Mammals appeared on
the earth long before the extinction of the dinosaurs; in fact, they originated
in the early Jurassic 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. Some of
these animals survived the Cretaceous/Tertiary catastrophe and evolved into 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 (the last 65 million years) is called the Age
of Mammals or Cenozoic.
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. Some of them were very large, and those
weighing more than about 100 pounds are referred to as the Megafauna.
Most of these species are extinct.
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.
GEOGRAPHIC
CHANGES AND EVOLUTION
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, 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 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.
EVOLUTION OF
MAMMALS ON DIFFERENT CONTINENTS
Mammalian
evolution in the Northern hemisphere.
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
Age of Mammals. It allowed the entry from Asia
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.
Mammalian Evolution in
South America.
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.
Mammalian
Evolution in Australia.
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.
During
the Cenozoic era, 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.