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

length of the body

Arthropod appendages