Lesson 3
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Lesson Outline:
♦ Origin of the Chordates
♦ Early Theories
o Derivation from protozoa, annelids, arthropods, and
hemichordates.
o Garstang’s Theory
o Dipleuruloid Theory
o Gutmann’s Theory
o Kardong’s Theory
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Objectives:
At the end of this lesson you should be able to:
♦ Describe the strengths and weaknesses of the different theories of the
origins of the Chordates.
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References:
♦ Vertebrates: Comparative Anatomy, Function and Evolution (3rd edition,
2002) by K.V. Kardong - Chapter 2: pgs 71-79
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Reading for Next Lesson:
♦ Chapter 4
Origin of Chordates - Chordate Phylogeny
Origin lies amongst the invertebrates - but where?
The origin of the vertebrates is still unsettled and controversial. Their origin
certainly lies somewhere amongst the invertebrates but the question is where? Many
theories have been put forward and we will briefly look at a few of the more notable ones
as well as at the most recent theory supported by Ken Kardong.
Within the early chordates, the basic body plan was established, pharyngeal slits,
notochord, dorsal hollow nerve cord, endostyle and post-anal tail.
The pharyngeal slits in extant vertebrates serve to separate suspended food particles from
the water as well as to extract oxygen and excrete carbon dioxide. Oxygen is required to
oxidize food substances to obtain ATP from metabolism and carbon dioxide is the
primary waste product of this process. All three processes are related (obtaining and
metabolizing food and excreting waste).
As animals grow, filter feeding becomes replaced by active feeding which requires
locomotion. Effective locomotion requires large muscle mass and a coordinated nervous
system. It also requires a stiff body upon which to generate force and a large surface (the
postanal tail).
Thus the basic chordate body plan is seen as a response to increasing size and the need
for active predation.
What gave rise to these features?
Early Theories
Derivation from protozoa to ....
At one time or another almost ever group of invertebrates has been cited
as the evolutionary source of the chordates.
The most plausible theories are based on anatomy and embryology.
Derivation from annelids and arthropods
Initially proposed in the early 1800's this theory maintains that the
chordate body plan is an inverted version of that of the annelids
and arthropods.
All are segmented
All have similar forms of gross brain regionalization
The basic chordate body plan is an inverted form of that seen in the
annelids and arthropods.
However, most similarities are analogous and not homologous
Segmentation occurs differently
Cleavage is different
Mesoderm formation is different
Coelom formation is different
Blastopore derivation is different
Derivation from echinoderms and hemichordates
(acorn worms)
There are several theories that are based on similarities seen in
echinoderms (starfish, sea cucumbers and sea urchins),
hemichordates (acorn worms),
urochordates, (tunicates or sea squirts) and
cephalochordates (amphioxus).
The strongest support for all of these theories is that all are deuterostomes.
Garstang's Theory
At the end of the last century, Garstang put forward the idea that chordates
evolved from echinoderms through the hemichordates (acorn worms) and urochordates
(tunicates) to cephalochordates and vertebrates. While the adults of these various phyla
are extremely different and share few if any structural similarities, an evolutionary
progression can be seen in their larvae. His theory was that selection acted on the larval
stages to produce new forms. In the case of the hemichordates and urochordates, the
"new' larvae ultimately gave rise to sessile adults that were very different. Ultimately,
however, through paedomorphosis (or the retention of larval traits into sexual maturity)
they gave rise to the cephalochordates and vertebrates
Dipleuruloid Theory
This theory, put forward by Malcolm Jollie, is
similar to that put forward by Garstang with the primary distinction that rather than
maintaining that each group arose successively from the next, it maintains that each arose
from a common ancestor - a small ciliated, bilateral organism now extinct. He proposed
that this ancestor gave rise to the echinoderms, hemichordates and a prechordate that was
a predator with a differentiated head, a pharynx with gill slits, and a large mouth. The
urochordates and cephalochordates evolved such that their active larval form gave rise to
a sessile filter feeding life style while the vertebrates evolved towards an even more
active lifestyle.
Derivation from cephalochordates
Gutmann's Theory
In opposition to this, Gutmann proposed that the ancestral form of the
chordates were the cephalochordates (amphioxus like) and that the tunicates,
hemichordates and echinoderms are advanced forms all derived from this ancestor to fill
specific niches. He proposed that the vertebrates also arose from the cephalochordates
along another distinctive line
Derivation from a protochordate ancestor
Current (Kardong's) Theory
The most current theories now not only employ anatomical and
embryological data but also employ new molecular data obtained from DNA analysis.
This data would suggest that the ancestor of the vertebrates first gave rise to two groups.
One group ultimately gave rise to both the echinoderms and hemichordates while the
other group gave rise to the urochordates, cephalochordates and vertebrates.
Except for details, this theory does not differ significantly from the Dipleuruloid theory
put forward by Jollie. Remember, however, that even these theories remain controversial.
Note:
Biochemical evidence also supports this view. Enzymes also undergo
evolutionary change and biochemical similarities can be used to support phylogenetic
linkage. In this light:
Invertebrates possess arginine kinase
Vertebrates and Cephalochordates possess creatine
kinase
Urochordates, Hemichordates and advanced
Echinoderrms possess both.
Thus, vertebrate evolution has tended to progress from:
sessile groups to active groups
filter feeding to active feeding involving muscular
action
soft body support to bony support
This appears to have occurred through paedogenesis and led to increased
locomotion and activity. A return to a sessile life style is rare amongst the vertebrates
although it does occur.
Vertebrat
Garstang’s
Urochordat
Echinoder
Cephalochorda
Hemichorda
Echinoder
Hemichorda
Urochordat
Cephalochorda
Vertebrat
Dipleuruloid
Echinoder
Kardong’s
Hemichorda
Urochordat
Cephalochorda
Vertebrat