Origin of Life in Precambrian(3.800 MY to ~1000 MY)

- Frumlífsöld -

Jarðsaga 1- Saga Lífs og Lands –

Ólafur Ingólfsson

A time of developingenvironments and life

Many of the most exciting events in the history of theEarth and of life occurred during the Proterozoic:

• Stable continents appeared and accreted. Stable continents favor development of life.

• Chemical composition of the Oceans and theAtmosphere change – oxygen levels increase

• First abundant fossils of living organisms appear, mostly bacteria and archaeans, but by about 1.8 billionyears ago eukaryotic cells appear as fossils too. Animals appear towards the end of the Proterozoic

• Episodes of mountain building and glaciationscontribute to the developing environments.

First signs of a modern-style orogeny• Cratons developing < 3.000 MY. The generalperiod between 3.0-2.5 BY ago, was the period of maximum continent formation. 70% of continental shield landmasses date from this period

First signs of modern-styleorogeny is the Wopmay orogeny (~ 2.000 MY) in NW CanadaThe North American Craton is sometimes called “TheUnited Plates of America”. It is composed of at least 6 proto-continental plates that accreated during late Archean and Proterozoic Eons

Early Proterozoicglaciations

The “Gowganda Formation” inCanada consists of tillites andvarved sediments withdropstones, dated to ca. 2.300 MY. Its thickness is ~1200 m. Glacial deposits of similar age have been recognized in South Africa, Australia and Finland

Huronian tillites

Some of the oldest rocks in the Worldoccur in South Africa. Four separatehorizons of metamorphized glacial-till have been recognized in these rocks. They have been dated to 2,720 to 2,360 MY.

The cause of the early Huronian glaciation not well understood...

Huronian varved siltstoneOne theory states a combination of weaker sun and decrease in greenhouse gasses (greatly reduced CO2) in the atmosphere caused the Huronian Ice Age...

What are the Origins of Life?

This is a key question, occupying all civilizations!

What is needed for developing life?The neccessary chemical elements for making organic compounds have to be in place (H2O, N2, CO2

, NH4 (joð), NH3 (ammóniak), as well as a source of energy.

Minimum criterion for defining a living organism:

• Must be able to reproduce

• The organism must exist as a species

• The organism must have metabolism (efnaskipti)

Environmental requirements:

• There has to be a relatively stable Earth’s crust

•There must be protection against ultra-violet radiation/sunwinds

• There has to be liquid water

• There has to be available oxygen for respiration, free O2 in the atmosphere, formation of ozone (O3)

Origin of life

– Atmosphere vs. oceans vs. interplanetary particles/objects (lots of debate)

– Clear that organic molecules can be created chemically in settings that are similar to the early earth

– Similar abundance of elements in the ocean and in the biota

Did life come to Earth from outer space?In 1969, the Allende Meteor-ite landed in Mexico. It was found to contain amino acids, the building blocks of pro-teins. This led to ides that life originated in space and came to Earth with a meteor-ite. The amino acids from meteorites are in a group known as exotics: they do not occur in the chemical systems of living things.

Biogenic-looking features in a Martian meteorite (Image from:

http://ares.jsc.nasa.gov/astrobiology/biomarkers/images.html)

This does not solve the origin of life. It just moves the problem elsewhere!

Origin of Life: Current Theory

• Chemical Evolution• .....The idea that long ago complex

collections of chemicals formed the first cells.

• Life began in the oceans 4 bya from simple chemicals joining together in a “primordial soup”

• Complex chemicals evolved into living cells

Things to consider…• What was Earth like 3.9 billion years

ago?– Are there still places on Earth that

resemble these conditions?• How could life originally arise from non-

living matter?• What did it take to form the first life?• What is “life”?

What is “life” anyway?• There are really no “biological” reactions that

can’t take place abiotically under correct conditions

• Organisms, however, catalyze many reactions that wouldn’t normally occur due to energy required, probability of encounter, accumulation of product, etc.

• Life is the compartmentalization, orchestration, and choreography of chemistry– Not all reactions can or should occur at same time– Ordering and containing reactants and reactions is key to

life– Capturing and containing energy is key to life

• Earth was very different then!

• A hot reducing (afoxandi)environment

• High temperatures• Essentially no

atmospheric O2– Highly corrosive,

destroys molecules• Highly energetic

– Lightning, volcanic activity, UV radiation high

– Provide energy for chemical reactions

Conditions on Early Earth

Miller ExperimentIn 1950, Stanley Miller designed an experiment in which he discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere.

From the water receptacle, designed to model an ancient ocean, Miller recovered some amino acids.

Where did life originate?

• The prevailing thought was shallow water or moist sediments.

• Some have proposed that life originated in deep-sea vents– These sites have energy, produce some organic

compounds, and have inorganic iron and nickel sulfides that can catalyze some organic reactions

– Modern phylogenetic analyses indicate that ancestors of modern prokaryotes thrived in very hot conditions and may have lived on inorganic sulfur compounds that are common in deep-sea vent environments

Figure 26.14 Deep sea vents, a window to early life?

Could life also originate elsewhere?

• As our understanding of our own solar system has increased, the hypothesis that life is not restricted to Earth has received more attention.– The presence of ice on Europa, a moon of Jupiter, has led

to hypotheses that liquid water lies beneath the surface and may support life

– While Mars is cold, dry, and lifeless today, it was probably relatively warmer, wetter, and had a CO2-rich atmosphere billions of years ago

• Some think Mars’ subsurface may still be capable of having life

– Many scientists see Mars as an ideal place to test hypotheses about Earth’s prebiotic chemistry

• Why not outside our solar system?

Biologically produced carbonate in Isua Formation, 3.800 MY ago?

It has been suggested that the greenstoneIsua Belt (Greenland), contains signatures of biologically produced carbonate already 3.800 MY ago. This is highly controversial, as the genesis of the rocks is not well understood.

Oldest known fossils...?Electron microscope images of whatis supposed to be about 3.500 MY old bacteria from Australia. 11 diffrent species have been recognized.

The evidence for Earth's oldest fossilshas recently beenquestioned. Maybe they are secondary artefacts formed from amorphous graphite...?

http://www.earth.ox.ac.uk/research/geobiology/geobiology.htmNature 416 (2002), 76 - 81

Prokaryotes were the first organisms on earth

3.5 billion yr old bacterial fossils Modern filamentous bacteria

If bacteria were this advanced 3.5 billion years ago, prokaryotes probably arose much earlier!!

Early Proterozoic LifeThe cyanobacteria have an extensive fossil record. They are relatively easy to recognize because they have remained unchanged for billions of years, and they leave chemical traces in form ofbreakdown products of pigments.

The cyanobacteria weretremendously important in shaping the course of evolution and ecological change throughout earth's history. The oxygen atmosphere that we depend on was generated by numerous cyanobacteria photosynthesizing during the Archaean and Proterozoic Era.

Fossil cyanobacteria

Stromatolite mats appear- The oldest stromatolites are 3.500-3.200 MY old, but were

first common after 2.500 MY -

They are colonial structures formedby photosynthesizing cyannobacteriaand other microbes. Stromatolitesare prokaryotes (lacking a cellular nucleus) that thrived in warm aquatic environments and built reefs much the same way as coral does today.

A stromatolite reefThe layers of the stromatolite mat/reef were produced as calcium carbonate precipitated over the growing mat of bacterialfilaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation.

Stromatolites reached their maximum diversity about 1.200 MY, but remained important reef-builders until the Devonian

Hamelin Pool Marine Nature Reserve, W Australia- Living Stromatolites; the most ancient organisms on Earth -At Hamelin Pool, the world's oldest organisms can be seen - stromatolites. Stromatolites are the result of primitive life forms that first existed on earth 3.5 billion years ago. The dome shaped structures reach up to 60cm in height and are formed by cyanobacteria. The process continues today.

http://www.calm.wa.gov.au/national_parks/hamelin_pool_mnr.html

Early life on earth… 3 billion years of microbes

Life began ~3.9 billion years ago

• Prokaryotes were the only life for billions of years

• Atmospheric O2 was first made by cyanobacteria– Plants couldn’t exist yet– Microbes still account for

~half of O2 produced globally

• Early eukaryotes were also single-celled and microscopic

• Plants and animals have only been here for ~600 million years– Australopithicus 4 million

yrs – Homo sapiens 0.5 million yrs

The Oxygen shock 2.200-1.800 MY543

Oxygen levels in the Archaean had been less that 1% of present levels in the atmosphere, but by about 1.8 billion years ago, oxygen levels were greater than 15% of present levels and rising. Hadean

Archaean

Neo-

Proterocoic

Meso-

Proterozoic

Paleo-

Proterozoic

4500

3800

2500

1600

900

Oldest animalfossils

Oldest Fungi

Origin of eukaryotes

Transition to oxygen

atmosphere

Oldest known fossils

Oldest known rocks

Atmospheric oxygen began accumulating 2.7 billion years ago

• Photosynthesis probably evolved very early in prokaryotic history.– Early versions of

photosynthesis did not split water or liberate O2

• Cyanobacteria, photosynthetic organisms that split water and produce O2 evolved over 2.7 billion years ago.

• This early oxygen initially reacted with dissolved iron to form the precipitate iron oxide.– This can be seen today in

banded iron formations.

Banded iron formations are evidence of Banded iron formations are evidence of oxygenic photosynthesisoxygenic photosynthesis

Oxygen accumulation• This “corrosive” O2 had an enormous impact

on life, dooming many prokaryote groups– Some species survived in habitats that remained

anaerobic (these are “obligate anaerobes”)• Other species evolved mechanisms to use O2

in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules

• Thus, prokaryotes altered the planet through O2 evolution, making aerobic respiration possible and paving the way for other forms of life (eukaryotes – “kjörnungar”)

Development during Early and Middle Proterozoic...

Even though the eukaryotes (protista) had appeared ~1.800 MY, the prokaryotes ruled pretty much the organic world through most of the Proterozoic. The steady build-up of oxygen, however, led to increased supply of nitrate (NO3

-) which fertilized eukaryotic algae.

The apperance of red algae and green algae about 1.000 MY shows increased diversification of life... These absorb diffrent wavelengths of light than do the bluegreen algae, and can thus colonize diffrent environments.

Eukaryotes (kjörnungar) are structurally more complex than Prokaryotes (dreifkjörn-ingar), and can be either unicellular or multicellular (Prokaryotes always unicellular)

Development of EukaryotesEukaryota include all animals, plants, fungi, and protists.

Mitochondrion = hvatberi, orkukorn; Chloroplast = grænukorn

It appears that the eukaryotic cell arose from the union of two prokaryotic cells, one of which came to reside within the other.

The cell that lived within the other was altered in minor ways to form a structure called mitochondrion. This structure allows cells to derive energy from food by means of respiration

Animalia

Fungi

Plantae

Bacteria EukaryaArchaea

Animalia

Fung

i

Plan

tae

Archaea

The origin of sexual reproduction: A continuing mystery

”The invention of sex was pherhaps the the most crucial development in the history of life”

Exactly how sexual diffrentiation occurred is still hotly debated – It was probably an ancient, Precambrian, innovation. The sexual imperative runs into deep time...

Roughly 1 billion years after the first organisms romped in the hay (Fungi), the origin of sex remains one of biology's greatest mysteries. Scientists can't say exactly why we do it, or what triggered those initial terrestrial flirtations. Before sex, life seemed to manage fine by employing asexual reproduction - the cloning of offspring without the help of a partner.

Two different kinds of reproduction, with diffrent inheritance patterns

• Single celled organisms reproduce by mitosis (jafnskipting)- Offsprings have the same traits as the parents

- Exception is due to mutation (random change – stökkbreyting)

• Multicelluar organisms reproduce by meiosis (rýri-skipting), the process underlying sexual reproduction

- Offsprings have a combination of the traits of the parents

- Mutation also occurs

Summing up the Precambrian

References, web-resources etcStanley, Earth System History, chapters 3, 7, 10, 11

http://www.ucmp.berkeley.edu/bacteria/bacteria.html

http://www.nbii.gov/issues/biodiversity/species.html

http://www.calm.wa.gov.au/national_parks/hamelin_pool_mnr.html

http://www.earth.ox.ac.uk/research/geobiology/geobiology.htm

Nature 416 (2002), 76 - 81

http://www.scotese.com/climate.htm

http://www.palaeos.com/Timescale/Precambrian.htm

http://www.palaeos.com/Archean/Archean.htm#continents

http://mason.gmu.edu/~bbishop1/chem663/6