The history of life

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The history of life

• How to organize our observations regarding
  changes in the biosphere over time

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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

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• 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

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Characters are distributed hierarchically

• Ex, mammals have hair/fur therefore this
  character distinguishes this group.
• Having fur, however, wont distinguish cats and
  dogs. We need another character (in this case,
  skull structure) to distinguish these.
• Two types of characters
• specific a character thats diagnostic of all
  members of a group
• general a character thats non-diagnostic of
  that group
• The same character will be specific for one group
  but general for a subset within that group. ex
  fur

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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
dont, ex, a trunk specific character of cars
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Add a motorcycle Two of these items have more in
common than with the other, therefore theres a
hierarchy
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Note that we can choose the character were using
to construct the cladogram.
This cladogram implies that motorcycles and
cars have more in common than cars and trucks
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How do you decide which characters to use?
B
A
The clade shouldnt change if you add characters.
Ex, you cant add chassis or steering
wheelto B. Likewise, you can also add other
motor vehicles, like SUVs or RVs to A.
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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.
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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.

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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
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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.
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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?
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When is a wristwatch a wristwatch? An example of
the use of cladograms
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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
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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 isnt correct wind-up watches have
moving parts and internal gears. digital watches
contain microcircuitry with no moving parts,
i.e., Its a small computer! 3 is a
more accurate cladogram of the degree of watch
similarity
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A watch is a small instrument for telling time
(a function), so it is not a great analogy of
living organisms, which we dont 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?
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Distinguishing between multiple options of
cladograms
parsimony a philosophical principle developed
by theologian William of Ockham. Often called
Ockhams 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 its not simple!
Cladograms are hypotheses of phylogenetic
relationships they make predictions which are
testable. Data includes fossils, DNA
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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.
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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.
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The cladogram for vertebrates
1 nerve cord within a sheath, bilateral
symmetry, pharyngeal gills 2 segmentation 3eyes
, kidneys, layered bone 4jaws 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 well return to Pikaia when
we discuss the Cambrian radiation of life).
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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?..
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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.