Choanoflagellata
Collar cell of a sponge
Porifera - sponges
Cnidaria - anemones
Cnidaria - jellyfish
Ctenophora - comb-jellies
Cnidaria - corals
Brachiopoda - lamp shells
Echinodermata
Chordata - sea
squirts,
lancelets,
vertebrates
Hemichordata - acorn
worms
Platyhelminthes - flatworms
Nemertea - ribbon worms
Annelida - segmented
worms
Mollusca
Annelida - earthworm
Annelida - polychaetes
Arthropoda - crustaceans
Arthropoda - arachnids
Arthropoda - insects
Arthropoda - centipedes
and millipedes (myriapods)
Nematoda - roundworms
Onychophora - velvet
worms
Significant evolutionary events
during the Cambrian explosion
! 1 - evolution of multicellularity - animals
can therefore become large
! 2 - evolution of true tissues
" The most primitive animals with true tissues
(the Cnidaria) are diploblastic " They have two embryonic (or germ) layers:
ectoderm and endoderm
Cnidarian cell types
Significant evolutionary events
during the Cambrian explosion
! 3 - evolution of triploblastic animals
! 4 - evolution of HOX genes
Figure 24.8
HOX genes
Sponge
Flatworm
Earthworm
Velvet worm
Crustacean
Centipede
Fruit fly
Amphioxus
Mouse
Zebrafish
Figure 24.9
Figure 24.9a,b left
(a) Early in
development
FISH LIMB BUD
Hoxd-11 transcripts
in hind part of limb
bud
FISH LIMB BUD
Early in
development
MOUSE LIMB BUD
(b) Later in
development
Few Hoxd-11
transcripts, only
in hind part
Head
Tail
(c) Hypothesis
Hoxd-11 expression
along long axis of limb
Fin
Tail
Head
Foot
Hoxd-11 expression as in fish,
followed by expression toward head
Figure 24.9a,b right
Head
Tail
Significant evolutionary events
during the Cambrian explosion
MOUSE LIMB BUD
Hoxd-11 transcripts
in hind part of limb
bud
Hoxd-11 transcripts
along head-tail axis
Head
Later in
development
Tail
! 5 - the evolution of radial cleavage
Significant evolutionary events
during the Cambrian explosion
Significant evolutionary events
during the Cambrian explosion
! 6 - the evolution of deuterostomatous
development
! 7 - the evolution of a coelom
(body cavity lined with
mesoderm)
Deuterostomia
Significant evolutionary events
during the Cambrian explosion
! 8 - the evolution of spiral cleavage
Significant evolutionary events
during the Cambrian explosion
Protostomia
! 9 - the evolution of protostomatous development
Significant evolutionary events
during the Cambrian explosion
! 10 - the evolution of a coelom (body cavity lined
with mesoderm)
Significant evolutionary events
during the Cambrian explosion
! 11 - the evolution of
segmentation
Significant evolutionary events
during the Cambrian explosion
! 12 - the evolution of a calcified exoskeleton
Arthropod appendages