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  13 C However non-biological reactions can change  13 C ?????

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 non biological origin. Similar structures produced in recent rocks from Mt Vesuvius.

Foraminifera - single celled marine organisms that produce mineral shells.

Dawson continued to champion ‘one of the brightest jewels in the scientific crown of Canada’.

Eozoön

as 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 Genome sequencing is allowing huge advances in our understanding of evolution of metabolism and cellular structure.

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.

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!

Reasons not to be Genome comparison cannot tell us anything before the LUCA. 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.

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...