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 - a third
germ layer, the mesoderm, appears
Mesoderm is used to make muscles, gonads,
circulatory systems, some excretory structures
4 - evolution of HOX genes
These are the homeotic genes we read about in
Ridley earlier
These genes are found in many animals and build
similar structures
The HOX genes are sometimes referred to as a
“genetic tool kit”
HOX genes
Figure 24.8
Sponge
Flatworm
Earthworm
Velvet worm
Crustacean
Centipede
Fruit fly
Amphioxus
Mouse
Zebrafish
Figure 24.9a,b left
FISH LIMB BUD
Hoxd-11 transcripts
in hind part of limb
bud
Early in
development
Few Hoxd-11
transcripts, only
in hind part
Later in
development
Head
Tail
Figure 24.9a,b right
MOUSE LIMB BUD
Hoxd-11 transcripts
in hind part of limb
bud
Hoxd-11 transcripts
along head-tail axis
Head
Tail
Figure 24.9
(a) Early in
development
FISH LIMB BUD
MOUSE LIMB BUD
(b) Later in
development
Head
Tail
Head
(c) Hypothesis
Hoxd-11 expression
along long axis of limb
Fin
Tail
Foot
Hoxd-11 expression as in fish,
followed by expression toward head
Significant evolutionary events
during the Cambrian explosion
5 - the evolution of radial cleavage
Significant evolutionary events
during the Cambrian explosion
6 - the evolution of deuterostomatous
development
The mouth formed early in development
becomes the adult anus
The adult mouth is formed secondarily
(deutero - second, stoma - mouth)
Significant evolutionary events
during the Cambrian explosion
7 - the evolution of a coelom
(body cavity lined with
mesoderm)
This cavity can serve as a means
of circulation
This cavity can also serve as a
hydrostatic skeleton to permit
burrowing through sediments
Significant evolutionary events
during the Cambrian explosion
8 - the evolution of spiral cleavage
Significant evolutionary events
during the Cambrian explosion
9 - the evolution of protostomatous development
The mouth formed early in development becomes
the adult mouth
The adult anus is formed secondarily (proto - first,
stoma - mouth)
The lineage to which arthropods, annelids and
other related phyla belong is called the
Protostomia
Significant evolutionary events
during the Cambrian explosion
10 - the evolution of a coelom (body cavity lined
with mesoderm)
The coelom of protostomes forms in a different way
from that of deuterostomes and is hence convergent
This cavity also serves as a means of circulation
This cavity can also serve as a hydrostatic skeleton
to permit burrowing through sediments
Significant evolutionary events
during the Cambrian explosion
11 - the evolution of
segmentation
Different segments are
often specialized for
particular functions
(e.g., reproduction,
excretion)
In annelids,
segmentation permits
localized shape
changes and hence
efficient burrowing
Significant evolutionary events
during the Cambrian explosion
12 - the evolution of a calcified exoskeleton
A pair of jointed appendages is associated with each
segment
Muscles attaching to the inside of each joint allow
great flexibility
The appendages vary in form and function along the
length of the body
Arthropod appendages