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 - nervesb) 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)

7

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 32.5 Body symmetry

14

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

Figure 32.10 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 ExplosionMyth 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

< Spriggina

Annelid Worm?

Arthropod?

Cyclomedusa >

Benthic Polyp

1-5 mm in diameter

Nemiana >

Sea anemonea?

Algal?

31

< Dickinsonia

Annelid Worm? Cnidarian?

Charnia >

Sea pen- up to 1m in length

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

34

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

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?

37

A B C

10

20

0

Millions of years

Fossil record by itself

at face value, group is about 10 Myr old

38

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

39

A B C

10

20

30

0

Mill

ions

of

year

s

40

If rate of molecular evolution has been constant, then group is about 40 Myr old

Extrapolation

40

“Phylogenetic Fuse”Molecular dating suggests origins prior to diversification.

Cambrian phylogenetic fuseCooper & 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

42