Evolutionary Perspectives Origins of multicellularity Animal Origins Reference Presentation Best of luck

1. MULTICELLULARAND TISSUE LEVELS OF
ORGANIZATION

2. OUTLINES
Evolutionary Perspectives
Origins of multicellularity
Animal Origins
Reference

3. EVOLUTIONARY PERSPECTIVE
◦Animals with multicellular and tissue levels of organization
have captured the interest of scientists and laypersons alike.
◦A description of some members of the phylum Cnidaria, for
example, could fuel a science fiction writer’s imagination:
◦From a distance I was never threatened, in fact I was infatuated
with its beauty.

4. ◦A large, inviting, bright blue float lured me closer. As i swam
nearer i could see that hidden from my previous view was an
infrastructure of tentacles, some of which dangled nearly nine
meters below the water’s surface!
◦The creature seemed to consist of many individuals and I
wondered whether or not each individual was the same kind of
being because, when I looked closely, I counted eight different
body forms!

5. ◦I was drawn closer and the true nature of this creature was
painfully revealed.
◦The beauty of the gas filled float hid some of the most hideous
weaponry imaginable.
◦When I brushed against those silky tentacles I experienced the
most excruciating pain. Had it not been for my life vest, I
would have drowned. Indeed, for some time, I wished that had
been my fate.

6. ◦Swimmers of tropical waters who have come into contact with
Physalia physalis, the
◦Portuguese man-of-war, know that this fictitious account rings
true in organisms such as Physalia physalis, cells are grouped
specialized for various functions, and interdependent.
◦This chapter covers three animal phyla whose multicellular
organization varies from a loose association of cells to cells
organized into two distinct tissue layers. These phyla are the
Porifera, Cnidaria, and Ctenophora.

7. Fig:1 Physalia physalis, the Portuguese Man-of-War.

8. ◦ The tentacles shown here can be up to 9 m long and are laden with
nematocysts that are lethal to small vertebrates and dangerous to
humans.
◦ A bluish float at the surface of the water is about 12 cm long. It is
not shown in this photograph. The entire organism is actually a
colony of polypoid and medusoid individuals.

9. ORIGINS OF MULTICELLULARITY
◦Multicellular life has been a part of the earth’s history for
approximately 550 million years.
◦Although this seems a very long time, it represents only 10%
of the earth’s geological history.
◦Multicellular life arose quickly in the 100 million years prior to
the Precambrian/Cambrian boundary, in what scientists view as
an evolutionary explosion.

10. CONT..
◦These evolutionary events resulted not only in the appearance
of all of the animal phyla recognized today, but also 15 to 20
animal groups that are now extinct.
◦Since this initial evolutionary explosion, most of the history of
multicellular life has been one of extinction.
◦The evolutionary events leading to multicellularity are
shrouded in mystery.

11. Many zoologists believe that multicellularity could have arisen
as dividing cells remained together, in the fashion of many
colonial protists.
Although variations of this hypothesis exist, they are all treated
here as the colonial hypothesis (fig 2)
A second proposed mechanism is called the syncytial
hypothesis (fig 3)

12. Fig 2 Fig 3

13. ◦A syncytium is a large, multinucleate cell.The formation of
plasma membranes in the cytoplasm of a syncytial protist
could have produced a small, multicellular organism.
◦Both the colonial and syncytial hypotheses are supported by
the colonial and syncytial organization that occurs in some
protist phyla.

14. ANIMAL ORIGINS
◦A fundamental question concerning animal origins is whether
animals are monophyletic (derived from a single ancestor),
diphyletic (derived from two ancestors), or polyphyletic
(derived from many ancestors).
◦ The view that animals are polyphyletic is attractive to a
growing number of zoologists.

15. CONT..
◦The nearly simultaneous appearance of all animal phyla in
fossils from the Precambrian/Cambrian boundary is difficult to
explain if animals are monophyletic.
◦If animals are polyphyletic, more than one explanation of the
origin of multicellularity could be possible, and more than one
body form could be ancestral.

16. ◦Conversely, the impressive similarities in animal cellular
organization support the view that all or most animals are
derived from a single ancestor.
◦For example, asters form during mitosis in most animals,
certain cell junctions are similar in all animal cells, most
animals produce flagellated sperm, and the proteins that
accomplish movement are similar in most animal cells.

17. ◦These common features are difficult to explain, assuming
polyphyletic origins.
◦If you assume one or two ancestral lineages, then only one or
two hypotheses regarding the origin of multicellularity can be
correct.

18. Reference
◦Miller−Harley: Zoology, Fifth Edition