Transcript Document

A brief history of life
Peter Shaw
We are lucky enough to emerge from the most remarkable lump of
stone in the universe (at least as far a we know).
We know that the universe exploded from a Big Bang about 14
billion years ago, and is widely if thinly seeded with hydrogen,
helium and lumps of stone. As far as we know, all the ones we
have studied have their surfaces in simple accord with basic
physical chemistry.
The surfaces of the moon, and Mars,
contain simple inorganic chemicals in
utter equilibrium with their surroundings.
Earth is different.
The air is not only thick (thanks to
earth’s magnetic field), but an unstable
mix of N2 and O2. These should end up
as nitrate in the sea. Some bits of the
earth’s surface burn given half a chance!
The sea is oddly deficient in some ions, notably
ammonium, nitrate and potassium compared to
chemical equilibria.
All these are the signatures of Life. Biological life, filtering air
and water into bodies, and cycling bodies through food chains
into soils.
Today I want to give you a brief history of this most
remarkable transformation.
We rely only on evidence: fossils to tell us what
conditions were like, extant life forms to infer common
ancestry, chemical simulations to explore what might
have happened. Sadly these will probably never ell us
the whole story.
The most important sections concern details of
biochemical organisation in proto-life, and we simply
do not know, sorry.
How old?
We can get reasonably reliable dates commensurate with the
planet’s age using very slow radioactive decay processes. The
best is the decay of K40 to Ar40, which has a half life around
1.270,000,000 years (1.27E9 years). Ar is a highly inert gas so
gases off – unless locked inside a glassy volcanic stone. Thus the
Ar40:K40 ratio of volcanic stones is an excellent clock for when
the lava cooled.
The oldest stones on the planet – tiny grains of the gem zircon inside
sandstone formed in an 4BYBP ocean (probably before life) give
dates around 4.3BYBP. Note that this is about 1/3 age of the
universe. Current thinking suggests true earth age around 4.5 BYBP.
Age of your planet: 4,500,000,000 years.
Hadean Eon
Earth assembled from plantary rubble, and
experienced a partial melt when something
(theia) bashed into us to knock off our moon.
Models suggest that this helped dense iron-rich
materials sink to the core, giving us a magnetic
field.
Oceans are projected to have started to form c.
4.4 BYBP, with the gradual settling down of a
surface crust. With little atmosphere and intense
instability we assume earth’s surface was lifeless
for most of this phase.
In the beginning
Then something remarkable happened. We had waterbased seas with organic compounds deposited from
comets, CO2 + CH4 in the atmosphere, irradiated with
UV. We know that in the lab simple building blocks of
life appear - sugars, amino acids. Somewhere in this
mix, a chemical reaction occurred which ultimately
promoted its own copying.
Nowadays these copying reactions centre on DNA; early
versions may well have relied on RNA, earlier still maybe
clay crystals. It is quite possible that the reactions involved
first appeared on Mars, which later seeded us with a spalledoff meteor. Somehow, an early reaction took ambient energy
to copy itself.
LUCA
We can only go on evidence, and the evidence is that
modern-looking filamentous prokaryotes (loosely;
algae) appeared very early on, within 700,000,000
years of earth’s formation. We do not know about their
biochemistry, nor about the intricate dance of chemical
reactions that preceded them. The evidence of modern
life forms is clear; all extant life forms came from one
ancestral design. LUCA - the Last Universal Common
Ancestor. One system bundled itself into phospholipid
bubbles and coded itself on DNA with an arbitrary
mapping that has permeated all life.
A question that repeatedly gets thrown at biologists is something
along the lines of
“What is the purpose of life?”
(Often by people who insist on disbelieving evolution..) Isolate
out the dual meaning of ‘life’ - your personal goals are your
personal choice and irrelevant to evolutionary theory - and the unask the question.
There is no evidence to suggest that the set of self-copying
chemical reactions we call ‘Life’ has or ever had a purpose.
Purpose implies planning, a conscious overview, which is why it is
so important to deny theists the use of the term.
Analogies
Take a great pile of randomly assorted pebbles and dump them on
the edge of the sea, somewhere with wave action. Their size
distribution will be random.
3 scenarios now;
1: Too little energy is applied; the stones remain unsorted. No
pattern persists.
2: Too much wave energy destroys the system, here by removing
stones. No patterns persist.
3: The wave energy is enough to move the stones around. A pattern
appears - big stones at the top, smallest down the base. This is the
usual pattern observed on most stony beaches by the sea.
What is the purpose of that pattern?
Carbon chemistry
Take a sterile planet with organic molecules in liquid water in its
oceans, and apply UV radiation capable of ionizing molecules.
3 scenarios now;
1: Too little energy is applied; no interesting reactions take place, no
new chemistry appears. No pattern visible.
2: Too much energy destroys the system, here by destroying
molecules or boiling the water. No patterns persist.
3: The solar energy is just enough to allow a self-copying reaction
based on solar-derived chemical changes. A pattern appears - proto
life. What is the purpose of that pattern?
Archean and proterozoic Eras
This period – the majority of earth’s history – ran from 3.8-0.7 BYBP.
Only stromatolites, no signs of animals.
The fossil record tells us that the majority of earth’s history it supported
oceans with slimy algae forming dirt-trapping hummocks. About 3
Billion years, at least 20* the rule of the dinosaurs. Just stromatolite
fossils, and slowly increasing oxygen levels.
Stromatolites
These are our oldest fossils but can still be found (notably Shark Bay in
Australia). Basically they are mounds made by photosynthetic algae
which entrap sediment so accrete, and can become quite large (a metre or
so) in shallow water. Humble, but we owe our existence to their slow
diligent production of oxygen.
About 3800 million years ago…
The earth had oceans, life, but the air was CO2/CH4 without
oxygen. The oceans contained vast amounts of dissolved Iron II
(blue-back). The earliest fossils are of stromatolites (+ strange
planktonic remains called acritarchs).
Age of planet earth, billions of years
4.5
4
3.5
3
2. 5
2
1.5
1
Photosynthesis
Oxygen builds up in air, removing
Iron 2 from oceans as rust and
filtering UV
0.5
0
Animals
Dinosaurs
Endosymbiosis
One more really important thing happened sometime in the
proterozoic, and we don’t know when. One cell
swallowed another, but instead of digesting it as dinner,
the intruder took up permanent residence using up a toxic
waste gas oxygen and creating high energy compounds.
This gave us the eukaryotic cell, with a nucleus and
organelles. All later multicellular life came from one
ancestral eukaryotic fusion.
This process has happened again, notably when a
eukaryotic cell ingested a cyanophyta cell and kept it to
evolve into chloroplasts - the light capturing organs of all
higher plants.
Endosymbiosis
For many years people knew that chloroplasts looked like some bluegreen algae (cyanophyta, which are prokaryotes) down to the
organisation of their stroma. But they weren’t invading pathogens,
they are well defined cellular organelles, copied faithfully down the
generations.
Oddly, another well-defined organelle, the mitochondrion, also looks a
bit like a bacterial cell. Odd that, but just one of those oddities…
Then in the 1960s Lynn Margulis produced her thesis suggesting that
these organelles really are prokaryotes, living as endosymbionts inside
their host cell. (And was laughed at for about 20 years..)
Endo – inside
Symbiosis – a mutually beneficial relationship between two life
forms.
Lynn Margulis predicted that these autonomous organelles would
have independent DNA, and to widespread amazement by the early
1980s it became clear that chloroplasts and mitochondria DID have
their own DNA, in a circular loop just like a prokaryote. The
endosymbiosis heresy became established fact. (This is where
science differs from religion.) They also have their own ribosome.
Specifically, chloroplasts can be visualised as a free-living
cyanophyta which has been swallowed (invaginated) by a host cell
but remained undigested.
Large cell,
perhaps of an
archaean
Photosynthetic
cyanophyta
Photosynthesis continues,
inside a vacuole and
inside its own cell wall.
A Mitochondrion (the tubby-sausage silhouette is
typical).
Note the dual
membrane system,
with the inner
membrane equating
to the ancestral cell
wall of the
endosymbiotic
bacteria.
Chloroplasts
Again note the double membrane, the
inner a relic of the prokaryotic
endosymbiont.
4-10μm
Bodies!
The buildup of O2 had several profound consequences.
1: It protected from UV, enabling life to emerge from under
deep water.
2: It enabled the rapid metabolisms of multi-cellular
organisms.
Ediacaran or Vendian era
First fossils appearing to animal are the bizarre Ediacaran creatures
that briefly flourished 700-450 MYBP. They only persist as
enigmatic fossils, fundamentally unlike anything since.
The strange name comes from the Ediacara range in S
Australia where the definitive faunas come from. The first
however was found by a schoolboy in Charnwood forest
(UK), at a time when everyone knew that fossils in precambrian deposits were impossible. The genus is called
Charnia to this day.
We know nothing
about these animals?
The basal bulb
probably anchored
them in a mat of
microbial filaments?
The Cambrian explosion
The fossil evidence of animals first appears with shelly forms at the
dawn of the Cambrian, c. 540-490 MYBP. We know from a truly
remarkable deposit called the Burgess Shales that diverse fauna, justabout recognisable as modern forms in most cases, existed about 500
MYBP. The key point is the appearance of hard shells, allowing good
fossilisation, but we have enough Vendian fossils by now to be confident
that there were few if any hard-bodied anythings then.
Age of planet earth, billions of years
4.5
4
3.5
3
2. 5
2
1.5
1
Photosynthesis
Oxygen builds up in air, removing
Iron 2 from oceans as rust and
filtering UV
0.5
0
Animals
Precambrian
Aysheaia – our ancestor
Burgess shales fauna
Hallucinigenia
In the early days of animals the seas held tadpole-like
protofish, shrimp-like arthropods, jellyfish corals etc,
molluscs (and lamp shells, not molluscs at all). Probably lots
of algae, hard to know. On land: nothing.
The devonian was the age of fish - the most advanced life
forms on the planet were armoured fish, co-predating with
giant sea-scorpions. Corals though present were often
superceded by sea lilies (crinoids).
Land colonisation
Our 1st land fossils come around 390MYBP, notably the
amazing rhynie cherts of Scotland, showing us the cells and
stomata of plants before leaves evolved, and how little
springtails have changed.
Asteroxylon mackei, an early lycopod.
X = xylem, t = leaf traces, scale bar 1mm
A modern club moss Lycopodium for comparison. They
also have striking xylem (see slide).
Geological periods
The recent (last 700 MYBP) history of the planet is named by
geologists, but using biologists’ data. Geological eras are
defined by their fossil assemblages, which is observed to
under occasional massive abrupt changes. These are Mass
extinction events - MEEs, and they set boundaries; silurian,
devonian, carboniferous, permian, triassic, jurassic,
cretaceous, palaeocene, eocene, .. holocene.
MEE – mass extinction events. Punctuation marks in the
earth’s history. We tell of slow evolutionary processes and
geological time scales, just as we talk of societies
advancing gradually towards [happiness, democracy,
equality…]
Then recall Baghdad May 2003; no
society, no structures, collapse and
meltdown.
MEEs are when the biosphere does
the same thing.
Geological Boundaries
Geologists divide the planet’s history up into bite-sized divisions. These
are NOT merely to satisfy vanity – “Let’s make up a name for the period
from 400-300 MYBP”. These reflect meaningful differences in the
stones based on their fossil content.
If you see a trilobite in the shales the rock’s age lies between two clear
boundaries. No hard-bodied animals before the Cambrian, no Trilobites
after the Permian.
The names were largely coined before isotopic dating had been
developed, and latterly there was huge tension between biologists
(who needed long timescales) and physical geologists (who
calculated how long the earth would take to cool from sun surface
temperatures; c. 20 million years)
Geological time scales
coded as: Era: Period: Epoch
Precambrian: = most of history + Ediacaran (Vendian), 4500570 MYBP
Paleozoic: 570-225 MYBP; Cambrian Ordivician Silurian
Mesozoic:
Cenozoic:
Precambrian
Devonian Carboniferous Permian
Triassic Cretaceous Jurassic Cretaceous
Tertiary (Palaeocene, Eocene, Oligocene Miocene Pliocene)
Quaternary (Pleistocene holocene)
Palaeozoic
Mesozoic Cenozoic
shelled animals
700
600
500
Coal
400
300
Age MYBP
?
Anthropocene
Oil
200
100
0=now
Geological boundaries
As explained, are marked by indicator fossils. These can be whatever
is fossilised well and often, invariably hard bodied forms: These
include a roll call of taxa that were world-dominating in their time,
and have now vanished, either absolutely or in ecosystem dominance:
Trilobites, Conodonts, ammonites – extinct
Graptolites, sea lilies, brachiopods –
survive at a low level.
The Permian MEE
The Palaeocene / Mesozoic boundary occurs between the Permian and
the Triassic, 251 MYBP. It marks the greatest mass killing in the Earth’s
history, with >95% of known marine species vanishing at the boundary.
The most famous were the Trilobites. (Also 70% terrestrial spp,
including gorgonopsian reptiles)
Sediments around the boundary show signs of anaerobiosis, and there is
a strong suggestion that oxygen tensions fell so low that life became
impossible for many life forms. We don’t really know why, but the
evidence below points one way. There is no meteor crater whose date
fits, little chemical evidence from the boundary suggesting asteroid
material (some contested argon ratios in 2 sites), but definite
synchronous large volcanic eruptions (forming the Siberian traps).
CO2 trends over time
RCO2 (1* = 350 ppm)
0 5 10 15 20 25
= methane catastrophe
P/T
600
500
400
Toarcian
183mybp
PETM 55mybp
300
200
100 0=now
Age MYBP
Palaeozoic
Mesozoic Cenozoic
The K/T boundary
The mesozoic / cenozoic boundary is marked by the K/T boundary (=
Keuper / Tertiary). This is dated to 64MYBP, and is marked by a
faunal shift nearly as great as the Permian. The boundary is observed
around the world as a homogenous ashy deposit, a few cm – 10s cms
deep. This is in itself unique: there are NO OTHER global boundaries
that manifest as a horizon over the whole planet.
Below the layer are mesozoic reptiles. In Hell Creek you can find
bones right up to the layer (some claim density falls away near the
boundary, but this looks like a sampling artefact; bones that big take a
long time to get covered over.)
Above the ashy layer, no mesozoic reptiles, and not much else: a spike
of ferns, some insects, then mammals start to radiate.
For this event we know that a major extraterrestrial impact was culpable, in part. There
is a chemical signal, detected and identified by
the Walter and Luis Alvarez (father and son). It
concerns the element iridium, a siderophilic
mineral rarely found on earth but relatively
enriched in asteroidal material. (I don’t know
why BTW). Its isotopic signature matched an
asteroid too.
The KT boundary in
Alberta badlands
The K/T boundary turned out to be enriched in Iridium 10-15*
background levels, all over the planet. This was convincing
evidence that an asteroid had hit earth at the K/T boundary, leaving
detritus in the ashy layer. The iridium values gave the asteroid a
10km diameter
The unifying theme
Time and again after a MEE there is a lull while new
species evolve, then the system settles down to land plants
supporting large herbivorous land animals, that in turn
support predators (and presumably scavengers and
parasites).
This didn’t apply in the carboniferous, but has done
every time since. Spot the modern parallels:
Permian 290-250 MYBP
Land plants
herbivores
carnivores
dicynodonts
Gorgonopsids
Seed ferns (here
Glossopteris)
(note sabre teeth)
Jurassic-Cretaceous 210-64 MYBP
Land plants
Gingo, cycads
herbivores
sauropods
carnivores
Tyranosaurus rex
Recent food chain (where?)
Land plants
grass
herbivores
litopterns
carnivores
Sabre tooth marsupial
tiger
Thylacosmilus (replaced
by Smilodon)
Life in an MEE
We are probably living in the early stages of a MEE to rival the
K/T boundary. The future biosphere + climate system will be the
anthropocene, dominated by an intense heat pulse (CO2 -> CH4
release).
Recovery from an MEE takes c. 10 million years.
Remember that when telling your kids that you recall the Chinese
river dolphin being found to be extinct.
Lipotes vexillifer
Extinct 2006