Chapter 2: THE AGE OF MAMMALS
|EXTINCTION OF THE DINOSAURS|
|Registered UCI students: view the slide show for this chapter or download it: http://darwin.bio.uci.edu:80/~sustain/protected/chap2slides.ppt|
(picture by NASA). At the end of the Cretaceous, 65 million years ago, not only did the dinosaurs disappear completely, but so did flying reptiles (pterosaurs), and marine reptiles (ichthyosaurs and plesiosaurs). In fact, between 60 and 80% of all animal species, including many marine forms, disappeared. Most turtles, crocodiles and primitive birds also disappeared but some survived to give rise to modern forms.
There have been numerous theories to account for the extinction of dinosaurs. But during the 1980's strong evidence was obtained to support the idea, originally proposed by Luis Alvarez, that a global catastrophe, caused by the impact of an asteroid, comet or meteorite, was responsible. Enormous amounts of debris would have been thrown into the atmosphere, making the Earth so cold and dark that cold-blooded animals like dinosaurs were unable to survive.
Supporting the impact theory, about 150 Impact Craters have now been discovered on the earth. One of the most spectacular is the Barringer Crater in the Arizona desert. (picture by NASA). The Barringer crater was formed about 30,000 years ago (much too young to have anything to do with dinosaur extinction). It is 7/10 mile across and 560 feet deep, and is thought to have been caused by an iron meteorite 200 feet in diameter, weighing one million tons, that hit the earth at a speed of 30,000 mph and released an amount of energy equivalent to the most powerful nuclear devices yet made. But it is tiny compared to some older craters.
The impact responsible for the extinction of the dinosaurs would have produced a crater at least 100 miles across. The entire planet shows a 3 mm-thick layer of rock at the appropriate level (i.e. exactly at the boundary between Cretaceous and Tertiary), containing several kinds of evidence for the impact:
|a high concentration of the element iridium, which is rare in terrestrial rocks but common in meteorites|
|"shocked" quartz grains showing parallel, colored markings which are an indicator of violent impact|
|unusual amino acids, thought to be extraterrestrial in origin|
In North America the 3 mm layer is underlain by a similar layer 2 cm thick, and it is thickest (46 cm) at sites near to Haiti and Cuba, suggesting an impact site in the Caribbean Sea. This layer contains many glass-like beads, mm to cm in diameter, that were probably formed by melting of rocks during the impact. The molten rock would have been ejected, and then would have fallen back to earth. The composition of these layers suggests an ocean impact.
Over the past few years geologists have discovered a 200 mile-diameter submarine crater (the Chicxulub crater), of exactly the right age but buried under 2 km of sediment, off the coast of the Yucatan peninsula in Central America. There are actually two layers, at least in western North America, suggesting that there were at least two impacts, possibly from different parts of a comet that broke up under the influence of the sun's gravitational field. It has been estimated that after the collision there was so much dust in the atmosphere that the entire planet was completely dark for 1-3 months. The lack of sunlight would have caused the extinction of many plants and animals. Among the animals, cold-blooded forms like dinosaurs would have been affected more than the warm-blooded mammals.
Geologists have found that the rock in the Chicxulub crater is unusually rich in sulfur. They conclude that the impact could have produced a global sulfur dioxide fog that could have caused darkness and acid rain for over a decade.
For a long time people found it hard to believe in such spectacular collisions. But impact craters are very common on many planets and the moon. And in 1994 a comet that had already broken into 13 pieces crashed into the planet Jupiter. Although the crash was not visible from earth, we were able to see the effects of the impact soon after it happened. This made it much easier to believe that comets can crash into planets. However, not everybody believes that this kind of catastrophe finished off the dinosaurs.
A meteorite impact was observed in Greenland in December 1997. A flash of light seen from fishing trawlers turned night into day, seismic signals were detected in Norway, and satellite images showed a cloud that suddenly formed and then blew away.
If you are interested in whether the Earth may be hit again by an asteroid or comet, NASA has a web site on impact hazards for you, including a list of the close predicted close approaches!
The survivors of the Cretaceous/Tertiary catastrophe included some small, squirrel-like animals that were to give rise to the dominant life forms of the next era - the fur-bearing, warm-blooded mammals that eventually gave rise to the human species. The period between the extinction of the dinosaurs and the present day is called the Age of Mammals or Cenozoic.
Mammals appeared on the earth long before the extinction of the dinosaurs; in fact, dinosaurs and mammals originated within 10 million years of each other, in the late Triassic about 200 million years ago.
By late Cretaceous small primitive marsupials (mammals that brood their young in a pouch, like opossums), and insectivores, similar to shrews and hedgehogs, were quite abundant and widespread. But only after the dinosaurs were gone did the mammals begin their great diversification and become the dominant land animals. Then, within 10 million years, there were mammals of all kinds living in many different habitats on land, in the sea and in the air. There were herbivores, carnivores, whales, bats. During the Cenozoic there was also tremendous radiation in other groups including birds, reptiles, amphibians and fish, leading gradually up to the peak of biological diversity that occurred in the recent past.
The geography of the world changed dramatically during the time when animals and plants were evolving. The major continental land masses were initially fused together into one giant continent named Pangaea during the Paleozoic era.
In the Mesozoic, Pangaea gradually broke up into the present-day continents, which have been moving apart from each other, by continental drift, ever since. This idea of continental drift was first based on the remarkably close fit between the coastlines of major continents, most notably the west coast of Africa with the east coast of South America. It is now supported by measurements, which show that the continents on either side of the Atlantic Ocean are still moving apart from one another, at the rate of several centimeters per year. Continental drift was actually a little more complicated, with the North American plate drifting around in the Pacific Ocean for quite a long time. A large chunk of the North American plate was recently found in Argentina, left there after the two continents bumped into each other then moved apart. Learn more about This Dynamic Earth.
The separation of the great land mass into different continents allowed biological evolution to take quite different paths in different parts of the world. And the formation of oceanic islands, often by volcanic activity, produced many more isolated areas where evolution could experiment with different forms.
Breakdown of this isolation, either by geological changes or by transport of organisms between the isolated areas, has often led to extinction of the endemic forms, and so loss of diversity.
During the Cenozoic era (the last 66 million years), there was a gradual lowering of temperatures as well as the gradual establishment of different climatic zones of the earth -the tropics, the temperate zones and the cool climates of the higher latitudes. The culmination of the cooling trend was the Pleistocene epoch, or Great Ice Age, of the last 1.8 million years. During this time vast expanses of North America and Eurasia were periodically covered with enormous continental glaciers. These glaciers advanced during the four ice ages (glacial periods) and retreated during the three interglacials. We are probably now living in the fourth interglacial stage. During the glacial periods the sea level became much lower because so much water was converted to ice. Consequently land bridges, especially the Bering land bridge across the Bering Sea joining Asia with North America, became available for animal migrations.
During the Cenozoic the mammals
reached their peak of evolution, producing a tremendous variety of species, many
of them very large. The segment of the fauna containing these large creatures
(those weighing more than about 100 pounds) is called the Megafauna. Most of
these animals are extinct.
We know about the Pleistocene mammals not only from fossils but also from carcasses, especially of the woolly rhinoceros, which have been found in petroleum deposits in Romania. Mammoths, complete with flesh, hair and stomach contents, have been found frozen in the ice in Siberia.
Some members of the Pleistocene megafauna were restricted to certain areas. For example, the woolly rhinoceros, giant deer, the moose-like giraffe shown in the slide, and the cave bear were found only in Eurasia and Africa. But the Bering land bridge has been present intermittently through the entire Cenozoic period. It allowed the entry of many of the large mammals that were to subsequently dominate the North American fauna, including the woolly mammoth, imperial mammoth, mastodon, bison, deer, sheep, cattle and many large carnivores. Slide shows a reconstruction of a Pleistocene scene in North America. Notice the saber-toothed cat with its enormous canine teeth.
The Bering land bridge also allowed animals that evolved in North America to colonize Asia. Horses and camels originated in North America and from here spread to Asia and South America. They subsequently (8,000 years ago for horses) became extinct in North America. Horses did not reappear in this country until Columbus' second voyage in 1493. See slide of the evolutionary history of the horse - one of the most complete fossil series available, often used as a classical example of evolution.
At the beginning of the Cenozoic era, there was a land bridge between North and South America, as there is today. This land bridge allowed primitive mammals to colonize South America from the North. This land link was later (during the Eocene) broken, and those animals which had settled in South America then evolved in complete isolation from the rest of the world. Over about 40 million years these primitive groups diversified in many unique ways.
The placental mammals (those that carry their young in a uterus, using a placenta to provide nutriment) of South America evolved as herbivores, many of them large and slow moving, like the ground sloth Megatherium which reached up to 29 feet tall when standing upright on its hind legs. The ground sloth was the largest and heaviest of all land mammals (there may have been a sighting in 1994!). Other South American mammals evolved single-digit feet ending in strong hoofs, exactly like those of modern horses, and others possessed a well-developed trunk and massive legs like the elephant's. These are examples of convergent evolution.
The marsupials (mammals that carry their young in a pouch, like kangaroos) evolved in the other direction, becoming carnivores. One of them bore an amazingly close resemblance to the independently evolved saber-toothed cat of North America, which was a placental mammal. This is another example of convergent evolution.
New forms of wildlife continued to arrive in South America even after the continent was cut off from North America. These "island hoppers" were able to cross the stretches of water then separating the continents because of their small size and/or their ability to swim. They included primates which gave rise to the New World monkeys, including howler monkey, marmosets, capuchins, woolly monkeys and spider monkeys; and rodents which evolved into a number of families, several of which are found nowhere else in the world. These include the capybara, the agouti, the coypu, the cavy (Guinea pig) and the chinchilla. In South America there is a greater variety of rodents than anywhere else in the world.
The Invasion of South America. In the upper Pliocene, 3 million years ago, the isthmus of Panama reappeared as a result of changes in the earth's crust. This was a disaster for many of the animals that had evolved in isolation in South America. South America was invaded by deer, camels, raccoons, tapirs, horses, mastodons, bears, peccaries, rabbits, shrews, cats, dogs, weasels and rodents. For some reason these animals were able to displace many of the South American species, driving many of them to extinction.
Some of the new arrivals (e.g. mastodons and horses) survived only for a brief period. Others were very successful, for example the camel family which has given rise to the vicunas, guanacos, alpacas and llamas. The camels as well as the horses subsequently became extinct in North America where they originated.
Another group of uniquely South American mammals, the edentates (sloths, armadillos and anteaters), survived the competition with the invaders and are still abundant in South America. The armadillos, like their primitive ancestors, are armor-plated mammals in which the armor plating is composed of separate shields and hinged bands. But the related species of one extinct group, the glyptodonts, had a single-piece carapace similar to that of tortoises. These glyptodonts, some of which were as big as a Volkswagen and armored like a tank, survived up until quite recent times and may have been hunted by primitive Indian tribes; piles of glyptodont bones have been found alongside various human artifacts.
A few of the animals that had evolved in South America migrated in the reverse direction, becoming established in North as well as South America: the anteater, porcupine, opossum and armadillo.
South America provides a spectacular example of how evolution can take off in novel directions when a region is isolated for a long enough period of time. It also provides a dramatic lesson in how apparently well adapted species can often be driven to extinction when exotic species (those coming from outside) are introduced.
The mammalian fauna of Australia also evolved in isolation since the early Cenozoic (Eocene), but in this case the isolation remained complete. For unknown reasons, Australia was apparently originally populated entirely by marsupials rather than placental mammals. Today the native mammalian fauna of Australia is made up of marsupials of many different kinds, that occupy ecological niches similar to those occupied by placental mammals in other parts of the world. Evolution produced marsupial mice, a marsupial mole, and, most impressive, a marsupial wolf (almost extinct) and a marsupial lion (extinct), all of which bear striking resemblances to the corresponding placental forms. These are examples of convergent evolution. Other Australian marsupials occupy the same ecological niches as certain placental mammals in other continents, but are rather different in appearance. For example the wallabies and kangaroos occupy the niche of browsing and grazing mammals which is occupied by the ungulates (hoofed mammals) in other parts of the world. The Koala, a tree-climbing, slow-moving herbivore, occupies the same niche as the tree sloths of South America. The Koala is now being considered for Endangered Species listing in this country; its population has plummeted because of destruction of eucalyptus forests in Australia. Around Christmas 2001 over 100 fires, most deliberately set, burned 1.2 million acres in southeast Australia, including huge areas of koala habitat. Thousands of koalas were lost out of less than 100,000 remaining.
Australia did produce some giant forms such as giant kangaroos, which are now extinct.
The Pleistocene is the time when humans evolved in the old world. The first hominids (i.e. creatures more closely related to humans than to apes) lived from 4 to 3 million years ago. These (called Australopithecus) lived in Africa. They had a protruding jaw, prominent eyebrow ridges and a small braincase. They walked upright.
1.8 million years ago, Homo erectus appeared in Africa, with a brain as big as the smallest modern human brain. H. erectus differed from modern humans by the prominent brow ridges and receding chin. They made sophisticated stone hand-axes with sharp edges, possibly made spear points, and probably used fire. They spread over Africa and Asia and survived until about 400,000 years ago.
The first fossils that are classified in the modern species Homo sapiens date from about 200,000 years ago (Nat. Geog. , Jan. 1996) and are called neanderthals (a subspecies of Homo sapiens). The neanderthals still looked primitive, with prominent brow ridges, low foreheads, and receding chins, but their brains were, on average, slightly larger than ours. They hunted woolly rhino and cave-bear and disappeared about 30,000 years ago.
About 30,000 years ago, fully modern humans called Cro-Magnon evolved from the neanderthal-like forms of the Near East and spread into Asia and Europe, rapidly replacing the more primitive neanderthals. They had domed heads, smooth brows, and prominent chins. They made precision tools, including definite spearheads, and they produced spectacular works of wildlife art on the walls of caves, which provide some glimpses of how the big game was hunted - with spears and rocks and probably also traps and fire. One painting shows an eviscerated bison about to gore a human.
How do we know the ages of things? Radiocarbon dating
During the last ice age (20-11,000 years ago), so much sea water was frozen into the ice caps that the sea level fell about 300 feet. This exposed a 1,000 mile-wide strip of land connecting the two continents called the "Bering Land Bridge", but at the glacial maximum of 18-20,000 years ago the continental glacier (the Laurentide Glacier) stretched all the way to the Pacific Ocean, blocking animal and human movement between the continents. At about 14,000 years ago a warming trend opened up an ice-free corridor between the main Laurentide glacier and the smaller Cordilleran ice sheet along the coast. This made it possible for early humans to migrate from Asia into the unglaciated lands of North America. Then during the present interglacial, melting ice caused the sea level to rise, cutting off the land connection between Asia and North America.
Early humans are believed to have arrived in North America about 11,500 years
ago, soon after the ice-free corridor opened up. These people are known as the
Clovis people, named after the town in New Mexico where their distinctive spear
points were found in 1932. Clovis points have been found in association with
mammoth bones at several locations in North America, suggesting that the Clovis
people hunted these animals. At some sites (e.g. Murray
Springs) bones of other large mammals have been found, including Horse,
Camels, Bison, Lion, and Dire wolf.
It has long been assumed that the Clovis people were the ancestors of American Indians, but this assumption is now being questioned. There are no human skeletal remains from the earliest arrivals, and only about ten sets of remains more than 9,000 years old. Surprisingly, physical anthropologists have concluded that the skulls of two of the oldest skeletons (Spirit Cave Man from Nevada, 9,400 years old and Kennewick Man found in Washington State in 1996, 9,300 years old) are quite different from those of modern American Indians. Their cranial vaults are long and narrow rather than round, their faces are slender rather than broad, and they do not have prominent cheekbones. These skeletons resemble those of Polynesians, Europeans, and the Ainu of Japan, more than they resemble American Indians, leading to the idea that American Indians may be derived from one or more distinct groups of settlers arriving some time after the Clovis culture. The third specimen (Wizards Beach Man from Nevada, 9,200 years old), is more similar to contemporary American Indians.
The base sequence of mitochondrial DNA (mDNA) from living American Indian populations is similar to that of samples from Asia and Siberia, confirming the earlier conclusions from archeology. But some mutations in American Indian mDNA are found elsewhere only among the aboriginal populations in Southeast Asia and in the islands of Melanesia and Polynesia, providing another clue for a possible immigration by sea. Unfortunately, three laboratories were unable to obtain any DNA evidence from Kennewick man.
requirement to return human remains to tribes that can show an affiliation,
so that they can be reburied, has led to
over how much scientific analysis of these samples will be
possible. In the case of Kennewick man, after the failure to obtain DNA
evidence the Department of the Interior in 2000 decided to abandon
scientific research and give
the skeleton to five Indian tribes who jointly claimed the remains for
Some evidence suggests human occupation of both North and South America earlier than the Clovis culture, leading to several fascinating alternative possibilities. Several sites in the southeastern part of the U.S. have been dated much earlier than Clovis, but the evidence from most of these sites has been seriously questioned for technical reasons. However, one remarkably informative early settlement in Monte Verde, Chile, makes it difficult to completely accept the traditional "Clovis-first" view. The site includes remains of dwellings and stone tools (not Clovis points) as well as medicinal plants, and dates from 12,500 years ago. This early date has led to suggestions that early migrants may have reached and explored North and South America by boat, rather than on foot. This would have allowed them to avoid the glacial barrier, it would have been faster, and it might have been easier, since food would have been plentiful. Archeological evidence shows that Pacific people used boats as early as 25,000 or even 40,000 years ago, so the technology may have been available. There were many unglaciated pieces of coastline that would have allowed breaks in the sea journey, and there are several sites along the coast of Canada, California, Peru, Ecuador, and Chile that show evidence for human occupation between 10,000 and 12,000 years ago. Unfortunately the sea level is much higher now than then, so much of the archeological evidence for seafaring at that time may now be deeply submerged and difficult to find.
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