1 Lecture 3: Origins of Animals Developmental, molecular and paleontological perspectives.
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Transcript of 1 Lecture 3: Origins of Animals Developmental, molecular and paleontological perspectives.
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Lecture 3: Origins of Animals
Developmental, molecular and paleontological perspectives
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What is an animal?• Multicellular adults, heterotrophic, eukaryotes. Most
feed by ingestion of food into a gut
• Cells lack cell walls, adhere by means of specialised junctions, secrete extra-cellular matrices using collagen.
• Most animals possess specialised cells for:a) signal transmission - nervesb) contraction for movement - muscles
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Animal Life History• Typically a small flagellated sperm fertilises a larger egg to form a
diploid zygote.
• Zygote undergoes cleavage.
• Formation of a blastula
• Blastula undergoes gastrulation during which embryonic tissue layers form
• Many animals have distinct larval stage
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Figure 32.1 Early embryonic development (Layer 1)
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Figure 32.1 Early embryonic development (Layer 2)
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Figure 32.1 Early embryonic development (Layer 3)
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Figure 32.2 A choanoflagellate colony
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Figure 32.3 One hypothesis for the origin of animals from a flagellated protist
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Figure 32.4 A traditional view of animal diversity based on body-plan grades
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First major split
Parazoans (no true tissues) versus
eumetazoa (all other animals)
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Second major split
• Radial versus bilateral symmetry• Cnidaria (jellyfish and anemones) and
Ctenophora (comb jellies) versus rest of animals
• Bilateral symmetry associated with cephalisation (development of a head)
• Some bilateral animals have acquired radial symmetry (e.g., some echinoderms)
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Figure 32.5 Body symmetry
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Third split: body cavities
• Acoelomate (no cavity)
• Pseudocoelomate (partially lined with mesoderm)
• Coelomate (cavity completely lined with mesoderm)
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Fourth split:Protostomes vs. Deuterostomes
• Different type of cleavage
• Different origin of coelom
• Different fate of blastophore (becomes either mouth or anus)
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Figure 32.4 A traditional view of animal diversity based on body-plan grades
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Figure 32.8 Animal phylogeny based on sequencing of SSU-rRNA
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Lophophorate animals
Bryozoans, brachiopods, and phoronids
Mouth surrounded by hollow tentacles (lophophore)
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Figure 32.9 A trochophore larva
Larva found in molluscs and annelid worms
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Figure 32.10 Ecdysis
Arthropods and nematodes peridically shed their cuticle.
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Figure 32.12 Comparing the molecular based and grade-based trees of animal phylogeny
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Why are animals so successful and so complex?
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What genes can tell us
• Sequence comparison tells us about evolutionary relationships
• Can also tell us about homology and development
• Question: what does genomics tell us about animal evolution?
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HOX genes
Regulate development of segmentation in animals
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Gene and genome
duplications
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The Cambrian ExplosionMyth or reality?
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Cambrian Explosion
• Nearly all major animal phyla appear in Cambrian rocks (545 to 525 million years ago)
• Many weird and wonderful creatures appear suddenly in the fossil record
• Question: Is the fossil record giving us a faithful picture of what happened.
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First appearance of animals in the fossil record
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< Spriggina
Annelid Worm?
Arthropod?
Cyclomedusa >
Benthic Polyp
1-5 mm in diameter
Nemiana >
Sea anemonea?
Algal?
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< Dickinsonia
Annelid Worm? Cnidarian?
Charnia >
Sea pen- up to 1m in length
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• British Colombia, Canada
• Discovered in 1909 by Charles Walcott
• Deep water deposit
• Around 515 million years (10 million years after Cambrian explosion)
Burgess Shale and the Cambrian Explosion
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Stephen J. Gould“Wonderful Life”
• Major body plans laid down very quickly
• Much more diversity in Cambrian than we see today
• Survival of a subset of phyla, due to luck more than anything else
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Molluscs Annelids Arthropods Echinoderms Jawless fish Gnathostomata Molluscs Annelids Arthropods Echinoderms Jawless fish Gnathostomata
"Cambrian explosion" model Molecular data
Today
500 Myr
1000 Myr
1500 Myr
Did the Cambrian explosion really happen?
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A B C
10
20
0
Millions of years
Fossil record by itself
at face value, group is about 10 Myr old
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A B C0
1
2
3
4
A B C0
1
2
3
4
molecular divergence
calibration
1% per 10 million years
%
Fossils + molecular data
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A B C
10
20
30
0
Mill
ions
of
year
s
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If rate of molecular evolution has been constant, then group is about 40 Myr old
Extrapolation
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“Phylogenetic Fuse”Molecular dating suggests origins prior to diversification.
Cambrian phylogenetic fuseCooper & Fortey 1998 TREE
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Summary
• Major divisions of animals defined by fundamental body plans
• Molecular data challenges some traditional views about relationships
• Much debate about when animals first evolved
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