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10/14/10
BY2204.Lecture 8
It’s all about SCALE
SF Evolution
Fossils and Extinctions
We can tip the
world into
meltdown leaving
it lifeless and
barren if we are
not careful.
On the timescale of evolution, we are negligible
Compared to what the
cyanobacteria did
(poisoned everything),
we have trivial effects.
True or False?
False
Man is puny
We CAN wipe out
everything we can name,
reduce the environment
to desert and kill
ourselves….
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Timescales beyond imagination
…but we will have no effect on
most insect species and microorganisms.
4600mya
Earth
formed
4000mya
Life
3500mya
Oldest
fossil (?)
2700mya
Definitely
Prokaryotes
Humans
Evolve
1- 2mya
Life will go on without us.
5000
With any luck, something more
sensible will evolve in a couple
of thousand million years.
Hey! Turn that
light off!
OK
Read “Earth: The Comeback” New Scientist
3rd Oct 2009
The fossil record works
on this timescale
But it is 1) far from complete,
and
2) hard to interpret.
In this lecture we will
explore why, then look at
mass extinctions, the most
obvious events in the fossil
record.
For
Geologists,
50,000 years
either way is
a reasonable
level of
inaccuracy.
4000
3000
3800mya
Oldest
rock
2100mya
Eukaryotes
543mya
Cambrian
Explosion
1500mya
Multicellularity
2000
1000
Now
Snowball Earths
Until 1500mya
1) Why is the fossil record so
incomplete?
It is very rare indeed for a body to
get fossilised.
Dead gazelles get torn apart,
eaten, the remains decomposed.
Only if they die and get covered,
perhaps in mud or sand, is there
any chance of fossilisation.
Only hard parts are preserved
(the rest rots) so shelled animals
or those with bones have a better
chance than squishy ones.
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Then the river bank (or
wherever) has to remain
undisturbed for hundreds of
years. If the body is
uncovered it disintegrates.
So fewer than 1 gazelle in a million gets fossilized, and
extremely few squishy things like jellyfish.
Where you live matters too – shallow
muddy shores, like estuaries, or lake
shores are much the most likely,
where sedimentary rocks are forming.
Once buried, the minerals in the
bones or shell get very gradually
replaced with minerals from the
sediment they are buried in.
In a few million years all the
original bone or shell is replaced
with rock minerals.
Meanwhile the sediment has
built up above the fossil, and the
sediment has become solid rock
under the pressure from layers
above.
Alternatively the shell may leave an imprint in the rock.
Footprints and worm casts can be preserved this way too
So the fossil record is
very biased towards
shallow water fish, and
shells.
Indeed fish are so common that there
is one caught in the act of eating
another (must be rare!)
And another!
By far our best fossil sequences are of
snails as they live in the right place,
and are hard on the outside so break
down very slowly.
Very occasionally soft parts are preserved
•  In ice, like this perfectly preserved baby
mammoth, gut contents and all;
•  In amber, the petrified sap of
pine trees. This produces perfect
insect preservations and shows
how little they’ve changed;
•  or through carbonization.
A leaf in an anaerobic swamp may not decay. If it gets buried in
silt and subjected to heat and pressure, most of the leaf's
organic material is released as
methane, water, and CO2. A thin film
of carbon remains, showing the
imprint of the leaf. Insects and fish can
also be preserved in this way.
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The fossil must then reach the
surface again before it reaches
the edge of the tectonic plate
on which it sits, and is
subducted into the molten rock
below.
2) Why are fossils hard to
interpret?
a) Stratographic dating of fossils
THEN it must be found by
someone who recognises what
it is, and describes it for
science.
The layers of sediment
build up to give strata,
which give a relative time
for the fossils in them. Old
fossils are in deep strata,
newer ones in the top
layers.
So the fossil record is both biased
towards certain species, and only
records a tiny tiny fraction of life
on earth.
BUT, the strata move, bend and squish at different rates,
making dating this way very difficult.
Same age rock at
different distances
form the surface
Faults and breaks in
strata allow slippage
Different pressure on the
same rock makes different
parts of the same layer
thicker or thinner
The ages of
geographically separated
fossils are hard to
compare by stratigraphy
Different rocks squish
different amounts, and
the sediment will build up
at different rates in any
two regions, so
contemporary fossils will
end up at different
depths.
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Radiometric Dating methods
Radioactive isotopes decay to a daughter product at a steady
rate. If you know the original level of the isotope in the
material, and you can measure the current level, then you
know how long the gap between has been.
The carbon in living things originates in the CO2 in
the air, via photosynthesis, and this is rich in 14C.
Non-living things have mostly 12C.
After death 14C decays, so the ratio 14C :12C left indicates how long ago
it died.
Half life = 5730 years, so useful only in specimens less than
50,000 years old E.g. organic material, Egyptian mummies,
mammoths, NOT most fossils
b) i) The speciation event itself is unlikely to get recorded
These two species appear very similar and would have
identical fossils. They tell themselves apart by song.
Fossils will not record their speciation event until they become
different enough that it shows in the bones, e.g. one gets
bigger than the other
Chiff Chaff
Speciation
event
Colour
differences
Good for bones, teeth, and corals
Other radioactive isotopes are used
to date the rocks themselves, going
right back to the oldest rocks on
earth.
So radiometric dating can help stratigraphy out considerably
now with difficult cases.
b) ii) The speciation event itself is unlikely to get recorded
Evolutionary change happens much faster in
small populations than in large ones, so new
species often form in isolated sub-populations.
Because fossilisation is rare, small populations
are unlikely to leave any fossils.
Main population (leaving fossils) doesn’t
change during the gradual formation of the new
species, so all new fossils are of the old type
Bone differences
e.g. size, shape
The new species then outcompetes the old,
and spreads over the whole area, leaving
suddenly different fossils of the new species.
Now
Fossils different enough to
recognise as different species
Half life = 0.25 million years
For dating fossils more than 1million years old (that is,
most of them)
Dating using 14C (carbon 14)
Songs different
so no more
interbreeding
Dating using 234Uranium
Willow Warbler
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What really happened
(dark bits are fossils)
c) Incomplete record means
rates of change are inaccurate,
and some information (a whole
species here) is missing.
What the patchy fossil record
shows
Rarity of fossils means you
also can’t tell the difference
between a gradual change and
a stepped one.
This has led to great controversy as
to whether evolution goes gradually,
or in leaps, or both.
Constant, gradual
rate of change
Evolution by means
of sudden leaps
Faster and slower rates
of change depending on
conditions
How do we measure the rate of evolution?
Unit = Darwins
1 Darwin = a proportional increase of 2.718 per my
That means:
Your arm lengthens at 1 Darwin if, after 1 million years it is
2.718 times its original length.
Sheldon 1987 showed that even though his trilobites
gradually gained more ribs over time, they did so in fits and
starts, sometimes even going backwards.
So evolution is gradual at both the smallest and
largest scales, but not constant in rate or direction.
Or
LN (arm length 2) – LN (arm length 1)
Time 2-Time 1
=
Rate of
evolution
in Darwins
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What caused the K-T extinction (killed the dinosaurs)?
Mass Extinctions
  Asteroid
10km across hits earth: equivalent to all atomic
bombs exploding
In the history of multicellular life,
there have been 5 really big
extinction events, so far.
  Dust cloud hides sun – nuclear winter
  Explosion fumes make acid rain
All associated with sea level changes
  Acid rain kills forests and increases greenhouse gasses
  Greenhouse effect causes global warming
Date
% genera
lost
% species
lost
Main groups going extinct
End of
Ordovician
61
85
Major groups of trilobites, brachiopods, corals,
echinoderms etc.
End of
Devonian
55
82
21% or marine families
End of
Permian
84
96
57% of marine families; all reefs, all trilobites, 27
families of tetrapods
End of
Triassic
47
76
58 families of cephalopods, many reptiles, all large
amphibians, many insect families
End of
Cretaceous
47
76
Dinosaurs and ammonites gone. Flowering plants and
marine groups decimated. Few land animal phyla left.
  Ecosystem collapse
Caution in interpreting mass extinctions from fossils
They appear sudden, but dinosaurs
were already dying out before the
asteroid hit.
Whole event took 400 000 years, with
asteroid in the middle of that time, so it
was only one of several causes.
A gap in the fossil record can look like
an extinction event
Looks like lots of
species dying,
and then new
ones appearing
The K-T extinction of Brachiopods looks like this.
If you look closely, only 8 species actually go extinct on the
boundary. 7 just before and 5 just after. Lots of new species way
later.
6 species die out but reappear! = Lazarus species. Probably in a
“refuge” small population which then re-expanded.
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Major origination events: the Ediacaran Fauna
Major origination event: the Cambrian Explosion
After the last ever “Snowball
Earth”, find suddenly a
whole fauna/flora of flattened
multicellular species and
bacterial mats.
…loads of very diverse
species with complex
bodies, heads, guts,
mouths, legs, hard
carapaces.
500mya version of “Eden”
No predation as all feeding
was by absorption, no guts!
What sparked this
diversity?
Prevented snowball earth
returning somehow?
Disappeared very suddenly
and replaced by…
The Burgess Shale shows all the basic
body designs we have now, and many
which have gone extinct…. Everything
was invented in the Cambrian Explosion
Or was it?
Read “When we were worms”
New Scientist 18th Oct 1997
pp30-35 for a nice synthesis of
this intriguing story
Necessary Reading;
New Scientist 3rd Oct 2009 pp32-35 “Earth: The
Comeback” about what will happen to life on earth if we carry
on causing global warming.
New Scientist 18th Oct 1997 pp30-35 “When we were
worms” about the Cambrian explosion and Hox genes.
And at least 2 of:
Freeman and Herron “Evolutionary Analysis” 3rd Edition
Chapter 17 on fossils and fossilisation
Cambell and Reece “Biology” 6th Edition
Chapter 25 for a brief overview of the fossil record.
Skelton “Evolution: a Biological and Palaeontological
Approach”
Chapter 10 for the fossil record
Chapter 15.4 for Mass Extinctions
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If you are interested (or want a first):
New Scientist 26th July 2003 pp 38-41 “The Collector”
about one of the gurus of punctuated equilibrium Niles
Eldridge.
Nature 26th June 2003 Vol. 423 p929 “The battle between
the sexes” gives new evidence that large populations may
evolve faster than small ones.
Skelton “Evolution: a Biological and Palaeontological
Approach”
Chapter 9 for information on speciation
Hall and Hallgrimsson 2008 “Strickberger’s Evolution” pp
84-86 for nice simple description of radiometric dating.
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