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
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Significant evolutionary events during the Cambrian explosion z1 - evolution of multicellularity - animals can therefore become large z2 - evolution of.
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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 structures4 - 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
Sponge
Flatworm
Earthworm
Velvet worm
Crustacean
Centipede
Fruit fly
Amphioxus
Mouse
Zebrafish
Figure 24.8
Hoxd-11 transcriptsin hind part of limb bud
Head Tail
Later indevelopment
Early indevelopment
FISH LIMB BUD
Few Hoxd-11 transcripts, onlyin hind part
Figure 24.9a,b left
Hoxd-11 transcriptsin hind part of limb bud
Head Tail
MOUSE LIMB BUD
Hoxd-11 transcripts along head-tail axis
Figure 24.9a,b right
Foot
FinHoxd-11 expressionalong long axis of limb
Hoxd-11 expression as in fish,followed by expression toward head
Head TailHead Tail(c) Hypothesis
(b) Later indevelopment
(a) Early indevelopment
FISH LIMB BUD MOUSE LIMB BUD
Figure 24.9
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