar, the giant bird Moa of New Zealand and the giant deer of Asia should have become extinct? On islands, even small mammals became extinct. In oceans also the destruction has been of equal magnitude. Permian extinctions could be attributed to the ice age but the late Triassic and end-Cretaceous extinctions occurred in the absence of global refrigeration.
Catastrophe as a possible cause of extinctions was proposed by a French naturalist. George Cuvier as early as in 1815. What kind of catastrophe, which seems to have occurred periodically, could have caused such mass destructions on earth? Alvarez (1980) in his Impact Hypothesis opined that iridium concentrations and shock metamorphosed quartz on earth were created as a result of the impact of a giant asteroid or comet on earth.
In 1990, a crater 180 km in diameter was discovered near a town called Chicxulub in Yucatan peninsula in Mexico. It is believed to have been caused by the hit of a meteorite 10 km in diameter about 66 million years ago and caused the extinction of dinosaurs.
Our solar system, moving at a speed of 150 miles/second takes 200-240 million years to complete one orbit around the nucleus of the Milky Way galaxy. It passes through the spiral arms or density waves every 50 million years where interstellar planetismals are captured by the sun’s gravity, causing cometary or asteroid bombardment on earth. Sepkoski and Raup (1986) proposed that the solar system also oscillates vertically every 67 million years, which increases the X-ray and Ultra-violet radiation intensity on earth, thereby causing mass extinctions as well as megamutations.
Recently, through the satellite images, a 480 km wide crater has been discovered beneath the ice sheets in Antarctica, which is 250 million years old and perhaps was the cause of Permian mass extinction.
The Tenth planet
Today scientists accept the possibility of existence of tenth planet beyond Pluto moving in a highly inclined orbit. If so then this planet would occasionally come close to the cometary cloud that surrounds our solar system and would pull comets to hurl them towards the sun. Geological evidences point to the large body impacts on earth and other planets from time to time. Were these extinctions caused by such occasional impacts of stray asteroids or comets, or a catastrophe of much larger proportion caused them?
Visit of a star or planet: The theory that a giant planet or a star passes periodically across our solar system, causing disturbances and destruction not only on earth but also on other planets, gains currency because of many abnormalities found in our solar system. For instance, what force could have created in earth a tilt of 23.5 degrees against its plane of orbit, and what kind of gigantic explosion could have produced the asteroid belt in which thousands of pieces of a shattered planet move in retrograde direction to the rest of the planets? How did Mars lose 70% of its crust and 90% of its water and attained a tilt of 24 degrees? Martian surface shows that once it had oceans. While Mars has lost all its water, earth has too much of it and so has Saturn. Why do six of the Jupiter’s 14 satellites move against the planet’s rotation? Strangely, Uranus is inverted by 92 degrees, its north pole pointing directly to the sun. Pluto is not only too small to be a planet but has a highly elongated orbit, the eccentricity being so great that at times it moves within the orbit of Neptune.
Apparently a larger body having a stronger gravitational pull has visited our solar system from time to time and disturbed the arrangement here. Lewis A Mason in his books, Out of the grey mist and The birth of the moon, proposes that every 200 million years our solar system encounters a star larger than our sun, which passes across within 10 million miles of the sun. In one of its earlier passages, its strong gravitational pull snatched matter from the sun, creating planets. Its subsequent visits created anomalies in different planets and produced the moon. Zacharia Sitchin in his book, The twelfth planet, proposes that a planet larger than Jupiter makes periodic visits to our solar system in a highly elliptic reverse orbit, and that its visits are marked by catastrophe, extinctions and major changes on earth. The strong gravitational pull of such a body, depending upon its proximity would tilt some planets, remove atmosphere and water from some of them and shatter some into pieces. The drastic atmospheric disturbances created by these bodies would have produced megamutations leading to the emergence of larger groups of animals and plants. One of the passages of such a star during Permian period caused mass extinctions but also produced mammals and dinosaurs.
Before Cretaceous, the earth was perpendicular to its orbital plane. There were therefore no seasons and no snow. Evidences indicate that the climate in Greenland was 30 degrees F warmer than at present. The earth was covered with shallow sea and lowland, and rich in animal and plant life. Then the passage of the star and its gravitational pull removed southern hemisphere’s crust like the skin of a ripe fruit to produce the moon. The strong jolt gave the earth 23 degree tilt. The remaining crust broke at several places to form continents, which started drifting towards the vast chasm, floating over the molten mantle. The star passed right through the solar system in about 60 weeks, leaving a trail of destruction and chaos. The strong gravity of the star also pulled water and 70% of the crust from Mars and hurled it towards the earth and Saturn, shattered the planet between Mars and Jupiter into Asteroid belt and placed Pluto into an eccentric orbit.
The catastrophe brought about by the visit of a giant planet during late Pleistocene swept large number of plants and animals in extensive floods and buried them deep into the earth and snow, as the temperature suddenly fell down. Extensive beds have been unearthed where bones of a variety of animals are mixed together. Such large scale burials deep into the earth also produced coal and petroleum. The fact that animals capable of living in torrid zones are buried in the frozen soil of the Polar Regions indicates catastrophe of gigantic proportion. As the star’s gravity pulled earth’s atmosphere from the South Pole side, a quick vacuum was created in the northern hemisphere imparting sudden death to animals by asphyxia, giving them no time to struggle. Coagulated blood in the lungs and grasses still in the mouth of frozen woolly mammoth fossils are indicative of that. It is strange that ancient tablets, scrolls, pictographs and folktales over the past thousands of years also refer to the catastrophe and floods that had befallen earth.
Fossil records indicate periodicity of 26 and 50 million years in minor extinctions and of 200 million years in major extinctions. Apparently such large scale destructions could not have been caused by the stray cometary or asteroid collisions. Much larger heavenly objects with tremendous gravitational power seems to have visited our solar system periodically to alter our destiny, depending upon the distance they passed through.