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1 Lecture 3: Origins of Animals Developmental, molecular and paleontological perspectives.

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1 1 Lecture 3: Origins of Animals Developmental, molecular and paleontological perspectives

2 2 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 - nerves b) contraction for movement - muscles

3 3 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

4 4 Figure 32.1 Early embryonic development (Layer 1)

5 5 Figure 32.1 Early embryonic development (Layer 2)

6 6 Figure 32.1 Early embryonic development (Layer 3)

7 7

8 8 Figure 32.2 A choanoflagellate colony

9 9 Figure 32.3 One hypothesis for the origin of animals from a flagellated protist

10 10 Figure 32.4 A traditional view of animal diversity based on body-plan grades

11 11 First major split Parazoans (no true tissues) versus eumetazoa (all other animals)

12 12 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)

13 13 Figure 32.5 Body symmetry

14 14

15 15 Third split: body cavities Acoelomate (no cavity) Pseudocoelomate (partially lined with mesoderm) Coelomate (cavity completely lined with mesoderm)

16 16 Fourth split: Protostomes vs. Deuterostomes Different type of cleavage Different origin of coelom Different fate of blastophore (becomes either mouth or anus)

17 17 Figure 32.4 A traditional view of animal diversity based on body-plan grades

18 18 Figure 32.8 Animal phylogeny based on sequencing of SSU-rRNA

19 19 Lophophorate animals Bryozoans, brachiopods, and phoronids Mouth surrounded by hollow tentacles (lophophore)

20 20 Figure 32.9 A trochophore larva Larva found in molluscs and annelid worms

21 21 Figure Ecdysis Arthropods and nematodes peridically shed their cuticle.

22 22 Figure Comparing the molecular based and grade-based trees of animal phylogeny

23 23 Why are animals so successful and so complex?

24 24 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?

25 25 HOX genes Regulate development of segmentation in animals

26 26 Gene and genome duplications

27 27 The Cambrian Explosion Myth or reality?

28 28 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.

29 29 First appearance of animals in the fossil record

30 30 < Spriggina Annelid Worm? Arthropod? Cyclomedusa > Benthic Polyp 1-5 mm in diameter Nemiana > Sea anemonea? Algal?

31 31 < Dickinsonia Annelid Worm? Cnidarian? Charnia > Sea pen- up to 1m in length

32 32 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

33 33

34 34

35 35 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

36 36 MolluscsAnnelidsArthropodsEchinodermsJawless fishGnathostomataMolluscsAnnelidsArthropodsEchinodermsJawless fishGnathostomata "Cambrian explosion" modelMolecular data Today 500 Myr 1000 Myr 1500 Myr Did the Cambrian explosion really happen?

37 37 ABC Millions of years Fossil record by itself at face value, group is about 10 Myr old

38 38 ABC ABC molecular divergence calibration 1% per 10 million years % Fossils + molecular data

39 39 ABC Millions of years 40 If rate of molecular evolution has been constant, then group is about 40 Myr old Extrapolation

40 40 Phylogenetic Fuse Molecular dating suggests origins prior to diversification. Cambrian phylogenetic fuse Cooper & Fortey 1998 TREE

41 41 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

42 42

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