Chapter 17: The Evolution of Animals
Big Ideas
Biology 160, Murkowski
Overview:
We’ll keep moving forward with our survey of the diversity of life, this time taking a
look at a familiar kingdom, the animals! As you read, remember that the words in bold are
vocabulary terms I would expect you to understand in a quiz or exam. If they’re not clear as
you read, check your textbook, ask a colleague or drop me a note!
Introduction: Thankfully, animals are a pretty easy group to define! We belong to the kingdom
that contains all the eukaryotic, multicellular, heterotrophic organisms that feed by ingestion.
This relatively simple definition, however, includes a LOT of diversity! As we explore the animal
kingdom, we’ll examine a handful of phyla. (A phylum is a subcategory within a kingdom.) As
always, we’ll try to take a mostly evolutionary route, starting with the simplest, most ancestral
groups and moving towards more recent arrivals on the evolutionary scene.
Thinking About Animal Bodies: While we do often now use DNA evidence to figure out
relationships between groups of animals, the simplest method to start to make sense of all the
diversity is to start observing animal body plans. Let me get us started by introducing some
common terms.
Among animals, we observe two primary types of symmetry.
Some animals, like a jelly fish or the anemone shown at right, are
radially symmetrical. These animals have no clear “head” but are
instead ready to meet the world from any direction. This is an
important adaptation for animals that don’t move much as it allows
them to interact with their environment from any direction! You and
I, and the lobster at left, however, display bilateral symmetry. We have a clear head (usually
anyway!), a useful adaptation for animals that move actively in their environments!
Another distinguishing trait you may be aware of is that you and I are vertebrates, or
animals with backbones. This is a pretty recent development in the history of the animal
kingdom. We are vastly outnumbered by very successful and abundant groups of
invertebrates, or animals that lack a backbone.
The final body plan feature we’ll worry about is the coelom, or body cavity. You and I
possess a true coelom or fluid-filled space separating the digestive tract from our outer body
wall. We use this space to
package and protect our internal
organs. Many animals lack a
body cavity; these are known as
acoelomates. And a few groups
create a partial cavity, known as
a pseudocoelom (or “false”
cavity).
A Quick Tour Through Major
Animal Phyla: We’ll start our
quick tour with some of the
simplest animals, the sponges.
Sponges are multicellular, but
lack the true, specialized tissues
characteristic of most multicellular
organisms. Most sponges are
marine, living on the ocean floor
as filter feeders. Because their bodies can vary, we don’t describe them as either radially or
bilaterally symmetrical. (They’re asymmetrical!)
The next major group, the Cnidarians, does evolve true, specialized tissues. This is a
radially symmetrical group including the jellyfish, sea anemones, and corals. They possess a
dead-end, sac-like gut, meaning that everything, food as waste, goes in and out through the
same opening! (Blech!) Perhaps most interestingly, all Cnidarians possess stinging cells that help
them capture their prey. If you’ve ever tangled with a jellyfish, you know they’re more than
capable of stinging you, too!
From here on out in our survey, all the animals we’ll meet are
bilaterally symmetrical. Among the simplest of these are the
flatworms. These bilaterally symmetrical worms lack a true body
cavity, and typically possess a sac-like gut, like the Cnidarians. Their
body plans are typically flattened. (Who would have guessed!?)
There are several infamous flatworms, including the flukes and
tapeworms (shown at right), both of which can colonize the human
digestive tract.
The next major group is the roundworms. These worms are named after their cylindrical
body. This is a tremendously diverse group of animals that are particularly abundant in soils.
They, too, are bilaterally symmetrical, but now possess a complete digestive tract that has two
openings! They also possess a body cavity (although it is a false cavity, or pseudocoelom)
allowing them to develop and protect a greater diversity of internal organs. While most
roundworms are benign, useful creatures, several cause well-known human diseases including
pinworms, hookworms, and the parasite occasionally found in undercooked pork, Trichinella.
Next up are some of the most familiar worms, the segmented worms or
Annelids. These worms, like our earthworms, also have complete digestive tracts
and now also have a true coelom or body cavity. This phylum is also shared by a
wide variety of marine worms and everyone’s favorite, the leeches!
Next in our story are the Arthropods. This is a very diverse and successful
group. They, too, are bilaterally symmetrical, have a true body cavity, and a true
gut. In addition, all the members of this group possess an exoskeleton or external skeleton. This
exoskeleton is handy for protection, but makes growth tricky! Thus arthropods must occasionally
shed their old exoskeletons and secrete larger ones, a process known as molting. While we
won’t worry too much about classification within this very large group, you should recognize
many of its members, including the spiders, crustaceans, millipedes and centipedes, and the
insects!
The mollusks also include some very recognizable animals including the slugs, squids,
snails, and clams. These are all soft-bodied animals that protect themselves with a hard shell.
(A few, however, have lost or reduced their hard shells in their evolutionary
history.) Many of them feed with a characteristic tongue-like organism, the
radula.
The second-to-last phylum we’ll explore is the echinoderms. This is a
fascinating group of deceptive, marine organisms. Common members you
might be familiar with include the sea stars (also known as starfish), sea urchins,
and sea cucumbers. Most of these have a water vascular system and an
endoskeleton made of hard plates beneath their skin. While their adult bodies
don’t look very similar to our own, their bilaterally symmetrical larval stages and developmental
processes look surprisingly like our own!
At long last, we reach our own phylum, the Chordates! As chordates, we all share four
key, but occasionally cryptic, traits: a dorsal, hollow nerve cord, a notochord, pharyngeal slits,
and a post-anal tail. While you might not find all of these in the adult form, all are present in
early embryos.
To preserve our sanity, we won’t go deeply into the diversity of groups within the
chordates, but it is important to recognize some key evolutionary trends. First, note that there
are two groups of invertebrates in our phylum, the tunicates and lancelets
(shown at left)! Thus, while all vertebrates are chordates, not all chordates
are vertebrates! Within the vertebrates, the earliest groups are the aquatic
fish. We also recognize amphibians, which possess a mixture of aquatic and
terrestrial adaptations. And finally, several major groups, including the
reptiles and mammals that are fully terrestrial due to the evolution of the
amniotic egg which allows them to successfully reproduce on land. Whew!