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

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. 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. Figure 32.1 Early embryonic development (Layer 1)

5. Figure 32.1 Early embryonic development (Layer 2)

6. Figure 32.1 Early embryonic development (Layer 3)

8. Figure 32.2 A choanoflagellate colony

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

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

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

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. Figure Body symmetry

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

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

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

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

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

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

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

22. Figure 32.12 Comparing the molecular based and grade-based trees of animal phylogeny

23. Why are animals so successful and so complex?

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. HOX genes
Regulate development of segmentation in animals

26. Gene and genome duplications

27. The Cambrian Explosion Myth or reality?

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. First appearance of animals in the fossil record

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

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

32. Burgess Shale and the Cambrian Explosion
British Colombia, Canada
Discovered in 1909 by Charles Walcott
Deep water deposit
Around 515 million years (10 million years after Cambrian explosion)

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. Did the Cambrian explosion really happen?
"Cambrian explosion" model
Molecular data
Today
Molluscs
Annelids
Arthropods
Echinoderms
Jawless fish
Gnathostomata
Molluscs
Annelids
Arthropods
Echinoderms
Jawless fish
Gnathostomata
500 Myr
1000 Myr
1500 Myr

37. Fossil record by itselfA B C 10
Millions of years 20
at face value, group is about 10 Myr old

38. Fossils + molecular dataB C A B C 1 1 % 2 2 3 3 4 4
calibration
molecular divergence
1% per 10 million years

39. A B C
Extrapolation 10 20
Millions of years
If rate of molecular evolution has been constant, then group is about 40 Myr old 30 40

40. “Phylogenetic Fuse”
Molecular dating suggests origins prior to diversification.
Keep in mind serious concerns about molecular dating.
Cambrian phylogenetic fuse
Cooper & Fortey 1998 TREE

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