Re: EVOLUTION - TRUTH

On Jun 17, 10:00 pm, 9 Trillion Dollar Republican National Debt
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HISTORY OF LIFE ON EARTH

The purpose of Conservation Biology is to find ways of maintaining the
high levels of biological diversity that are seen in today's world.
But first, in order to appreciate that diversity, we need to
understand how it has evolved, and what a complicated series of
geological, climatic and biological events have led up to the present
day situation. Therefore, we will review briefly the history of life
on earth. Visit the virtual Museum of Paleontology at the University
of California at Berkeley to find more information and illustrations
regarding any of the topics or geological periods mentioned.

Geological Time

The chart shows the life forms that have been present on the earth
since Cambrian times, and whose fossils are therefore found in
different layers of rocks. Each time a new layer of rock is deposited
(usually by sedimentation) it is on top of the older layers, so the
chart reflects this order.

Rise of biological diversity

The history of global biological diversity is best seen in the marine
animals since the ocean is where life started, and marine animals are
the best represented in the fossil record. The chart in the slide show
shows the number of families of marine organisms on the earth at
different times in biological history.

See Measuring Past Biodiversity, by Jeremy B. C. Jackson and Kenneth
G. Johnson. Science 2001 September 28; 293: 2401-2404. [Summary]
[Full Text] [Supplemental Data]

Multicellular animals first appeared about 600 million years ago in
the early Paleozoic and there was a rapid rise in number of families
during the Cambrian and Ordovician. Diversity remained relatively
constant (perhaps even declining) up until about 200 million years ago
and then it rose again to its current all-time high of close to 800
families.

Four eras are recognized, and are characterized by typical life forms:

1. Precambrian: The origin of life.

2. Paleozoic ("Ancient Life"): The origin of plants, most
invertebrate types, the first vertebrates (back-boned animals,
including fishes, amphibians, and reptiles).

3. Mesozoic (the "Age of Reptiles"): The origin of flowering plants,
dinosaurs, birds, and mammals.

4. Cenozoic (the "Age of Mammals"): The diversification of flowering
plants, insects, birds and mammals, and the appearance of humans.

The eras are divided into periods. Biological diversity was
dramatically depleted by five mass extinction episodes at the ends of
the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods. At
each of these times a large fraction of existing species was wiped
out, leaving the survivors to repopulate the biological world. The
most famous of these was the extinction at the end of the Cretaceous
because this ended the age of dinosaurs and made possible the
evolution and dominance of mammals. But it was not the most
devastating of the mass extinctions. Now we will review the history
of life in more detail.
THE DAWN OF LIFE
Precambrian

The origin of life: the Earth was formed about 5 billion years ago. A
series of giant meteorites ("the late heavy bombardment") essentially
sterilized the planet about 3.8 billion years ago. Rocks 3.5 billion
years old contain microfossils of primitive one-celled organisms
without a nucleus ("prokaryotes") resembling bacteria and blue-green
algae, and carbon isotope ratios characteristic of biological
materials, representing the earliest clear signs of life. The first
cells with a nucleus ("eukaryotes") appeared 2 billion years ago, and
the first organisms made up of many cells (multicellular algae)
appeared about 1.8 billion years ago.

In addition to prokaryotes and eukaryotes, a third major group of
organisms, called Archaea, consisting of about 500 species but making
up about 30% of the biomass on Earth, was not discovered until 1977.
They live in the most extreme environments on Earth - the hottest,
coldest, and highest-pressure environments, so they are sometimes
called "extremophiles". Most of their known biomass is in the
Antarctic.
ANCIENT LIFE (PALEOZOIC); 600-230 M.Y.B.P.

Cambrian

After 3 billion years of very little change, all of a sudden during
the Cambrian period, there was a frenzy of evolutionary innovation
that generated just about all of the existing major types (phyla and
classes) of marine invertebrates plus many others that no longer
exist. Accurate dating methods used in 1993 (involving measurements of
uranium and lead isotopes) indicate that the explosion happened during
a very short period - from 533 to 525 million years ago. This is
sometimes called the Cambrian explosion. What caused it is not known -
one of the most plausible suggestions is that it followed shortly
after marine animals evolved protective shells and cuticles that
allowed them to exploit new ecological niches. Another possibility is
that more complex body organization became possible after the
atmospheric oxygen reached a certain threshold.

Many of the important fossils telling the story of the Cambrian
explosion were found in a fossil bed in the Canadian Rockies called
Burgess Shale, where both soft- and hard-bodied animals were buried in
an underwater mudslide and preserved in water so deep and oxygen-free
that they did not decompose. They are described in a book by Stephen
J. Gould called "Wonderful Life". View pictures of the actual
fossils.

Two different views on the role of chance and selection in evolution:

Wonderful Life: The Burgess Shale and the Nature of History by Stephen
Jay Gould (1990). W.W. Norton & Company.

The Crucible of Creation: The Burgess Shale and the Rise of Animals by
Simon Conway Morris (1998). Oxford University Press, 272 pp.

The Cambrian explosion probably generated over 100 major animal groups
with fundamentally different body plans, but only about 30 survived to
this day. Examples of those that survived are arthropods (insects,
spiders, crustaceans), echinoderms (sea urchins and starfish),
mollusks (snails, clams), and chordates, the latter represented in the
Cambrian by worm-like animals with a rod-like backbone but eventually
giving rise to the vertebrates. Although large numbers of new families
evolved in the later periods of earth history, these later innovations
did not include major reorganizations of the body plan, but rather
variations on themes that were established in the
Cambrian.

We will look at the kinds of animals and plants that were present on
the earth at different periods, starting with a jump to the
Carboniferous because at that time, both plants and animals had moved
out on to the land and were beginning to diversify.

Carboniferous (360-286 million years ago)

During the Carboniferous the climate was hot and humid, and there were
extensive swampy forests dominated by giant tree ferns and conifers,
club mosses, and horsetails. The decomposed remains of these plants
gave rise to the major coal and oil reserves of today. There were no
flowering plants and no grasses.

Unlike modern fish that breathe through gills, some of the
Carboniferous fish breathed air through two lungs, and they had
developed lobe-like fins. The fascinating search for a surviving
relative, the Coelacanth, has been recounted in Samantha' Weinberg's
"A Fish Caught in Time": click on the picture. These fish live in the
deep ocean, and since their discovery in 1938 more than 200 more of
them have been captured.? The other surviving relative of these
Carboniferous fish is the lungfish.

The Carboniferous fish gave rise to modern bony fishes.? In these the
fins have lost their lobes, and one lung has been lost and the other
converted to a swim bladder. They also gave rise to the amphibians, in
which both lungs have been retained and the limbs strengthened and
specialized for walking. These amphibians, like modern frogs and
salamanders, hatched from eggs and spent their larval period in the
water as tadpoles. They then went through a change of body
organization called metamorphosis, in which they lost their tail and
developed legs, and moved out on to the land as adults. They were the
first land-dwelling vertebrates, and some of them were massive animals
two or three feet long. In addition to the primitive amphibians there
were some very primitive reptiles that were totally terrestrial, like
today's lizards.? In these animals the egg gave rise directly to a
miniature version of the adult, which could survive on the land.

The arthropods of the Carboniferous had also moved onto the land and
had in fact taken to the air. Many of the ancient insects were
gigantic compared to present day ones, among them mayflies with a 14-
inch wing span, and giant six-winged insects, in which the first pair
of wings had been already reduced to nubs but not eliminated as they
are in all present-day insects. These insects had long sucking
mouthparts with which they sucked the juices out of large primitive
pinecones. Feeding on these insects were dragonflies with a 30-inch
wingspan!
Permian (286-245 million years ago)

The land was increasing in altitude in the Permian and the climate was
cooler and dryer than it was in the Carboniferous. The landscape was
also developing more topography, with low hills covered with primitive
vegetation such as ferns. Small streams and lakes were starting to
appear.

Reptiles such as the large sail-backed carnivore Dimetrodon, were
common at this time. Some of these early reptiles had elaborate sails
on the back. The purpose of these structures is not known although
they are thought to have been involved in temperature regulation; that
is, to radiate heat when the animal was too hot and to absorb it when
the animal was too cold.

The most successful land animals of the middle Permian were many kinds
of mammal-like reptiles, which were quite varied and included both
herbivores and carnivores. Most of them were large, heavy and slow
moving. None of them survived, so we can't tell if they were warm-
blooded or if they had mammary glands, like true mammals, but they did
have a jaw articulation like that found in mammals and they had
several types of teeth, which is a mammalian feature.

At the end of the Permian, 245 million years ago, huge numbers of
animals became extinct. About 96% of all marine animal species and 52%
of the families disappeared in the greatest mass extinction the world
has ever known. Recent data shows that 8 of 27 orders of insects, 21
of 27 families of reptiles, 6 of 9 families of amphibians and most of
the terrestrial plant life disappeared at this time. This happened
over a remarkably short period - about one million years. The most
popular theory is that it was caused by flood volcanism in Siberia -
huge continent-sized floods of scorching hot magma, about a mile deep,
poured out from fissures in the Earth's crust and spread over the
land. The eruption lasted for about a million years, generating about
2 million times as much lava as was involved in the eruption of Mount
St. Helens in 1980. This might have generated enough debris to block
out sunlight and trigger an ice age, enough sulfuric acid to acidify
the oceans, and/or enough carbon dioxide to cause global warming by a
greenhouse effect. However, studies reported in 2001 suggest an
alternative explanation - that the extinction might have been caused,
like the later one that finished off the dinosaurs, by a giant
meteorite hitting the Earth. It took about 100 million years for the
species diversity to recover from this extinction event.

THE AGE OF REPTILES (MESOZOIC): 245-65 million years ago

Triassic

Reptiles and mammal-like reptiles dominated the landscape in the
Triassic. Late Triassic was the time when the first true mammals,
descendants of the mammal-like reptiles, appeared. The first ones were
small, like Megazostrodon, resembling a modern shrew.
Jurassic and Cretaceous

At the end of the Triassic many of the amphibians, reptiles and most
of the mammal-like reptiles disappeared and were replaced by the
dinosaurs and crocodiles in addition to some early turtles, lizards
and frogs.

The Jurassic and Cretaceous together were the age of giant reptiles.
For 150 million years, the dominant vertebrates on the land were the
dinosaurs. (By comparison, humans have been on the earth for only
about 3 million years).

The dinosaurs are composed of two distinct orders:

The Ornithischians (bird-hip dinosaurs) included the duck-billed
dinosaurs or Hadrosaurs, Stegosaurs with their plate-like armor along
the back, Ankylosaurs which were heavily armored and flattened, and
the rhinoceros-like horned dinosaurs or Ceratopsians (example -
Triceratops).

The Saurischians (lizard-hips) included the Sauropods - gigantic
herbivorous dinosaurs with extremely long necks and tails, such as
Brontosaurus and Brachiosaurus - the largest terrestrial animals that
ever lived; and Theropods - carnivorous dinosaurs with enormous
skulls, powerful teeth and ridiculously small front legs,
Tyrannosaurus being the best known example but Giganotosaurus was
bigger. Fossils of a new species of theropod, about the size of a
dog, were recently discovered on Madagascar.

The dominant animals of the oceans were the ichthyosaurs, some of
which were as large as medium sized whales, the long-necked
plesiosaurs and some marine crocodiles. The dinosaurs also took to the
air and evolved some spectacular and huge flying reptiles called
pterosaurs. One of them, Quetzalcoatlus, was the largest flying
vertebrate the world has ever known, with a 40-foot wingspan!

Some cretaceous dinosaur fossils discovered in China had a downy
covering resembling primitive feathers, which probably functioned in
keeping the animal warm rather than for flight. But a recently
reported fossil shows feathers on all four legs, and these may have
been used in gliding. These small dinosaurs are the only ones to show
early wing development, so they may have been early ancestors of
birds. The next stage of bird evolution, where the front limbs are
wing-like but the hind limbs still leg-like, are seen in the Jurassic
fossils of Archaeopteryx and its relatives. Archaeopteryx has been the
focal point of many arguments between creationists and evolutionists,
mainly concerned with whether it was a true intermediate between
dinosaurs and birds, or just an unusual type of early bird. It had
claws on its wings, once suggested to be leftovers of the dinosaur
anatomy - but at least three modern species of birds also have wing
claws (hoatzin and touraco, only in young stages; ostrich, even in the
adult). Recent studies of the front limbs show that the dinosaur
digits correspond to the human thumb, index and middle finger, whereas
those of early and present-day birds (and alligators and turtles)
correspond to the index, middle and ring fingers. The feathers are
considered the defining characteristic of birds, and were apparently
fully developed in Archaeopteryx. All of these features show that
Archaeopteryx was an early bird, without any distinct reptilian
features.

The predominant land plants in the Jurassic were the Cycads, primitive
palm-like seed plants. A few species of cycads still exist in
tropical and sub-tropical regions today, and although they are used as
ornamental plants some of them are facing possible extinction. Early
Cretaceous saw the evolution of flowering plants (angiosperms). The
flowers provided a new food source for pollinators, mainly insects,
with profound effects on the evolution of those forms.

The end of the Cretaceous saw the culmination of dinosaur evolution.
Dinosaurs were more varied and adapted for more different modes of
life than any other group, before or since. There were at least 100
species of dinosaurs. Some of them even lived in the Arctic, which was
probably a lot warmer than it is now. They had achieved a level of
evolutionary success that guaranteed them a permanent place in
history, and yet they were soon to disappear entirely from the face of
the earth. Several different groups of bird-like dinosaurs had
evolved, and all except one - Archaeopteryx, the ancestor of all
modern birds - died out at about the same time as the rest of the
dinosaurs.

Further reading:

Species, Speciation, and the Environment by Niles Eldredge
The Sixth Extinction by Niles Eldredge
(CENOZOIC): 65-0 M.Y.B.P.

EXTINCTION OF THE DINOSAURS

(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.

An impact crater in northwestern Australia has been dated to the end
of the Permian period, suggesting a possible connection to the mass
extinctions that occurred at that time.

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 predicted close approaches!

EVOLUTION OF MAMMALS

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.

EVOLUTION OF HUMANS

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 in Africa about 7 million years ago. Scientists have now
identified about 14 species, of which the best known is
Australopithecus. They were 1-1.5 meters tall, walked upright, had a
protruding jaw, prominent eyebrow ridges and a small braincase.

2.5 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.

Last year scientists reported discovering remains of an entirely new
species of dwarf human, Homo floresiensis. These people were about one
meter tall and lived about 18,000 years ago on the island of Flores,
east of Java. Not only were they hobbitt-sized, their brains were the
smallest of any known hominid. Remains of dwarf elephants were found
in the same deposits as the humans.

How do we know the ages of things? Radiocarbon dating

HUMANS ENTER THE NEW WORLD

During the last glacial period (the Wurm glaciation of 120-20,000
years ago), so much sea water was frozen into the ice caps that the
sea level fell about 300 feet, and Asia and North America were
connected by a strip of land called the "Bering Land Bridge".
However, during this time a huge glacier (the Laurentide Glacier)
stretched all the way from the Arctic 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 *** along
the coast. This made it possible for early humans to migrate from Asia
into 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.

North and South America were free of humans until they arrived from
Asia about 11,500 years ago, soon after the ice-free corridor opened
up. These Clovis people are 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 a suggestion that the ancestors of
American Indians may have arrived by sea. Unfortunately, three
laboratories were unable to obtain any DNA evidence from Kennewick
man.

The legal requirement to return human remains to tribes that can show
an affiliation, so that they can be reburied, has led to enormous
controversy 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 reburial. But then a judge in 2002
ruled in favor of a group of scientists who sued to obtain the remains
for scientific study, since there was no proof for a "cultural
relationship" between the skeleton and present-day American Indians.

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.

LATE PLEISTOCENE EXTINCTIONS

In late Pleistocene, during the last 50,000 years, there were mass
extinction events in many different parts of the world, involving at
least 200 genera (plural of genus = a group of related species). But
this was different from previous episodes of mass extinction:

1. It was much more selective, involving mainly the megafauna: the
large herbivores (mammoths, mastodons, huge ground sloths, cave bears,
woolly rhinoceros, other rhinoceroses, etc.) and the carnivores that
fed on them, the dire wolves and saber-tooth cats. There was no
accelerated extinction of smaller terrestrial species, plants, or
marine organisms.

The following disappeared from America, Europe and Australia:

All herbivores

1000 kg

75% of herbivores

100-1000 kg

41% of herbivores

5-100 kg

< 2% of herbivores

< 5kg

2. It occurred at different times on different land masses:

Time of start of
major extinction episodes
(years before present)

Africa and S.E. Asia

50,000

Australia

50,000

North Eurasia

13,000

North America

11,000

South America

10,000

West Indies

4,000

New Zealand

900

Madagascar

800

This excludes any global catastrophe or climatic change as an
explanation.

In all of these cases except Africa, the extinctions occurred shortly
after the first arrival of prehistoric humans. The first humans were
faced with animals that had evolved in the absence of human predators,
and the animals were probably easily overcome. Therefore, the most
plausible explanation is that these extinctions were caused by
overexploitation by human hunters.

In Africa, massive extinction does not coincide with the arrival of
humans. Humans had been evolving there for millions of years without
causing mass extinctions (they may not have been as carnivorous as
their descendants in other parts of the world) but it does coincide
with the maximum development of advanced early Stone Age hunting
cultures.

Many authors have remarked that to see what the Pleistocene was like,
you should go to Africa. Africa still has more large herbivores
(including elephants, hippos, rhinos, etc.) than any other place on
earth. But, even in Africa, the big game we see today is only about
70% of the genera that were present in mid-Pleistocene. About 50
genera disappeared about 40,000 years ago.

It is paradoxical that the region where humans have existed the
longest (Africa) retained a wide variety of big game whereas the areas
where humans arrived more recently have suffered a more complete
loss. Perhaps the African big game had time to evolve defensive
behavior, whereas species elsewhere were caught defenseless and naive
by a newly arrived advanced hunting culture.

Australia once boasted a spectacular megafauna including giant wombats
as big as grizzly bears and giant kangaroos. But the continent was
colonized by humans (already Homo sapiens) around 55,000 years ago and
subsequently lost all of its large and medium-sized mammals; in fact
all except some kangaroos. All 19 species exceeding 100 kg and 22 of
38 species 10-100 kg disappeared, along with three large reptiles and
the 450lb flightless bird Genyornis. Miller et al. used eggshell
dating to show that Genyornis disappeared suddenly around 50,000 years
ago, very shortly after the first arrival of humans. This does not
necessarily mean that the animals were simply hunted to extinction.
The humans brought to the continent the use of fire as a hunting tool,
and this may have destroyed so much vegetation that many herbivores
were deprived of their food and could not survive. Although some
authors have claimed that the Australian megafauna was wiped out
quickly after the arrival of humans, careful analysis of the ages of
various remains suggests that man may have coexisted with the
Australian megafauna for over 10,000 years. More discussion.

North America. 12,000 years ago, North America had an amazing
Megafauna including condors with a sixteen-foot wingspan, ground
sloths as big as hippos, three kinds of elephants, three kinds of
cheetah and five other kinds of big cat, several kinds of pronghorn
antelopes, long-legged, antelope-like pigs, an assortment of camel,
llama, deer, horse, and bison species, giant wolves, giant bears and
giant armadillos.? North America has been called a "super-Serengeti"
with more big animal species than you would find in Africa. But 11,000
years ago, nearly all of these big animals - 70 species or 95% of the
megafauna - disappeared completely. This is exactly the time when
humans (Paleo-Indians) colonized North America, and their arrival and
skill as hunters at that time is documented by the appearance of
artifacts. The disappearing mammals in North America included all of
the following:

*Mammoths

*Mastodons

*Horses

*Tapirs

*Camels

*Four-horned antelopes

Ground sloths

Peccaries

Giant beaver

Dire wolves

Giant jaguar

Saber-tooth cat

The carnivores on the list were probably not hunted directly, but were
dependent on the large herbivores for food, so soon followed them to
extinction. In some cases accurate dating methods have shown that
certain species became extinct at exactly the times that humans
arrived. Giant ground sloths and mountain goats in the Grand Canyon
both went extinct 11,100 years ago, which is the time that the human
hunters arrived (within the accuracy of dating methods, which is +200
years). There is also direct evidence for killing by humans. The human
archeological sites from 11,000 years ago have stone projectile
points, which were presumably used in hunting the large mammals. One
mammoth skeleton has eight stone spear points among its ribs. Some of
the large mammals were trapped in pits, and some were cornered using
fire. La Brea tar pits and the Page Museum is an excellent place to
see the fossils and reconstructions from this period. Mammoth Trumpet
(a newsletter about the first Americans).

Detailed study of late Pleistocene extinctions in North America
(Martin, 1986) suggests that they happened over just a few hundred
years. This explains why there is so little archaeological evidence
for hunting of mammoths in the New World. The total number of mammoths
from archeological sites in North America is 38; in Asia, where
mammoths were hunted for many thousands of years, there are many more
mammoth remains -e.g. remains of 1000 mammoths at just one site in
Czechoslovakia and of 100,000 horses at another site. Paul Martin has
suggested that the human population quickly expanded south from the
Bering land bridge, and exterminated the big game as they went
("Blitzkrieg" model). Martin, P. S.1986. Refuting late Pleistocene
extinction models. In Elliot, D.K. (ed) Dynamic extinction. Wiley and
Sons, NY. 1073-130.

Other authors have disputed the idea that human hunting finished off
the Pleistocene megafauna of North America. For example, Donald
Grayson, an archaeologist at the University of Washington, suggests
that climate shifts and associated vegetation changes could have been
responsible. Grayson disputes two aspects of the overkill hypothesis:

1. Out of the 35 genera that became extinct around this time, only 15
have been shown to have survived beyond 12,000 years ago. So 20 genera
may have disappeared before human arrival.

2. There is good evidence for mammoth kills by the Clovis people, but
no evidence that they hunted any other large mammals (he does not
mention the evidence that they hunted two kinds of buffalo).

Paul Martin responds that the Pleistocene megafauna had survived
several climatic changes during the previous million years, some more
severe than the one that occurred at the end of the Pleistocene. Yet
these changes did not cause multiple extinctions.

South America was also colonized by humans about eleven thousand years
ago, and since that time it has lost 80% of its genera of large
mammals, including ground sloths, horses, and mastodons.

SURVIVORS FROM THE PLEISTOCENE

In North America, the only surviving herbivores of the megafauna are
bears, elk, moose, buffalo and mountain lion. The horse also survived,
but only through its domestication and preservation overseas. The
moose was hunted to near extinction but has recovered to a population
of about 1 million. Yellowstone moose decline due to hunger, not
predators, ENN Daily News -- 10-6-1999

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