||Zoology 250 (2017)
CONCLUSIONS: The Tree of Life
(click on tree to see full cladogram)
- 1) The Tree of Life -- one of humankind's greatest accomplishments -- reveals that life has a history: It is a grand vision
- 2) The Tree of Life can be read partly from traits of living organisms:
a) multicellular 'animals' likely arose from colonial protists
b) early branches yielded cellular, then tissue (diploblastic, radial), then organ-system (triploblastic, bilateral) grades of organization
c) bilaterians split into two major branches (Protostomia and Deuterostomia) within which coeloms and a complete gut (anus) evolved independently
- 3) The Tree of Life can also be read partly from fossils and molecules:
a) A literal reading of the fossil record suggests animal phyla arose in an 'explosion' near the beginning of the Cambrian (~580 MYA)
b) But recent molecular evidence suggests protostomes diverged from deuterostomes much earlier!
- 4) The study of comparative anatomy and development continues to yield essential insights into the evolution of animal form and diversity
a) Invertebrate Zoology remains a vibrant and exciting field
b) Much interesting and important research remains to be done
- 5) Major questions about the Tree of Life remain:
a) What is the correct phylogenetic placement of sponge classes? ctenophores? acoel flatworms? 'pseudocoelomates'? lophophorates?
b) What was the form of the ancestral deuterostome? ancestral chordate?
c) Excitement about phylogenetic relations is as high now as it was in the years following the Darwinian revolution.
d) Many fascinating things remain to be learned about the history of life on this planet.
- 6) Now it's your turn -- heed the words of the poet Walt Whitman (from Leaves of Grass, 1900):
"When I Heard the Learned Astronomer"
When I heard the learn'd astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts and diagrams to add, divide,
and measure them,
When I sitting heard the astronomer where he lectured
to much applause in the lecture-room,
How soon unaccountable I became tired and sick,
Till rising and gliding out I wander'd off by myself,
In the mystical moist night-air, and from time to time,
Look'd up in perfect silence at the stars.
(substitute 'zoologist' for 'astronomer' and 'invertebrates' for 'stars', and you'll see the point)
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Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)