Download SF Ev L5 Fossils and cambrian explosion

10/12/11
It’s all about SCALE
SF Evolution Lecture 5
Fossils and the Origins of
Diversity
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
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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•  Snowball
earths
The History of Life
…but we will have no effect on
most insect species and microorganisms.
4600mya
Earth
formed
4000mya
Life
3500mya
Oldest
fossil (?)
2700mya
Definitely
Prokaryote
s
Life will go on without us.
5000 million
years ago
With any luck, something more
sensible will evolve in a couple
of thousand million years.
•  Photosynthesis
starts
•  Multicellularity
evolves
OK
Read “Earth: The Comeback” New Scientist
3rd Oct 2009
The History of Life
5000 million
years ago
4000mya
Life
4000
2700mya
Photosynthesis
starts, producing
oxygen
3500mya
Oldest
prokaryote (?)
3000
3/4 of life’s
history
was single
celled!
•  Oldest rock
2100mya
Eukaryotes
543mya
Cambrian
Explosion
1500mya
Multicellularity
2000
1000
Now
Snowball Earths
until 1500mya
The fossil record works
on this timescale
But it is 1) far from complete,
and
2) hard to interpret.
3800mya
Oldest
rock
Refer to 1st yr
notes for
differences
between
prokaryote and
eukaryote cells
•  Cambrian
explosion
•  Earth formed
•  Eukaryotes
appear
3000
3800mya
Oldest
rock
•  Oldest
prokaryotes
Hey! Turn that
light off!
4600mya
Earth
formed
4000
•  Life started
2100mya
Eukaryotes
543mya
Cambrian
Explosion
1500mya
Multicellularity
2000
Repeated snowball
Earths
1000
Now
In this lecture we will
explore why, then look at
one of the most obvious
events in the fossil record.
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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.
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.
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
lower 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
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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;
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.
THEN it must be found by
someone who recognises what
it is, and describes it for
science.
•  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.
2) Why are fossils hard to
interpret?
a) Stratographic dating of fossils
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
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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 considerably
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
The rate of evolution carried on
more or less the same for
millions of years, producing new
species and losing some others.
Then suddenly there was a huge
increase in diversity; hundreds of
new forms in the fossil record
looking like nothing we would
recognise.
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.
These are the Ediacaran Fauna
known from a few deposits
scattered worldwide. The name
comes from one of these, the
Ediacara hills in Australia
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A whole fauna/flora of
flattened multicellular
species and bacterial mats.
The Cambrian Explosion – amazing diversity
Suddenly we have complex
bodies: heads, eyes, guts,
mouths, legs, hard carapaces.
All feeding was by
absorption. No guts. No
predation. No heads or
legs.
Ways of feeding: predators,
filter feeders, grazers,
browsers, detritivores.
Prevented snowball earth
returning somehow?
Types of movement: pelagic,
sessile, motile, drifting
This diversity then disappeared
suddenly.
Another even more diverse
collection of fossils very quickly
appeared.
Fossils resemble early members
of all the main groups alive now.
= “Cambrian Explosion”
We know about the Cambrian Explosion
from fossil assembalges.
The most famous deposit of fossils
showing the Cambrian Explosion is the
Burgess Shale in the Rockies in USA.
See “This Wonderful Life” by S.J.
Gould. – an easy and fascinating
read.
All of our current diversity
started here…
… or did it?
Read “When we were worms” New Scientist 18th Oct
1997 pp30-35 for a nice synthesis of this intriguing story
Interpretation of the
Burgess Shale
animals is difficult.
e.g. Anomalocaris
Antennae identified as an
“unusual Shrimp”
Huge diversity of
unknown forms
makes
reconstruction of
whole animals from
partial fossils tricky.
Hallucigenia
Mouth identified
as some form
of jellyfish
Body
identified as
some form of
sponge
Now
recognised as
one animal
“Anomalocaris”
Hallucigenia was originally considered a worm
and drawn the other way up. It’s name
indicates how confusing the scientists found it!
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What Caused the Cambrian Explosion?
Many explanations suggested:
(see Wikipedia “Cambrian Explosion”)
1) Ecological:
Larger planktonic animals fell through the water
column supplying nutrients to the sea floor, opening
many new ways of life.
Buried ones took carbon out of circulation,
increasing concentration of oxygen in the water.
Increase in oxygen levels allowed large sized
animals which didn’t have to be flat.
Led to more complex food chains and predator/ prey
interactions, leading to more complex adaptations.
Evolution of eyes in particular considered important
for causing diversity.
What stopped the diversity expanding?
No new major taxanomic group has evolved since the
Cambrian Explosion… Why not?
Once every
ecological niche is
are filled, strong
competition in
each niche stops
new forms
evolving to take
over.
(not very
convincing
argument!)
2) End-Ediacaran mass extinction
Mass extinctions leave many ecological
niches empty, but usually brief bloom
then reduce again. Why did the diversity
stay so high?
3) Snowball Earths
Ediacaran biota appear soon after the last
"Snowball Earth“, but long before the Cambrian
Explosion. These cold periods may even have
delayed the evolution of large size.
4) Developmental explanations
Map-making genes e.g. hox genes mean a few
gene changes can cause radical body change. But
hox genes may have been present in the
Ediacrians.
So the Cambrian Explosion is still a
matter of debate:
1) Whether it happened fast enough to
consider it an “explosion” of new life forms.
2) What caused it.
3) Which features of complex bodies really
evolved then, and which earlier.
4) Whether we’ve got the reconstructed
animals correct.
5) Why no new major taxanomic groups have
appeared after this time.
6) “Wonderful Life” by S.J.Gould also
explores the role of chance events in
evolutionary history; another major debate in
current evolutionary biology.
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Please Read:
If you are interested:
New Scientist 18th Oct 1997 pp30-35 “When we were
worms” about the Cambrian explosion and Hox genes.
Hall and Hallgrimsson 2008 “Strickberger’s Evolution” pp
84-86 for nice simple description of radiometric dating.
And at least 2 of:
NewScientist 6 February 2010 “Life’s a gas” importance of
oxygen levels in the rise of life.
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
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.
Wonderful Life: The Burgess Shale and the Nature of History
(1989) by Stephen J Gould – interesting about the role of chance
in evolution as well as exploring the Cambrian Explosion.
A bit off-topic:
NewScientist 17 October 2009 “Cradle of Life” newest theory
on the origins of life in deep sea vents.
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