Transcript Life, Evolution and all that…

Fossil record of life
Planets formed about 4.6 Billion years ago
= 4600 Million years
Oldest rocks on Earth – 4.0 Billion years
(most of surface < 1 Billion years old)
Oldest rocks on Moon – 4.4 Billion years
Oldest meteorites – 4.6 Billion
A time scale for life
4.6-3.8 Hadean Eon – Earth Still Solidifying.
Frequent Impacts. Can vaporize oceans.
Life Impossible.
3.8-2.5 Archaean Eon – Prokaryotes (probably)
existed since before 3.5 Bya.
2.5-0.55 Proterozoic Eon – Significant increase in
atmospheric oxygen. First Eukaryotes.
550 Mya -present - Phanerozoic Eon – period of
visible life. First Multicellular organisms.
http://www.palaeos.com/Timescale/default.htm
Time scale for life in the Phanerozoic
Before 550 Mya – Precambrian
550 – Cambrian explosion. Diverse fossils
appear.
550-250 – Palaeozoic. Invertebrates. Fish.
Amphibians.
250-65 – Mesozoic. Reptiles.
65-present – Caenozoic. Birds. Mammals.
2 Mya – bipedal ancestral humans
50,000 – genetically modern humans
Oldest Rocks on Earth
Oldest Igneous – 4.0 Ga – Acasta, Northwest Territories
Oldest Sedimentary – 3.8 Ga – Isua, Greenland
These rocks may be metamorphosed. No structural information
Some claims for life based on 13C
However non-biological reactions can change 13C ?????
Thought to have been formed in the ocean  ocean already at this stage
Oldest sedimentary rocks claimed to contain fossils
3.5 Ga – Pilbara, Western Australia (Warrawoona/ Apex chert)
3.4 Ga – Barberton Greenstone belt, South Africa
Two kinds of evidence: stromatolites and microfossils.
Stromatolites – some of the oldest fossils
Stromatolite means layered rock.
Stromatolites are structures
formed by colonies of
microorganisms. They are not
individual organisms.
Section through a
fossil stromatolite
Modern stromatolites at Shark Bay, Australia
Formed by communities of
micro-organisms.
Surface = cyanobacteria.
Filamentous photosynthetic
bacteria.
Lower layers = anaerobic
bacteria
When surface gets buried by
mud the cyanobacteria can
glide up towards the light
forming a new layer.
J William Schopf – Prof. of Palaeobiology at UCLA
Claimed to be the earliest known fossils
Apex Chert site – Pilbara,
Australia
Age 3.5 billion years
Fossilized cyanobacteria
preserved as individual
filaments in what was
formerly gelatinous
mucilage.
But are they really fossils?
But are they fossils?
Both the stromatolites and the microfossils might be non-biological look-alikes.
Cyanobacteria are advanced aerobic photosynthesizers – late stage of metabolic
evolution. If they existed at 3.5 Ga, life came very far very fast.
Schopf (2006) latest position:
Consider cases where microfossils occur with stromatolites. Carry out small scale
chemical analysis and show that position of carbon compounds corresponds to
observed structure position. (Only about 1% of stromatolites have microfossils
associated with them.)
Says that stromatolites are extremely widespread in Proterozoic and that nearly all
must be biological. Acknowledges that it is more problematic in the Archaean.
Although similar to cyanobacteria, acknowledges that the earliest microfossils may
be other kinds of filamentous bacteria.
Concludes that the evidence for life back to 3.5 Ga is strong.
Brasier et al (2006)
Argues for non-biological null hypothesis
Co-occurrence of biology-like morphology and carbonaceous chemistry is not
enough.
Ask what they are, not what they remind us of.
Argues that the microfossils are non-biological. Spherulite mineral crystals form
and push carbonaceous material to the boundaries. Artefacts of symmetry
breaking during crystallization.
Concludes that cyanobacteria probably
present by 2.6 Ga
Early microbial mats may have been
anaerobic photosynthesizers or
chemosynthetic
Concludes that life may still prove to go
back to 3.5 Ga but this is not yet certain.
The earliest known Eukaryotes.
Han & Runnegar - 1992
Empire Mine, Michigan.
Found fossils of filamentous eukaryotic algae called
Grypania - Age 2.1 Billion years
Vendian period (end of Precambrian)
around 580 Million years ago
Multicellular animals found
Also called Ediacaran Fauna, because they were
discovered in the Ediacara hills in Australia in 1946
Possibly related to early metazoa (e.g. Jellyfish) or could be entirely
different branch of evolution (see Seilacher in OI book)
Burgess shale fossils discovered in the Rockies in 1909.
Age approx 515 Mya – around the Cambrian explosion.
Canadia spinosa
(‘thorny Canadian’)
An annelid worm
Canadapsis perfecta
(‘completely arched
Canadian’) - a crustacean
Hallucigenia – nothing
like it nowadays
Eozoön canadense - the dawn animal of Canada
An early controversy
John William Dawson – Principal of McGill. President
of Royal Society of Canada 1882. Knighted by
Queen Victoria.
Discovered Precambrian limestone age 1.1 Ga
containing green and white layered patterns.
Claimed to be remains of foraminifera. Earliest
fossils then known.
Shown in 1894 that these samples were of nonbiological origin. Similar structures produced in
recent rocks from Mt Vesuvius.
Foraminifera - single celled
marine organisms that produce
mineral shells.
Dawson continued to champion Eozoön as
‘one of the brightest jewels in the scientific
crown of Canada’.
Religiously motivated. Wanted to show that
there was a huge gap in the fossil record and
hence that gradual Darwinian evolution could
not be true.
The Martian metorite – a recent controversy
Martian meteorite ALH84001
found in Antarctica in 1984.
Age approx 4.5 Billion
years.
Science (1996) – ‘possible
relic biogenic activity’.
Contains polycyclic
aromatic hydrocarbons and
possible ‘nanobacteria’
fossils, 20-100nm across.
Much smaller than any
earth bacteria, therefore
very controversial.
The Red Rain of Kerala – a current controversy
Fell in India in 2001 - covered a large area.
Associated with loud boom of a meteorite(?)
Contain organic molecules but no DNA
Claimed to be extraterrestrial life
????
Dr Godfrey Louis – Cochin, India
http://education.vsnl.com/godfrey/
Dr N C Wickramasinghe - Cardiff
http://www.astrobiology.cf.ac.uk/redrain.html
Electron micrograph of sections of these particles
3 micrometres in diameter
So we’re about at the end then... What do we make of all this?
Reasons to be cheerful
Reasons not to be
Genome sequencing is allowing huge
advances in our understanding of evolution
of metabolism and cellular structure.
Genome comparison cannot tell us
anything before the LUCA.
We continue to discover new life in unusual
places on earth and to improve our
understanding of the diversity of
microorganisms.
New chemistry. e.g. artificial ribozymes.
LUCA was already complicated and
may be far removed from the origin of
life.
Not clear that we will ever produce life
in a test tube.
Many other planets now discovered. Almost
certainly we will find earthlike ones soon.
Spacecraft and eventually people will
explore Mars, Europa etc.
There is a new interdisciplinary field of
Astrobiology. This is a science. We are not
looking for a miracle.
There must be life out there!
Exploration outside the solar system
will not be possible without a miracle
in our understanding of physics.
Maybe it is simply impossible.
Maybe we will never meet them...
Maybe that’s a good thing after all...