Lecture 3: Origins of Animals

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)
7
(No Transcript)
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
(No Transcript)
15
Third split body cavities

• Acoelomate (no cavity)
• Pseudocoelomate (partially lined with mesoderm)
• Coelomate (cavity completely lined with mesoderm)

16
Fourth splitProtostomes 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
Nemiana gt Sea anemonea? Algal?
lt Spriggina Annelid Worm? Arthropod?
Cyclomedusa gt Benthic Polyp 1-5 mm in diameter
31
lt Dickinsonia Annelid Worm? Cnidarian?
Charnia gt 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)

33
(No Transcript)
34
(No Transcript)
35
Stephen J. GouldWonderful 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 itself
A
B
C
0
10
Millions of years
20
at face value, group is about 10 Myr old
38
Fossils molecular data
A
B
C
A
B
C
0
0
1
1
2
2

3
3
4
4
calibration
molecular divergence
1 per 10 million years
39
A
B
C
Extrapolation
0
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.

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

42
(No Transcript)