Download Early Life on Planet Earth - University of Evansville Faculty Web sites

1
Early Life on Planet Earth
Overview
It was early in the Archaean that life first appeared on Earth.
Our oldest fossils date to roughly 3.8 billion years ago, and consisted of bacteria
microfossils.
The atmosphere during the Archaean Eon (3.8 to 2.5 bya) was very different from what
we breathe today; at that time, it was likely a reducing atmosphere of methane,
ammonia, and other gases which would be toxic to most life on our planet.
Also during this time, the Earth's crust cooled enough that rocks and continental plates
began to form.
Earth’s Oldest Rocks
The oldest rocks on Earth are from northern Canada; they are 3.9 billion years old and
There are also some very old rocks in the Isua area of west Greenland, dating at
approximately 3.85 bya
Rocks from both sites contain no evidence of life.
Carbon isotope evidence at 3.8 Bya from Isua, Greenland
The Isua rocks contain quite a lot of carbon in the form of the mineral graphite (a type of
elemental carbon).
During photosynthesis, RUBISCO generates organic matter enriched in 12C.
The carbon at Isua is in the form of “light” graphite - as if it had been produced by
RUBSICO via photosynthesis, providing indirect evidence that there may have been life
on earth before 3.85 bya
The Oldest Rocks with Life
There is clearer isotopic evidence for biological carbon life at about 3.5 bya
Archaen districts in both Australia and Africa provide evidence of stromatolites, which
are low mounds or domes of finely laminated sediment composed of either calcium
carbonate (CaCO3) or chert (SiO2).
They represent fossilized microbial mats formed mainly by photosynthetic
cyanobacteria
2
Living Stromatolites
Stromatolites contain a consortium, a complex associations of interacting organisms.
Composed of interwoven mats of slime-covered, filamentous cyanobacteria and other
bacteria.
At the top, cyanobacteria do oxygenic photosynthesis.
Below the surface, bacteria that do photosynthesis without producing oxygen occur.
Finally, deeper in the mat, heterotrophic bacteria feed on the decaying organic matter
produced by photosynthesis at the top of the mat.
The minerals, along with grains of sediment precipitating from the water, are later
trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which
then continued to grow upwards through the sediment to form a new layer.
As this process occurred over and over again, the layers of sediment were created.
Banded Iron Formations (BIFs)
A peculiar rock type emerges between 3.5 to 2.0 bya - Banded iron formations or BIFs
BIFs are sedimentary rocks formed from alternating bands of chert (SiO2) and iron
oxide.
They are sometimes repeated millions of times in microscopic bands
Where does the iron come from?
Iron is and was probably dumped into the oceans from erosion down rivers and from
deep-sea volcanic vents
Q. Why are there no BIFs around in present geologic time?
A. There are no BIFs today because iron readily precipitates out of solution in the
presence of oxygen and organisms subsequently extract and use iron and silica (silica that
could have gone into chert, SiO2) in building protective shells and skeletons
Iron dissolves readily in water that has no oxygen and that there was apparently little or
no free oxygen when BIFs were being formed
Iron can only have precipitated from seawater in the amounts observed in the BIFs by an
oxidizing chemical reaction
It is in an oxygen-rich ocean, that iron is oxidized, forming minerals that are insoluble in
water, allowing iron oxide to dominate the ocean-floor sediments.
3
Evidence of no oxygen on the early Earth
Pyrite (iron sulfide) and Uranite (uranium oxide) occurring in riverbeds from 3 to 2 bya.
These minerals are not stable when O2 levels are high.
Their presence in rivers confirms our suspicions that oxygen levels were very low on the
early Earth.
How did iron then precipitate out of solution when there was apparently little or no
free oxygen available?
The alternating iron oxide/chert beds indicate that there must have been periodic waves
of O2 available
In an oxygen-poor ocean, iron is soluble in water, so chert dominates the sediments on
the ocean floor.
In an oxygen-rich ocean, iron is oxidized (it rusts), forming minerals that are insoluble in
water, so iron oxide dominates the ocean-floor sediments.
The oxygen could have been supplied by the photosynthetic cyanobacteria present in
stromatolites
Ultimately, this oxygen is used up by the "rusting" of this iron, and the ocean reverts to
its ocean-poor state.
Note:
This process is believed to have generated tremendous iron deposits, which are the source
of most iron ores that are mined today.
BIFs are deposited in bands that can be traced for hundreds of kilometers
They appear to be laid down uniformly, or at least continuously, over great areas
This suggests that the photosynthetic bacteria were floating across the ocean surface
The Oxygen Revolution
Stromatolites were flourishing along the continental shorelines, and the cyanobacteria
contained within them were engaging in photosynthesis and giving rise to O2
Presumably, stromatolites did not cover large expanses of earth; therefore it would have
taken a tremendous amount of time before significant amounts of O2 accumulated in the
air and water
Q. Why might photosynthesis have established itself on the early earth?
It is not hard to imagine that any cell containing chlorophyll could have trapped energy
from the sun and evolved photosynthesis
For bacteria, the advantages of photosynthesis may have occurred as soon as simple
organic molecules began to run low, and fermenters began to run low of food
4
The earliest photosynthetic cells probably used H from H2, H2S, or lactic acid
Perhaps later some of the bacteria began to break up the strong H bonds of water
molecules
H2O + CO2 + light ‡ (CH2O) + 2 O
Any bacteria that became capable of successfully breaking down water than H2S would
have immediately multiplied their energy supply 6-fold
However, there most certainly would have been a cost with this switch
The waste product of photosynthetic processes that used molecules other than H2O were
easy to manage
For example, the waste product of H2S photosynthesis is sulfur (S) which is easily
disposed of
The waste product of H2O photosynthesis is monatomic oxygen (O), which is a deadly
poison to a cell because it can break down vital organic molecules by oxidizing them
Thus cells needed to evolve a natural antidote to this oxygen poison before they could
consistently operate the new photosynthesis
We and other organisms evolved superoxide dismutases to serve as antidotes; they
absorb O as soon as its forms and create ordinary (O2) or diatomic oxygen (2 O)
Presumably as soon as cyanobacteria evolved an antidote to oxygen poisoning, they
could control the use of it, including the use in new processes such as respiration
The Advantage of Respiration
Aerobic respiration extracts considerably more energy from organic molecules
(C6H12O6) than does fermentation (anaerobic respiration)
Fermentation yields lactic acid which still has a great deal of energy
By using oxygen to break up a series of by-products all the way down t water and carbon
dioxide, a cell can release up to 18X more energy from a sugar molecule via respiration
than it can via simple fermentation
Cyanobacteria, especially those in stromatolites, appear to be the dominant forms along
the early ocean shorelines
Their success was likely due to the control over oxygen, which gave them an abundant
and reliable energy supply in 2 ways: 1) by mastering photosynthesis based on water and
2) by breaking down food molecules in respiration rather than fermentation
Stromatolites increase dramatically in the rock record with the beginning of the
Proterozoic Era at about 2500 mya
5
Summary and Overview of Oxygen Revolution
The increased oxygen supply by stromatolites in shallow water produced the first great
masses of BIFs
It is probable that by oxidizing the iron, the BIFs served as a sort of buffer, allowing
oxygen tolerance and utilization to evolve among some bacteria
But eventually BIF formation slackened and the oceans and atmosphere began to
accumulate small amounts of oxygen
Continental Red Beds
There is other geological evidence that confirms the oxygenation of the oceans around
this time
Beginning 2.3 Bya, iron minerals in soils on land began to be oxidized (rusted) during
weathering.! Soils turned red.!
The atmosphere must have contained O2 for this to occur.
Red-bed Blouberg Formation
Subvertical pebbly sandstones of the red-bed Blouberg Formation, about 1900 million
years old, overlain by horizontally-layered Waterberg sandstones, near Glen Alpine Dam,
Northern Province.
The appearance of red beds, characterized by red iron oxide minerals, in the geological
record marks the first appearance of significant quantities of oxygen in the Earth's
atmosphere.
Older rocks rarely show the effects of oxidation that produce the tell-tale reddish or
ochreous colours that accompany weathering.
An Ozone Shield
One important environmental effect of higher O2 levels: an Ozone Shield
With O2 levels in the1% range, the stratosphere would begin to develop an effect ozone
(O3) layer. Stratospheric ozone absorbs ultraviolet radiation.!
This ozone shield is not especially important to aquatic organisms that are protected by
water.!
For living things to colonize land an ozone shield is essential or the organisms would get
cooked by UV radiation.