The history of life
How to organize our observations
regarding changes in the
biosphere over time
Some definitions
• Hierarchy = a rank or order of features.
“biota” = the largest group of living organisms.
“vertebrates” are a subset of biota
• Characters = traits possessed by organisms.
Not a single feature of a single organism but
features common to all members of a group.
Ex, all mammals have 3 bones comprising
their ear. This is a character of mammals
• In living organisms, we can describe a wide
range of characters:
• Structural: muscles, bones, teeth, organs,
skin, feathers, fur,
• Genetic: DNA, chromosome structure
• Embryology: ontogenetic development
• Ecology/behavior: swim, fly, bipedal
For fossils, this list is much more limited
Characters are distributed
hierarchically
• Ex, mammals have hair/fur therefore this
character distinguishes this group.
• Having fur, however, won’t distinguish cats and
dogs. We need another character (in this case,
skull structure) to distinguish these.
• Two types of characters:
specific: a character that’s diagnostic of all
members of a group
general: a character that’s non-diagnostic of that
group
The same character will be specific for one group
but general for a subset within that group. ex: fur
Paleontologists and evolutionary biologists have developed
a grouping pattern to display hierarchical characters in the
biota: cladogram = a branching diagram depicting hierarchies
of shared characters
Ex: cars and trucks:
characteristics: used to carry people? No- this is a function
4 wheels, chassis, engine, steering wheel, headlights- Yes
The branch in the diagram indicates that there are some features
That cars have that trucks don’t, ex, a trunk = specific character of cars
Add a motorcycle:
Two of these items have more in common than with the other,
therefore there’s a hierarchy
Note that we can choose the character we’re using to construct
the cladogram.
This cladogram implies that motorcycles and cars have more in
common than cars and trucks
How do you decide which characters to use?
A
B
The clade shouldn’t change if you add characters. Ex, you can’t
add “chassis” or “steering wheel”to B. Likewise, you can also
add other motor vehicles, like SUVs or RVs to A.
Traits that appear early in the cladogram are called
“primitive.” Traits that appear later are called “derived.”
Wheels, engine, headlights are primitive traits of motor vehicles.
Chassis, steering wheel, passenger space are derived traits.
What has been the impact of cladograms
(and cladistics) to the study of the history
of life?
• A way of analyzing patterns
• Show groups of organisms that are
related to one another as a result of
shared traits; they also show how
closely related they might be.
A few more terms about characters:
Homologues = anatomical structures that can be traced back
to a common ancestor. Ex, forelimbs of pterosaur, bird, bat,
human
Analogues - anatomical structures that perform the same
function but have no shared ancestry.
Ex, wings of a bird and a fly or human and grasshopper legs
Both human and grasshopper legs can move to
produce the jumping motion (= similar function).
However, their internal structures are totally different.
Therefore, they did not arise from a common ancestor.
Phylogeny = the history of descent of an organism over time.
based on how characters have changed over time and on
hierarchies of shared characters, revealed through cladograms
A branch on a cladogram = a clade = a monophyletic group =
groups or organisms are more closely related to each other
than they are to others. Characterized by derived traits.
A cladogram is a “hypothesis of relationships.” How are they
tested?
When is a wristwatch a wristwatch? An example of the use
of cladograms
There are 6 possible
cladograms, but a & b are
redundant in that the groups
at a node share the
characteristics at that node.
In other words, in #1 both
wind-up watches and digital
watches are above the last
node.
1a &2a are appealing
because digital
watches are the
newest and most
advanced BUT a
cladogram is
established on the
basis of shared traits
1a and 2a say that a
digital watch shares the
most traits in common with
a quartz or wind-up watch.
Examine the watches
and you see that this
isn’t correct: wind-up
watches have moving
parts and internal gears.
digital watches contain
microcircuitry with no
moving parts, i.e., It’s a
small computer! #3 is a more
accurate cladogram of
the degree of “watch
similarity”
A watch is a small instrument for telling time (=a function), so
it is not a great analogy of living organisms, which we don’t
classify by functionality. That said, we can still use watch types
for a discussion of how cladograms work.
If a “watch” can include a tiny computer as well as a tiny
machine with moving parts, then the cladogram for watches
must also include all computers!
=
From an evolutionary perspective, the wristband and watch
case have evolved twice, once for moving parts and again for
a very tiny computer. Is this plausible? Likely?
Distinguishing between multiple options
of cladograms
“parsimony” = a philosophical principle developed by theologian
William of Ockham. Often called “Ockham’s Razor”: simplest
is best.
Ex, bird, bat, human: 2 cladograms are possible:
B is less
likely because
it requires the
loss of fur/hair
or mammary
glands; it’s not simple!
Cladograms are hypotheses of phylogenetic relationships;
they make predictions which are testable. Data includes fossils,
DNA
When we study the history of life we use both
cladograms and the more traditional phylogenetic study.
Cladograms suggest “degrees of similarity” based on shared
traits, with the implication that shared traits are a result of
common ancestry.
Phylogenies are the actual
representation of
ancestor-descendent
relationships based on
observed and measured
morphologic changes over
time.
Shown here is one of the
best documented
phylogenies: horse
evolution in the Cenozoic.
Interrelationships of Vertebrates
The chordates are monophyletic, having originated from a
Middle Cambrian organism, Pikaia, that looks like a worm, but
had a nerve cord within a sheath (notocord), bilateral symmetry,
gill slits (pharyngeal gills) in throat. The modern organism that
is very similar to Pikaia is called a “sea squirt,” whose larval
stage contains a primitive notocord.
“Strange as it sounds, the tubular
creatures are essentially
vertebrates without
backbones. They're much more
closely related to fish, birds and
people than to worms, starfish or
other invertebrates. Like humans,
they belong to a group of animals
called chordates.”
The cladogram for vertebrates:
1= nerve cord
within a sheath,
bilateral symmetry,
pharyngeal gills;
2= segmentation
3=eyes, kidneys,
layered bone
4=jaws
5= bone in
endrochrondal
skeleton
6= ray fins
7= fleshy pectoral
and pelvic fins
8= 4 limbs
Our survey of the history of life will focus above node 3, with organisms
with a developed vertebrae (although we’ll return to Pikaia when we
discuss the Cambrian radiation of life).
Evolution of fishes
The oldest fish fossils: Ordovician. Major diversification:
Devonian, often called the “Age of Fish”
Jawless fish: armored, relatively immobile poor swimmers
Jawed fish: the Gnathostomes
Within this group are the Chondrichthyes or cartilagenous
fish, including sharks, and also the
Osteichthyes, or bony fish, which include the
Actinopterygii,or ray-finned fish, which are what you
catch and eat. These are the largest (most numerous
group of vertebrates on Earth today).
Think you know what a fish is?……..
What makes a fish a fish?
• Live, breath, move and reproduce in water?
These are all behaviors, and are therefore not
good criteria for defining a group, or clade.
• It turns out that the term “fish” is not really
meaningful in a cladistic perspective. The
characters that unite all fish are also shared
by other “non-fish” organisms.