Invasion of Land by Vertebrates
I. Overview of Chordates
II. Origin
g of Limbs
III. Origin
g of Amniotic Egg
gg
Phylum Chordata
• Cephalochordata
• Urochordata
• Craniata (vertebrates)
– cranium
– axial skeleton
– elaborate
l b t b
brain
i
Cephalochordata
Urochordata
Larval Urochordate
Actinopterygii
(ray-finned fish)
S
Sarcopterygii
t
ii
(lobe--finned fish)
(lobe
Hypothetical Transition from Water to Land
Evidence for the Evolution
of Vertebrate Appendages
pp
g
• Comparative
• Fossil
• Genetic/Developmental
Ge et c/ e e op e ta
Ichthyostega
Lobe-finned fish
Extinct tetrapod
Acanthostega
g
Earliest evidence of reptiles
(315 mya)
“found by UK scientist Dr Howard Falcon-Lang in
fossil-rich sea cliffs at New Brunswick.”
Evidence for the Evolution
of Vertebrate Appendages
pp
g
• Comparative
• Fossil
• Genetic/Developmental
Ge et c/ e e op e ta
Clusters of Hox Genes in Vertebrates
Head
Tail
Carroll (1997)
Hox Genes and Limb Development
As in the development of head
head--to
to--tail axis
axis, Hox genes are
expressed in order—
order—from the shoulder to the fingers.
[Source: Davis et al. 1995, Nature 375]
Hypothetical Evolution of
Paired Appendages
• Hox clusters were duplicated in an early
vertebrate
• New Hox clusters became responsible for
b ildi lilimbs
building
b (d
(developmental
l
t l modularity).
d l it )
Superclass Tetrapoda
Class Amphibia
Class Reptilia
Class Mammalia
Class Aves
Amniotes
Amniotic
Egg
Amnion:: surrounds and protects embryo
Amnion
Chorion:: exchange gases and circulate nutrients
Chorion
Yolk sac:
sac: contains protein and lipid for energy
Allentois:: stores metabolic wastes
Allentois
Mammals
Monotremes
Marsupials
Placentals