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Animals
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Animals are multicellular
eukaryotes. They are
heterotrophs: they get their food
and energy from other organisms.
Animals are motile during at least
part of their life cycle. Nearly all
animals are diploid throughout
their life cycle, except for the
single-celled gametes.
The evolutionary ancestor of
animals is not clear: animals have
been in the fossil record from very
early times, and none of the
protists is an obvious ancestor.
We divide the animals into
vertebrates (animals with
backbones, like us) and
invertebrates (animals without
backbones). There are far more
invertebrates than vertebrates.
Animal Body Plans
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Several evolutionary trends are seen
among the animals: body symmetry
patterns, cephalization, development
of a gut, internal body cavities, and
segmentation.
Body symmetry. Most animals have
bilateral symmetry or radial symmetry.
Bilateral symmetry means that the
body halves are mirror images—we
are bilaterally symmetric. Radial
symmetry means that the body is
composed of several similar body
parts arranged like spokes on a wheel.
Cephalization. As animals get more
complex, the head becomes more
differentiated from the rest of the body.
It develops more sensors (like eyes
and feelers) and more nervous system
(brain). These developments allow the
animal to react more quickly to
changes in the environment.
Animal Guts
• The gut is a sac or tube
projecting into the body where
food is digested.
• In more primitive animals, the
gut has only a single opening,
that is used to take in fresh
food and to expel waste
products: it is both a mouth
and an anus.
• The development of a gut tube,
with a separate mouth and
anus, occurred later. This
system is more efficient: food
passed through in a one way
flow.
Body Cavities
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What is between the body wall
and the gut?
The simplest animals have this
regions packed with body organs,
but more complex animals have a
cavity separating the body wall
from the gut.
If this cavity has a lining that holds
the internal organs in place, it is
called a “coelom”.
Some animals have the cavity
without a lining. This condition is
called “pseuodcoelomate”.
Segmentation
• One way complex
animals are built is to
combine a series of
similar segments
together. The obvious
case is the segmented
worms, but insects and
vertebrates are built the
same way. The
segments in higher
animals show more
differentiation into
different functions.
Invertebrate Groups
• We are going to
examine several
groups of animals: the
sponges (porifera),
Cnidarians (jellyfish
and hydras),
flatworms,
roundworms
(nematodes), annelids
(segmented worms),
mollusks, arthropods,
and echinoderms
(starfish). Later we
will look at the
vertebrates.
Sponges
• Sponges are the phylum Porifera. They are the
simplest group of animals.
• Sponge bodies have no symmetry. The body contains
many pores that water flows through
• Just a few cell types, and they are relatively
independent of each other.
• . Flattened cells line the outside. The inside cells have
flagella that propel water through the pores and trap
food particles. In between these two linings is an area
filled with sharp glasslike spicules, that are a major
deterrent to predators. Amoeba-like cells roam this
interior space and secrete the spicules.
• Sponges can reproduce sexually or asexually. Sperm
are released into the water, while eggs are retained
within the sponge. After fertilization, a larval stage
develops. Sponge larvae swim away to find a suitable
environment to take root in.
Sponge Anatomy
Asexual Reproduction in Sponges
I can reproduce asexually….if I
wanted to….
Cnidarians
. Cnidarians are animals with
radial symmetry and tentacles.
Jellyfish, sea anemones, corals,
and hydras are common
examples.
Cnidarians have 2 basic body
forms: the medusa, which has a
bell-shaped floating body with
tentacles hanging below it, and
the polyp, whose body is a tube
anchored at one end and with a
tentacle-ringed mouth at the
other end. Larval and adult
cnidarians often switch between
these forms.
Cnidarian Life
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Unlike the sponges, cnidarians have a
nervous system. It is called a “nerve
net”, because there is no centralized
brain. The nerve net controls
movement of the body and the
tentacles.
There is only a single opening into the
gut, used as both a mouth and an
anus. Food is taken in, digested, and
waste products are expelled through
this opening. Cnidarians move by
rapidly expelling water out of the
mouth.
Cnidarian larvae are motile, but many
adult cnidarians are rooted to one
spot.
Cnidarians contain a special defensive
cell called a nematocyst. It contains a
coiled barb that shoots out and injects
a poison when triggered.
Flatworms
• Flatworms are the first group of
bilaterally symmetric animals
we will study.
• Examples of flatworms:
planarians (which can
reproduce by dividing in half),
flukes (parasitic worms), and
tapeworms (which live in the
gut as parasites).
• Flatworms have no coelom,
the internal cavity between the
gut and the outer body wall.
The entire space within the
flatworm is packed with
organs.
Flatworm Anatomy
• Unlike the sponges and
cnidarians, flatworms have
distinct internal organs that do
specialized tasks.
• Flatworms have a nervous
system with a primitive brain in
the head.
• Flatworms also have a
digestive system, but only a
single mouth/anus opening,
and an excretory system to
maintain ion balance. Also
separate male and female
gonads.
Roundworms
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Roundworms are also called
nematodes. They exist in almost
every habitat, feeding on bacteria and
decaying organisms. Some are
parasites, but many more are freeliving.
Trichnosis is a disease caused by
eating nematodes embedded in
uncooked pork meat. Elephantiasis is
another nematode-caused disease.
Nematodes are bilaterally symmetric,
with tube-shaped bodies.
Nematodes have a complete tube for a
digestive system, with a separate
mouth and anus.
Nematodes have a body cavity, but it
is unlined. Thus, nematodes are
“pseudocoelomate”.
Protostome/Deuterostome Split
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Animals that developed after this time all
have coeloms, lined gut cavities. Guts are
now tubes, not sacs. A major split occurs
between two major groups, depending on
which develops first, the mouth or the anus.
In protostomes (which means “mouth first”),
the mouth develops first, and the anus
develops after most of the body has formed.
Protostomes include the molluscs, the
annelids, and the arthropods.
In deuterostomes, the anus develops first
(deuterostome means “mouth second”).
Deuterostomes include the echinoderms
(starfish) and chordates, including the
vertebrates.
Animals past this point all have 3 basic body
layers: the ectoderm, the mesoderm, and
the endoderm.
Ectoderm forms the skin and nervous
system, mesoderm forms the muscles,
bones and most internal organs, and
endoderm forms the gut.
Mollusks
• Mollusks are animals with
bilateral symmetry, a soft
fleshy body, and often a hard
shell. The body is covered with
a soft tissue called a mantle;
the mantle secretes the shell.
• Snails, clams, and octopuses
are common mollusks.
• Mollusks are the most complex
invertebrates. The cephalopod
group (octopuses and squids)
have large eyes, well
developed brains, and the
ability to move very quickly.
Annelids
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Annelids are segmented worms.
Earthworms are a common type.
Annelids are composed of
numerous segments, each with its
own bristles, muscles, nerves,
blood vessels and kidneys. The
gut runs through all segments,
and there is a nerve cord running
through the body connected to a
brain in the head.
Annelids have a hydrostatic
skeleton: there are no hard parts,
but there is a tough flexible outer
wall that keeps its tube-like shape
due to water pressure from inside.
Arthropods
• Arthropods are probably
the most successful
group of animals: there
are more different
species of arthropod
than any other group.
• The largest arthropod
groups are the insects,
the crustaceans
(shrimps, crabs, etc.),
and the spiders.
Arthropod Characteristics
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Arthropods have a hard
exoskeleton. It is composed
mostly of chitin, a polysaccharide.
This serves as protection against
predators, and as a way to stand
up against gravity. It also has a
waxy coat to prevent desiccation.
One problem with a hard
exoskeleton is that as the
arthropod grows, it must shed its
exoskeleton and grow a new,
larger one. This process is called
molting.
Jointed limbs allow arthropods to
move around easily. The muscles
attach to the exoskeleton.
More Arthropod Characteristics
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Respiratory structures. Aquatic
arthropods (mostly crustaceans)
use specialized gills to breath.
Land-dwelling arthropods have
tubes called trachea that run
through their bodies, delivering air
to all cells. Some also have lungs
to pump the air through the
trachea.
Sense organs. Some insects
have very complex eyes. The
structure is very different from our
eyes, but they use the same visual
pigment, rhodopsin, that we use.
Insects also have well developed
chemical senses (smell) and
hearing.
Metamorphosis
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Many insects have a larval stage that
is very different from the adult stage.
This allows specialization for feeding
and for mating at different times during
the life cycle. The larvae feed in one
environment, but the adult can mate
and migrate in a different environment.
Conversion from the larval form to the
adult form is the process of
metamorphosis.
Metamorphosis involves a pupal stage,
a resting condition in which the body is
remodeled. The cocoon is the pupal
stage of butterflies.
Not all insects have metamorphosis:
many insects just have smaller,
immature forms for the young.
Successive molts bring them to the
adult size and maturity.
Echinoderms
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Echinoderms are the starfish, sea
urchins, and similar sea-dwelling
creatures
Echinoderms are deuterostomes:
the mouths develop after the
anus. This is also characteristic of
the vertebrates, but not mollusks,
annelids, or arthropods.
Echinoderms have spiny skins
and radial symmetry. However,
echinoderm larvae have bilateral
symmetry
Echinoderms have a nervous
system, but no central brain.
However, they arms are able to
share information to coordinate
their movements.
Chordates
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Chordates are the larger group that contains the vertebrates, animals with
backbones. Other chordates are the tunicates (sea squirts) and lancelets.
Chordates are bilaterally symmetric, coelomate, deuterostomes.
They are characterized by having a notochord, a long rod of stiffened tissue that
supports the body and runs along the back. In many chordates (including us), the
notochord vanishes: it is only seen in the embryo. It is replaced by the vertebrae of
the spine.
Chordates also have a nerve cord in their backs: our spinal column, for example.
Chordates also have a set of slits in the wall of their pharynx. The pharynx is a
muscular feeding tube (the throat), and the slits are used as gills in marine
chordates..
Invertebrate Chordates
• The tunicates are bag-like
marine organisms that are
rooted to one spot as adults.
They squirt water out a siphon
when irritated. Their larvae are
typical chordates, with bilateral
symmetry, a notochord,
nervous system, and pharynx.
After metamorphosis, most of
the nervous system is lost and
they are converted into filter
feeders.
• The lancelets are filter feeders
that look like very primitive fish.
Amphioxus is a common
example.
• Both tunicates and lancelets
use their pharynx and gill slits
to suck in water and filter out
the food.
Key Innovations in Vertebrates
• Much of basic vertebrate evolution can be understood in
terms of an arms race between predators and prey.
• Vertebrae instead of a notochord: a hard, flexible
backbone allows a stronger and faster body.
• Jaws: more efficient predation, and also support for gill
arches.
• Brains and sensory organs: to detect and react to
predators or prey.
• Paired fins along the body: for steering and additional
propulsion. Eventually evolved into legs.
• Gills: more efficient than taking oxygen directly across
the skin, as the lancelets do. Lungs developed from gut
wall pouches: better for direct breathing from the
atmosphere.
Early Fish
• The first fish were jawless.
They were scavengers and
filter feeders. Modern
examples include the lampreys
and hagfishes. They did not
have hardened bones or
paired fins along the body.
• The placoderms are an extinct
group of fish, the first group
that had jaws, with teeth in
them, and also paired fins.
Their heads were covered with
bony plates, but they still used
a notochord as support instead
of a bony vertebral column.
Modern Fish
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Two groups: the cartilaginous fish
(like sharks and skates), and the
bony fish (most other fish).
Cartilaginous fish have skeletons
made of cartilage, not hardened
bone.
Bony fish have hardened bones
throughout their skeleton. They
are the most numerous and
diverse of the vertebrates.
The lobed fin fish have fins that
are fleshy extensions of the body.
These eventually developed into
legs.
Lungfish developed lungs to take
oxygen directly from the air, a
useful trick when living in stagnant
water and ponds that dry up.
Amphibians
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A few fish can spend a little time
on land, enough to move from
pond to pond, for example. But,
without legs or efficient lungs, fish
don’t live on the land.
Amphibians have jointed legs and
a bony body. They have lungs
during at least part of their life
cycle, but their skin also
participates in gas exchange.
Amphibian skin must be kept
moist, so many amphibians live
most of their lives in the water.
Amphibian eggs are soft and jellycoated. They must develop in
water.
Common amphibians: frogs,
toads, salamanders.
Metamorphosis and Neoteny
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The larvae of most amphibians
develop in the water. They have long
tails for swimming and gills for
breathing.
Some amphibians, such as frogs,
undergo a radical shift in body form
when moving to adulthood. The tail is
absorbed into the body, gills are lost
and lungs develop, legs develop. The
animal is now capable of living at least
part of its life on land.
Some salamanders (axolotl) retain
some of their juvenile characteristics:
long tail, gills. However, they become
sexually mature. This process of
retaining juvenile features into
adulthood is called neoteny. The
more-or-less hairless condition of
humans is also an example of neoteny.
Apes are born hairless, and then hair
over the entire body develops as they
mature. We retain the hairless state.
Reptiles
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Reptiles were the first group of
vertebrates to live completely on dry
land. Most important development is
the amniote egg: an egg surrounded
by a waterproof membrane. Fish and
amphibian eggs are coated with jelly
and must be kept in water.
Reptiles also have a waterproof skin
(amphibians use their moist skin for
gas exchange), internal fertilization
(amphibians release sperm and eggs
into the water), and kidneys that
concentrate urine to conserve water
(amphibian kidneys mostly function to
excrete excess water, not retain it).
Nitrogenous waste in reptiles and birds
(from protein metabolism) is converted
into uric acid. Mammals convert it to
urea. Uric acid is not very soluble in
water, and it is excreted as a white
paste: bird droppings.
More Reptiles
• Common reptiles: lizards,
turtles, crocodiles,
snakes.
• Dinosaurs were the
dominant land animals
throughout the Mesozoic
period. Dinosaurs were a
major adaptive radiation
of the reptiles. They
became extinct at the end
of the Mesozoic.
Dinosaurs
• Dinosaurs and related
mammals lived over a
very long period of time,
between 250 and 65
million years ago. Many
species and groups arose
and became extinct
during this period.
• At least some dinosaurs
were probably warmblooded (like birds).
More on the Extinction
Birds
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Birds are the modern descendants
of the dinosaurs. Birds are warmblooded: they maintain a constant
internal temperature. This makes
enzyme action more efficient,
because the enzymes can always
function at their optimum
temperature.
Most birds can fly, and much of
their structure is based on this.
Bones are hollow to decrease
weight, the circulatory system is
well developed for rapid pumping
of oxygen and nutrients to the
muscles, lungs are very large.
Birds are covered with feathers,
which are lightweight insulating
structures modified from the
scales that cover reptiles.
Mammals
• Mammals are characterized by having hair and
mammary glands (organs in females that
secrete nutritious milk for their offspring).
• Mammals have more behavioral flexibility and
learning ability than other groups of animals.
• Mammals have teeth, which are quite different
from the teeth found in other groups such as
sharks.
Mammalian Groups
• Most mammals are
placental: they have a
placenta that feeds the
developing offspring in
the uterus. A placenta is
a tissue composed of
both fetal and maternal
tissues that is attached to
the uterine wall. It allows
the developing animal to
live in the uterus for a
long time, until it is
relatively mature.
Marsupials
• Marsupials are a smaller
group of mammals that
includes the opossums
and the kangaroos, and
most of the native
animals in Australia.
Marsupial offspring are
born at an earlier stage
than placental offspring.
After birth they crawl into
a pouch that contains
milk glands, where they
develop further.
Monotremes
• Monotremes are a
very small group: the
platypus, the echidna,
and the spiny
anteater are the only
living examples.
Monotremes lay eggs.
After hatching, the
offspring moves into a
temporary pouch for
further development.
Primate Evolution
• Humans are
mammals in
the primate
family, along
with monkeys,
apes, tarsiers,
lemurs.
• We are most
closely related
to the apes,
and then to the
Old World
monkeys.
Hominids
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The apes had a major adaptive
radiation in Africa during the Miocene
era, 25- 5 million years ago. Many
different species appeared and spread
throughout Africa, Asia, and Europe.
Around 6 million years ago, the lineage
leading to modern humans split with
that of the Great Apes (gorilla,
chimpanzee, bonobo, orangutuan). All
creatures on the lineage after this spilt
are called “hominids”. After that time
there have been a number of species
of hominid, mostly living in Africa.
First Hominids
• The largest group of prehuman species was
Australopithecus. Shortly after
the ape/hominid split the
species Australopithecus
afarensis lived. The best
known example is the skeleton
“Lucy”, which is almost
complete. These creatures
were 4 feet tall or less, but fully
bipedal (walked on 2 legs all
the time, unlike chimps and
gorillas). They had large jaws
and fairly small brains.
Hominid Evolution
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The hominid line split into 2 main
branches after this time. One
branch led to the modern humans,
and the species on this line are in
the genus Homo: Homo habilis,
Homo erectus, Homo sapiens.
The other branch contains several
species that are called
Paranthropus or continue to be
called Australopithecus, or the
“robust” Australopithicines. This
branch developed very large jaws
along with the sagital crest (on top
of the skull) to support the jaw
muscles. Big jaws, small brains—
they seem to have been
vegetarians. All species on this
line died out more than 1 million
years ago.
Our direct ancestral species was
Homo erectus.
Homo erectus
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learned to create stone tools as long as
2 million years ago. Use of fire is more
controversial. Some think fire use (if not
the ability to make fire) came quite early,
and that the gradual decrease in jaw
size is a response to the use of fire to
cook food. Others hold that fire use is a
very late development, Homo sapiens
only.
H. erectus did one other interesting
thing: walked out of Africa and populated
most of the Old World. This happened
perhaps 1.8 million years ago. The “Out
of Africa” theory really means Out of
Africa Twice: once by Homo erectus,
and them again by modern humans. The
multiregionalists also believe that Homo
erectus migrated out of Africa to the rest
of the world (although they consider H.
erectus to be primitive Homo sapiens
and not a separate species).
Could Homo erectus talk? The only real
evidence against it is that the spinal
column in the thorax in the best
preserved skeleton is quite narrow. It
has been argued that this implies an
inability to control breathing well enough
for speech.
Neanderthals
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In August 1856 workers in a limestone quarry in the
Neander valley in Germany came across some
bones that were undeniably human, but very odd
looking, especially in the skull. Scientists held two
differing views: they were either the bones of a
modern human distorted by disease (a Cossack
soldier fleeing Napolean’s army was a popular
theory), or they were the bones of an human
ancestor. Darwin’s Origin of Species was published
in 1859, considerably adding to the controversy.
“Neanderthal” means “Neander Valley”. Often
spelled without the h (Neandertal” to match the
German pronunciation.
As time went on, more similar skeletons were found
throughout Europe, and it became that the bones
were quite ancient.
In 1864 Irish anatomist WIlliam King decided they
represented a new species, christened “Homo
neanderthalensis”. They were considered to be the
ancestors of modern humans.
Neanderthal bones have been found across Europe
and the Middle East, but not in Africa or eastern
Asia. Neanderthals lived from approximately
200,000 years ago until about 30,000 years ago.
There are no human remains in the Americas older
than about 30,000 years.
What did Neanderthals Look Like?
More Neanderthal Appearance
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Physical description: short, stocky, heavy build, large
head, protruding brow ridges and a large nose. Their
brain was as large or larger than ours. The oldest
known was 40 years old when he died, and nearly all
Neanderthal skeletons show signs of injury: healed
bones.
Were they hairy like apes or smooth-skinned like us?
When fist discovered, Neanderthals were thought to
have been extremely primitive, closer to the apes than
to us. We now know that there were many other
human-like species that came between us and our
common ancestor with the apes. In recent times
Neanderthals have been thought to be very human-like
in appearance and behavior. Certainly living in cold
climates it seems likely that they wore clothing—animal
skins, probably, although no direct evidence for such
clothing exists.
A recent study of human lice bears on this subject.
Head lice live in the hair, and they have been with us
since long before we became human. Body lice, on
the other hand, live in clothing. Body lice are a subspecies of head lice. By examining he DNA of the 2
types, and comparing them to chimpanzee lice, it is
clear that they diverged from each other fairly recently,
about 70,000 years ago. This implies that early
humans and Neanderthals may not have worn clothing.
Neanderthals may have been hairy beasts after all. The
evidence is pretty indirect and does rely on a number
of assumptions.
Alternate Views
Neanderthal Behavior
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Could they talk? It’s a little late for a conversation!
An argument has been made that the structure of the
base of the skull would not have allowed the larynx
(voicebox) to produce the range of sounds that
modern humans have. Another contribution to this
controversy: in one skeleton, the hyoid bone in the
throat (connects the tongue to the lower jaw) has
been found. It is shaped like a modern human hyoid,
and not like the hyoid bone in gorillas and chimps.
Evidence for human-like behavior. Neanderthal
bones are sometimes found in what look like funeral
burials, arranged in a comfortable position. Some
evidence that flowers were used to cover one of
them. This evidence is controversial, however. In
one case, Shanidar (named after the site), the person
had had severe injuries, including destruction of an
eye socket. These wounds were healed, and they
were severe enough so that he wouldn’t have
survived without assistance.
A fragment of a flute has been found from
Neanderthal times (50 000 years ago) It is bone, with
holes spaced in a way that allows several modernstyle notes on it.
They definitely made stone tools and used fire.
What happened to the
Neanderthals?
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About 35,000 years ago, modern humans came into their
territory in western Europe. The modern humans are
sometimes called “Cro-magnon”, based on the first
archeological site they were found at. Although there is no
obvious evidence of conflict, after several thousand years of
co-existence, the Neanderthals apparently died out.
Two competing theories. 1. The Neanderthals were the same
species as modern humans, and the distinctive Neanderthal
type disappeared by interbreeding. This implies that people of
today carry Neanderthal genes. 2. Alternatively, the
Neanderthals may have been an entirely different species,
unable to produce fertile hybrids with modern humans. This
implies that people today carry no Neanderthal genes.
Theories are tied up in a larger context. The older theory ,
called the “Multi-regional hypothesis”, says that all of the
human-like creatures that lived in the past two million years or
more (including Homo erectus, generally considered to be our
ancestral species) are part of the same species, Homo
sapiens, and that they evolved worldwide from the primitive
forms into the forms we see today. The mechanism for the
spread of new genes was a slow process of interbreeding
between neighboring groups. This theory suggests that many
of today’s populations have lived in the same area of the world
for a very long time: the Chinese evolved in China, the
Africans evolved in Africa, etc.
The newer theory, called “Out of Africa” says that there have
been many different species of human-like creatures, with
Neanderthals just one of these species. Modern humans
evolved in Africa about 100,000 years ago, then spread out
from there. All other human species were eliminated.
Evidence
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The main evidence for the multiregional hypothesis comes from fossil bones.
Anthropologists of this school claim to see the same regional differences in ancient
bones as are present among the current inhabitants. Also, some skeletons are
claimed to show intermediate characteristics between modern humans and
Neanderthals. The out of Africa adherents say that bones are subject to
deformation, and that the differences are too subtle to be real. I can’t judge these
arguments.
The multi-regional hypothesis is currently losing ground due to DNA evidence.
The DNA from 3 different Neanderthals has been examined, and the variant forms
there are far outside the range of modern human DNA—at least twice as far from
any modern human type as any two modern types are from each other. This
implies that Neanderthals and modern humans last had a common ancestor
450,000 years ago, long before the encounters in western Europe.
Another aspect of DNA evidence is that modern human DNA is not very variable:
there is more genetic variation among the chimpanzees in the Gombe Stream
Reserve in Tanzania than there is among all human populations. This implies that
somewhere around 100,000 years ago the human population went through a
bottleneck—it was reduced to a very small number, from whom we are all
descended. Most of the human DNA variation is found in Africa, and modern
human remains have been found there of an appropriate age.
The DNA evidence mostly comes form the mitochondrial DNA, a small circle of
DNA found outside the nucleus, in the mitochondria, the organelle that provides
most of the energy to run the cell. This DNA is found in large amounts than
nuclear DNA, and it is tougher—as a circle it has no free ends to attack.
Mitochondrial DNA is inherited strictly through the mother, so it doesn’t give
complete information about inheritence patterns in the species. For instance, if
Neanderthal-human hybrids were only fertile with a modern mother and a
Neanderthal father, we could have Neanderthal genes in us now, but not
Neanderthal mitochondrial DNA. This type of situation; fertility in one direction but
not the other—is very common in crosses between closely related species.
Human Evolution