Where did we come from ?:

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Where did we come from ? A whirlwind tour of
evolutionary trends
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Pre-biotic Earth
During the Archean Eon, prior to About 3.5
billion years ago, the floor of the seas would
have looked like this.
Barren rock and sediment
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By at least 3.5. billion years ago, Oldest life
forms appeared Prokaryotes Cyanobacteria
(photosynthetic bacteria) were among these early
life forms
Stromatolite (structure constructed by
cyanobacteria) North Pole, Western Australia Age
Archean (3.5 billion years old)
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Additional evidence of early life prokaryote
microfossils
Filamentous Prokaryote Microfossils (Probably
Cyanobacteria) Apex Chert, Marble Bar, Western
Australia Age Archean (3.46 billion years old)
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From prokaryote to eukaryote
Acritarch (algal cyst)
By about 2 to 1.5 billion years ago (Proterozoic
Eon) the first eukaryotes appeared (organisms
will a well-defined nucleus and
organelles). These are distinguished in the
fossil record by their larger size.
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From prokaryote to eukaryote
cyanobacteria
plant cell
Symbiosis
Host prokaryote
animal cell
Symbiosis
purple bacteria
Organelles of eukaryotes (mitochondria in
animals,and both mitochondria and chloroplasts in
plants) are believed to have been produced via
the introduction of prokaryotes into larger
host prokaryotes and the establishment of a
symbiotic relationship between the host cell and
the smaller prokaryotic cells that they
engulfed. Purple bacteria became mitochondria,
cyanobacteria became chloroplasts
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First Experiment in Multicellularity
By about 600 to 575 million years ago (latest
Proterozoic), the first multicellular animals
appeared. All of these were soft-bodied, and are
known only from impressions
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Latest Proterozoic (Ediacaran Period)
Oxygen production by cyanobacteria over several
millions of years had, by this time, made enough
oxygen to sustain complex life forms in the sea
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But then, something happened in the Garden of
Ediacara
Something wanted to eat Everything !
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Early Cambrian Period (Paleozoic Era, Phanerozoic
Eon)
Between about 543 and 510 million years ago,
skeletonized organisms appeared in a huge
explosion of diversity. This event is called The
Cambrian Explosion.
Seascapes changed
to war zone with weapons and armour
From peaceful oasis
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A Short Note on Taxonomy
Before we proceed with the rest of this lecture,
we should consider how organisms are
classified. In essence, our standard method of
classification of life forms in the animal
kingdom considers five basic hierarchical
divisions Phylum (e.g. Chordata) Class (e.g.
Mammalia) Order (e.g. Primates) Family (e.g.
Hominidae) Genus (e.g. Homo) Species (e.g.
sapiens)
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Cambrian Explosion
Nearly all of the phyla (highest level of
classification for multicellular organisms) with
skeletons that are present on Earth today
originated in the Cambrian Period.
Why ? The worlds first arms race Organisms
that developed hard mouthparts preyed on other
animals. In turn, prey had to develop some means
of protection (hard skeletons or greater
mobility) Predators improved their weapons And
prey had to further improve their
defenses Etc., etc.
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Results of the Cambrian Arms Race
Different groups of organisms developed their
weapons or defense systems in their own way,
resulting in a wide array of body plans suited to
various ways of life.
Ediacaran Period
Cambrian Period
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Oldest abundant skeletal remains Earliest
Cambrian (543 million years)
(scale bar 1 mm) Hydroxyapatite
(phosphate-bearing mineral) was widely used for
skeletal construction, even among the earliest
skeletonized organisms. Why use such a scarce
material to make skeletons ? We will answer this
next day.
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Significance of Oddballs
In addition to the basic animal body plans that
appeared in the Cambrian and survived to the
present day, a number of oddball forms also
appeared.
Paleontologist Stephen Jay Gould boldly suggested
some Cambrian phyla were represented by few or
single species (the oddball forms) If so, it is
possible that the Cambrian Explosion produced
more phyla than are present today
extinction
Conventional view Gradual increase in number of
phyla through time
View of Stephen Jay Gould Sudden appearance of
phyla, removal of many by later mass extinction
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Oddball example 1
Opabinia (Phylum Lobopoda)
Combined characteristics of arthropods and
worms (does not fit into a present-day
phylum) From Middle Cambrian Burgess Shale, B.C.
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Oddball example 2
Hallucigenia (Phylum Onychophoraor something
completely different ?)
From Middle Cambrian Burgess Shale, B.C.
Original reconstruction
Current reconstruction
Hallucigenia is sufficiently weird that people
have argued which end was the head and which end
was the tail, and which side was the bottom and
which side was the top.
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One of the Many Creatures That Appeared During
the Cambrian Explosion
Pikaia earliest known Chordate
A notochord (an internal band of elastic tissue
that could be flexed by muscle packs down its
length) allowed early chordates to swim without
the burden of heavy external armour.
We ultimately evolved from an animal that looked
like this !
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An Aside Preservation of Burgess Shale Fauna
Burgess Shale organisms living on foot of
escarpment (and possibly on edge and top of
escarpment as well) smothered by due to
slumping, killed and buried instantly Rapid
burial low oxygen ( possible mineralization
during early decay) exceptional preservation
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Jawless Fishes
From the early chordates arose the first
fishes (abundant by the Ordovician Period)
These fishes were covered with bony armour and
lacked jaws
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Jawed Fishes
From the jawless fishes arose the jawed fishes
(abundant by at least the Silurian Period). Jaws
are thought to have developed via the
modification of one pair of gill arches
(structures that support gills slits).
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Lobefinned Fishes
Include Coelocanths (Latimeria)
One group of jawed fishes was the lobefinned
fishes. The Coelocanth is one of the only
lobefinned fishes that has survived to the
present day. The uniquely structured fins served
as precursors to legs
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Late Devonian Lobefinned Fishes with Lungs
Some lobefinned fishes developed lungs. One
group of these lung-bearing lobefinned fishes
became the ancestral group to the earliest
amphibians. Lobe-finned fishes such as
Eusthenopteron had evolved amphibian-like
features, including fins that were strong enough
to allow limited locomotion on land.
Eusthenopteron
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Fish to Amphibian Skeletal Modifications
Amphibians had to strengthen their shoulder and
pelvic girdles to support their body weight (did
not have the luxury of water support). Tongue
cartilage was transformed into the first earbone
(stapes).
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Amphibian to Reptile

• To make the final transition to land, a
  amphibians had to
• Free themselves from a dependence on water for
  reproduction (this was solved via the development
  of the hard-shelled amniote egg).
• Develop a water-tight body covering to prevent
  drying out (solved through the development of
  thicker skin and scales).

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Family Tree of Reptiles
From an ancestral reptile arose several
subgroups, including the archosaurs (including
turtles, crocodiles, dinosaurs, birds), flying
reptiles (pterosaurs), and marine reptiles. A
group of special significance to us is the
mammal-like reptiles (therapsids) that appeared
by the latest Paleozoic.
No, youre not expected to know all these names !
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Reptiles to Mammals hearing with our jawbones
Evolution of Ear stapes acquires stirrup
shape articular bone in jaw becomes
malleus quadrate bone in jaw becomes incus
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Early True Mammals
By a stroke of luck, one group of therapsids gave
rise to mammals (Class Mammalia) during the
Triassic (The first true mammals appeared on
Earth together with the earliest dinosaurs during
the Triassic)
Hadrocodium (Early Jurassic) Member of extinct
group of early mammals
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Divisions Within Class Mammalia
Multituberculates(extinct, rodent-like mammals)
Marsupials (pouched mammals)
Monotremes (egg layers)
Eutherians (placental mammals)
Mammals, in turn, diverged into 4 major
subclasses We belong to the eutherian (placental)
subclass.
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The Odd Primate
Within the placental mammals merged The Primates
(our taxonomic order).
During the Late Tertiary early hominids appeared
in the Africa savannas.
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Are We Not Men ? We Are Devo !
It would take about 5 million years of evolution
to reach the current state of development of
modern Homo sapiens.. 
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Why Consider Evolution in Medical Geology ?
We are the product of at least 3.5 billion years
of evolution. The functioning of the human body
embodies all the traits we have acquired during
the course of our evolution, how we have
interacted with other life forms during our
recent evolution, and the changes in our
environment as our circumstances change with the
progress of technological development. The next
lecture will deal with examples of how these
factors influence the current state of our health.
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END OF LECTURE