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Animal Evolution
“Welcome to your kingdom…”
Nearest
0.5 bya
~0.5
~1
Major Events
Colonization of Land, Cambrian Explosion, 1st Animal Fossils
start of Animal Lineage
~1.5
~2
~2.5
1st Eukaryote Fossils
Atmospheric O2 increases
~3
~3.5
1st Prokaryote Fossils (Photosynthesis?)
~4
Oceans, Water, (1st Life?) forms
~ 4.5
Earth forms
…
~ 14
“Big Bang”
1
The last 0.5 billion years:
• ~ 540-250 mya: Paleozoic
= era of invertebrates, fishes, amphibians
(+ seedless plants)
• ~250-65 mya: Mesozoic
= era of reptiles (+ Gymnosperms)
• ~ 65 mya – present: Cenozoic
= era of insects, mammals (+ Angiosperms)
Mass Extinctions
• Geologic eras typically separated by mass
extinctions (ME’s) followed by adaptive
radiations (AR’s) = large scale divergent
evolution of surviving forms
• Periods, epochs separated by smaller scale
extinctions and radiations
• Evidence from fossil record gave rise to the
geologic time scale
2
Ter
-tia
-ry
Precambrian
S&T
3
Pre-Cambrian/
Paleozoic eras
~ 540 mya
• Snowball Earth theory –
limiting diversity
• A.R.: Cambrian Explosion
– New predator-prey niches
ecological diversity, co-evolution
– Rise in O2 metabolic diversity
– HOX genes morphological diversity
Figure 32.6 A Cambrian seascape
Burgess Shale Fossils
4
Paleozoic/
Mesozoic eras
~250 mya
• Permian M.E.:
– Extreme volcanism (Siberia)
CO2 rise global warming, O2 deficit in oceans
– Formation of super-continent Pangea
draining of shallow seas, continental climate
• A.R.: plants (seed plants: Gymnosperms) +
animals (reptiles) adapted to dryer conditions
prevail
India collided with
Eurasia just 10 million
years ago, forming the
Himalayas, the tallest
and youngest of Earth’s
major mountain
ranges. The continents
continue to drift.
Cenozoic
0
Millions of years ago
65.5
m er
th A
Nor
ica
Eurasia
Africa
India
Madagascar alia
tr
us
Antarctica A
South
America
251
Paleozoic
135
Mesozoic
Laurasia
Go
nd
wa
na
a
ae
ng
Pa
By the end of the
Mesozoic, Laurasia
and Gondwana
separated into the
present-day continents.
By the mid-Mesozoic,
Pangaea split into
northern (Laurasia)
and southern
(Gondwana)
landmasses.
At the end of the
Paleozoic, all of
Earth’s landmasses
were joined in the
supercontinent
Pangaea.
5
Mesozoic/
Cenozoic eras
~ 65 mya
• Cretaceous M.E.:
– asteroid collision (Mexico)
debris cloud cooling, inhibition of
photosynthesis
– Pangea break-up habitat & climate change
• A.R.: co-evolution of Angiosperms & insects;
demise of dinosaurs opened niches for
descendants of surviving mammals
• Start of animal lineage ~ 1 bya
(last common ancestor of choanoflagellates & animals)
Choanoflagellates (colonial,
flagellated protists) = closest living
relatives of animals (metazoa)
6
X
• First animal fossils ~ 0.6 bya (575 mya)
= Ediacaran fossils (Australia)
7
Digestive
cavity
Somatic cells
Reproductive cells
Colonial protist,
an aggregate of
identical cells
Hollow sphere
of unspecialized
cells (shown in
cross section)
Beginning of cell
specialization
Infolding
Gastrula-like
“protoanimal”
Animals likely evolved from advanced colonial
stages of choanoflagellate-like ancestor
Figure 32.2 Early embryonic development in animals
(layer 3) All animals develop in stages, incl.
- blastula (hollow ball of cells)
- gastrula (invaginated ball)
Blastocoel
Cleavage
Cleavage
Zygote
Eight-cell stage
Blastocoel
archenteron
Blastula
Cross section
of blastula
Endoderm
Ectoderm
Gastrula
Blastopore
Gastrulation
8
Animal Development
Zygote blastula gastrula embryo
• (maturation) adult or:
• larva(e) (metamorphosis) adult
complete
incomplete
Animal Characteristics Ancestral
• Eukaryotic
• Heterotrophic
9
Animal Characteristics Derived
•
•
•
•
•
•
•
•
Ingestive
Multi-cellular
Lack of cell walls
Unique inter-cellular junctions
Unique tissues: nervous, muscular
Hox gene complex (developmental genes)
Diplontic life cycle
Gastrulation & tissue development from
germ layers
Animal Tissue Development
Germ Layers give rise to adult tissues
• Ectoderm outer covering,
(central nervous tissue)
• Endoderm digestive tract, (lungs, liver)
• (Mesoderm muscles, most organs)
Blastocoel
Endoderm
Ectoderm
Gastrula
Blastopore
10
Animal Body Plans
• Refers to shared organizational
complexity
• May be result of homologies or analogies
• Group of species sharing same body plan
= “grade” (not necessarily “clade”),
e.g. “slugs,” “worms”
Figure 32.7 Body symmetry
(a) Radial symmetry. The parts of a
radial animal, such as a sea anemone
(phylum Cnidaria), radiate from the
center. Any imaginary slice through
the central axis divides the animal
into mirror images.
(b) Bilateral symmetry. A bilateral
animal, such as a lobster (phylum
Arthropoda), has a left side and a
right side. Only one imaginary cut
divides the animal into mirror-image
halves.
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Symmetry
• Radial: top/bottom or oral/aboral
generally sessile, floating
• Bilateral: dorsal/ventral, left/right,
anterior/posterior
– Usually implies cephalization (= concentration
of brain and sensory organs in anterior end)
generally actively moving
Tissues
• Diploblastic: 2 germ layers (ecto- and
endoderm), e.g. jellies
• Triploblastic: 3 germ layers (ecto-, endoand mesoderm) – most animals
• Exception: sponges – no true tissues
12
Figure 32.8 Body plans of triploblastic animals
Coelom
(a) Coelomate. Coelomates such as
annelids have a true coelom, a body
cavity completely lined by tissue
derived from mesoderm.
Body covering
(from ectoderm)
Digestive tract
(from endoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Body covering
(from ectoderm)
(b) Pseudocoelomate. Pseudocoelomates
such as nematodes have a body cavity only
partially lined by tissue derived from
mesoderm.
Muscle layer
(from
mesoderm)
Pseudocoelom
Digestive tract
(from ectoderm)
Body covering
(from ectoderm)
(c) Acoelomate. Acoelomates such as
flatworms lack a body cavity between
the digestive tract and outer body wall.
Tissuefilled region
(from
mesoderm)
Digestive tract
(from endoderm)
Body Cavities
Coelom = fluid-filled space between digestive
tract and outer body wall,
lined by mesoderm
Functions:
• Suspends & cushions organs
• Independent movement of organs & body wall
• Hydrostatic skeleton in some, e.g. earth worms
13
Early Embryonic Development
• Protostome Development:
– Spiral, determinate cleavage
– Schizocoelous
– Blastopore mouth
• Deuterostome Development:
– Radial, indeterminate cleavage
– Enterocoelous
– Blastopore anus
Figure 32.9 A comparison of protostome and
deuterostome development
Protostome development
(examples: molluscs, annelids,
arthropods)
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Eight-cell stage
Spiral and determinate
Radial and indeterminate
Coelom
Archenteron
Mesoderm
Coelom
Blastopore
Schizocoelous: solid
masses of mesoderm
split and form coelom
Mesoderm
Blastopore
Enterocoelous:
folds of archenteron
form coelom
Mouth
Anus
Digestive tube
Mouth
Mouth develops
from blastopore
(a) Cleavage. In general, protostome
development begins with spiral,
determinate cleavage.
Deuterostome development is
characterized by radial,
indeterminate cleavage.
(b) Coelom formation. Coelom
formation begins in the gastrula
stage. In protostome development,
the coelom forms from splits in the
mesoderm (schizocoelous
development). In deuterostome
development, the coelom forms
from mesodermal outpocketings of
the archenteron (enterocoelous
development).
(c) Fate of the blastopore. In
protostome development, the mouth
forms from the blastopore. In
deuterostome development, the
mouth forms from a secondary
opening.
Anus
Anus develops
from blastopore
14
Phylogenetic Tree of the Animal Kingdom – under debate
“Radiata”
“traditional”
“Radiata”
“molecular”
15
“molecular”
condensed version
Points of Agreement
• All animals share a common ancestor
• Sponges (Porifera) are basal animals
• Eumetazoans = clade with true tissues
– Grade “Radiata” = Jellies (Cnidaria) & comb
jellis (Ctenophera) are basal eumetazoans
(radial symmetry, diploblastic)
…
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Points of Agreement (cont.)
• Most phyla in clade “Bilateria” (bilateral
symmetry, triploblastic)
• Chordates & Echinoderms are in clade
Deuterostomia
 re: Ecdysozoa:
Ecdysis = molting =
shedding of exoskeleton
re: Lophotrochozoa • Lophophore = crown of
ciliated tentacles used for
feeding (Lophophorates)
• Trochophore = planktonic
larval stage
(Molluscs & Annelids)
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