Chapter Eight – Primitive Life in the Solar System
Tools for the Study of Ancient Environments
8.1 Ancient Environments are revealed by Sedimentary Rocks
8.2 Stratigraphy reveals how Ancient Environments changed with Time
8.3 Internal Heat produced Intense Geologic Activity on Archean Earth
Evidence for Archean Life on Earth and Mars
8.4 The Origin of Proto-continents is revealed by Archean Rocks
8.5 Archean Rocks have Geochemical Properties that may be the result of Early Life
8.6 Fossil Evidence for Life emerges in Archean Rocks
8.7 A Meteorite from Mars contains possible evidence for Martian Life during the Archean Eon
Life in the Archean
8.8 Prokaryotic Cells provide clues to the nature of Archean Life
8.9 Genomes of Modern Organisms suggest evolutionary pathways of the distant Past
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Tools for the Study of Ancient Environments
We have no record of Hadean times from terrestrial, but evidence suggests that life on
Earth got started during that period. Clues in some of the oldest rocks from the Archean Eon
suggest that life was already abundant by that time and may have started much earlier. To
evaluate the evidence for ancient life on Earth, including that from Archean times, we will
expand our investigative tool kit.
The geological record of life makes use mainly of life’s signature in sedimentary rocks.
We briefly encountered these in Chapter 6 as hosts to ancient zircons. As is typical of Archean
rocks, the zircon host-rocks were metamorphosed. The older a rock is, the more likely that it has
experienced burial and uplift. These processes subject the rock to alteration by heat and pressure.
Metasedimentary rocks are sedimentary rocks that have been changed in this way.
These rocks contain clues to the nature of the environment at the time of their formation and may
also show evidence of life. But these signatures are altered by metamorphic influences, so we
have to understand both the sedimentary and metamorphic processes at work to decipher the
story that metasedimentary rocks have to tell.
8.1 Ancient environments are revealed by Sedimentary Rocks
Sedimentary rocks are classified according to basic properties that are a function of how
and where they accumulated. Sediments begin when tiny pieces of rocks and minerals,
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sometimes called clasts, are broken off from the exposed surfaces of existing rocks of any type.
This process of erosion is instigated by water, wind, and temperature changes, so it is often
referred to as weathering. Clasts are then transported by water, wind, landslides, or glaciers as
they find their way to lower elevations under the influence of gravity. Their final destination is
referred to as the environment of deposition and has an important influence on the properties of
the rock that eventually forms.
As sediment accumulates, it is buried underneath increasing loads and subjected to heat
and pressure. These compact the loose sediments into a hard rock in a process called
lithification. Chemical processes may aid the formation of cement between grains that helps in
the lithification process. Diagenesis refers to the combined physical, chemical, and sometimes
biological changes that result in the formation of a sedimentary rock.
Figure 1 – Sedimentary rocks form from different kinds of sediments. Clastic sedimentary rocks
are classified according to the size of their constituent grains. A mud made of tiny particles, for
example, will become a shale or mudstone when buried and turned to stone. Coarser sand grains
may become a sandstone, and pebbly gravels a conglomerate.
The properties of transported sedimentary grains or clasts provide clues to both their
rocks of origin and their method of transport. Swiftly moving water, for example, may carry
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relatively large clasts. Glaciers may transport huge boulders and drop them far from their point
of origin. In contrast, only tiny particles may be carried by a slow-moving stream or river.
Consequently, one of the first things we may notice about clastic sedimentary rocks is the size of
the grains.
Figure 1 shows an example of sediments with different grain sizes and rock types they
yield. Mudstones or shales, for example, are made up mainly of tiny particles that are smaller
than a tenth of a millimeter. Sandstones have grains that are about a millimeter in size. Since
fast-moving fluids carry larger particles than slow ones do, these two different kinds of rocks
form in different environments. Sand particles may drop out of suspension in high-velocity water
or wind as these slow down and lose energy. Slow-moving water may continue to carry mudsized particles until it reaches even more quiescent environments like a still lake or pond.
Figure 2 – Levels of sorting in clastic sedimentary rocks. Rocks with good sorting have grains size
distributions like those in the illustration at left. Those with poor sorting have a wider range of
particle sizes like the panel to the far right.
If transporting fluids change velocity with some regularity, they will deposit grains of a
particular size in one location. This will lead to a sedimentary rock in which all the grains are
similar in size, like the particles shown in the left panel of Figure 2. Fluids that carry grains with
a wide range of sizes and then suddenly deposit them all at once will yield rocks with a size
distribution like that shown in the right panel of Figure 2. Sedimentary petrologists have
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quantified this property and refer to it as sorting. Values of the degree of sorting may be labeled
as good, moderate, or poor.
Sedimentary rocks that are classified according to the size and sorting of individual
grains are often referred to as clastic sedimentary rocks. These make up the vast majority of
rocks in the sedimentary record. Clasts that are transported in water may undergo chemical
changes, and many of the elements in their constituent minerals may become dissolved in the
water. If conditions are right, these elements may recrystallize to form new minerals on the floor
of marine or lake environments. The resulting chemical sedimentary rocks are signatures of a
different environment than the one in which clastic sedimentary rocks form.
Carbonate rocks are chemical sedimentary rocks that form when dissolved calcium ions
react with CO2 in water to form calcium carbonate (CaCO3). These reactions involve
intermediate steps in which ions of bicarbonate (HCO3) are formed. The calcium carbonate often
precipitates on the sea floor together with some sediment. In modern oceans, the detritus of
shells from small marine creatures frequently accompanies the calcium carbonate. Sedimentary
rocks formed this way are called limestone and make up about 10% of all sedimentary rocks.
Limestone is easily dissolved again upon exposure to the elements. As a consequence, it is often
the setting for beautiful caves and host to weird topographical features like those shown in
Figure 3a).
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a)
b)
Figure 3 a) Limestone deposits in China have eroded to form eerie shapes. b) Evaporite
deposits were deposited on the floor of a dry lake bed in the Mojave Desert
Dissolved elements and minerals may recrystallize and precipitate out of water when
their abundance becomes too large for them to remain in solution. This situation occurs quite
readily in shallow warm ponds or lagoons where the water is evaporating rapidly and leaves
behind an increasingly concentrated broth of salts and dissolved elements. Minerals and rocks
that form as water is lost in this way are often referred to as evaporites. Evaporite minerals
include common table salt (otherwise known as halite - NaCl), gypsum (CaSO4-2H2O) , calcite
(CaCO3), and other mineral salts. They often precipitate in dry lake beds in desert environments
like the one shown in Figure 3b).
a)
b)
Figure 4 - a) Modern mudcracks occur in mud that has recently dried in the hot sun. b) Ancient
mudcracks were formed in the same way and are preserved in this billion-year-old rock from
Montana.
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Sedimentary structures are physical properties in rock units that are visible on larger
scales than constituent grains. These structures often reveal important information about the
origin of the original sediments. You may have seen ripples in sand or mud at the beach or in a
stream, for example. Or perhaps you’ve seen large cracks in mud that has dried in the hot sun,
like those in Figure 4a). Features like these are often preserved in the final rock and can be used
to infer the environment in which the sediments originally accumulated.
Figure 5 - Examples of Depositional Environments. Sediment tends to be transported from regions
of high elevation to lower elevations but can be deposited at many steps along the way. Glaciers,
lakes, rivers, beaches, desert dune fields, deltas, and off-shore marine environments are all
examples of places where distinctive sediment can accumulate and eventually be buried to form
sedimentary rocks.
The particular conditions and surroundings of sediment accumulation are referred to as
environments of deposition. Examples of different modern depositional environments are
illustrated in Figure 4. These are the main focus of study for many sedimentary geologists who
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study sediment in modern environments to interpret the properties of sedimentary rocks formed
in similar ancient settings. Deposits that are currently accumulating in locations such as desert
dunes, river deltas, lakes, glaciers, or continental shelves have characteristic grain-size
distributions, chemical compositions, and sedimentary structures that are unique to those
settings. A distinctive rock unit that forms in a particular depositional environment is called a
facies.
Geologic mapping of a particular rock facies reveals the regional extent of a past ancient
environment. But when did this setting exist? The sedimentary-rock record gives a detailed order
of how geological events are likely to have happened and how they relate. Where this record is
punctuated by igneous rock intrusions, radioisotope dates can be calculated. These dated rocks
are treated as markers in the record from which the ages of the intervening sedimentary rocks can
be inferred. The principles that establish the relative sequence of events are embodied in another
geologic specialty: stratigraphy.
8.2 Stratigraphy reveals how Ancient Environments changed with Time
The rhythmic deposition of sediment over long periods of time leads to a sedimentary
structure that is common in the rock record. Layers of rock, sometimes called strata (singular
stratum), are found throughout. Each layer represents a particular episode of deposition. As
illustrated in Figure 5, strata are easily seen in striking erosional features of the canyons of the
Colorado Plateau, including the Grand Canyon. Color and hardness variations reveal different
strata from a distance and horizontal bands with different degrees of slope. None of the rocks in
the Grand Canyon are as old as the Archean, but they illustrate principles that we will use to
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chronicle the history of older rocks. Detailed interpretation of the vertical and regional
relationships of strata like these is the focus of the science of stratigraphy.
a)
b)
Figure 5 – Sedimentary rock layers, or strata, are exposed in canyons of the Colorado Plateau,
including the Grand Canyon, Zion Canyon, and Bryce Canyon. Layer formations are identified in
geologic sections illustrated for each location. The formations are laterally continuous as illustrated
in the geologic cross-section at the bottom of the figure. [Note – Supai “fm” is now the Supai Group
(w/4 formations). Wasatch Formation is now called the Claron Formation].
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Strata vary widely in thickness and geographical extent. The fundamental geologic unit of
stratigraphy is the formation. A formation includes rock layers with similar or identical rock
types, facies, or other properties. The main formations of the Grand Canyon, Zion Canyon, and
Bryce Canyon are depicted in Figure 5 as a geologic section. Formations may contain smaller
subunits called members and still smaller units referred to as beds. Related formations may be
combined to form a larger group, and larger groups may be classified as a supergroup. Criteria
for classification of strata as one of these units often hinges on the usefulness of recognizing the
units in the field and placing them on geologic maps of their surface exposure.
Sedimentary layers of rock like those exposed in Figure 5 are analyzed by stratigraphers
with a handful of basic principles, some of which were articulated by Nicolas Steno in the 17th
century. These Laws of Stratigraphy illustrate an important point made in our discussion of
scientific laws in Chapter 1. They are generalizations that work for a well-defined set of
circumstances, but not in all situations. For instance, the Law of Original Horizontality holds
that most sedimentary layers were originally laid down horizontally. Exceptions may be
landslide deposits on a mountain slope or debris slides off the undersea edge of a continental
shelf.
When we look at the layers in Figure 5, it appears that most are still parallel to the horizon.
In fact, the canyons are exposed because layers were uplifted and warped to some degree. The
Law of Original Horizontality implies that this warping was not part of the depositional process
but occurred at a later time.
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a)
b)
Figure 6 – a) The Law of Superposition says that the layers are ordered in a time sequence from
bottom to top, with the oldest Layer 1 at the bottom and youngest Layer 4 at the top. The Law of
Original Horizontality maintains that gently folded layers illustrated here were originally laid down
horizontally. b) The Principle of Lateral Continuity says that the Layers 3 and 4 were originally
deposited as horizontally continuous layers and that the „valley” separating them was eroded at a
later time.
The Law of Superposition is the key to orienting the events of the rock record. It says that
sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the
youngest on the top. This is a simple consequence of the fact that, to put “something” on top of
“something else,” you have to put the “something else” down first! It is illustrated in Figure 6
where layers are numbered from bottom to top as 1 through 4 and are labeled as a time sequence
with layer 1 being the oldest and layer 4 the youngest.
Steno’s Law of Lateral Continuity says that sedimentary layers initially extended laterally
(“to the side”) in all directions. It points out that very similar rocks separated by a valley or
erosional feature were at one time connected. In fact, sedimentary layers do not extend
indefinitely, but have recognizable limits that were determined by the amount and type of
sediment available and the size and shape of the basin in which they came to rest. In Figure 6,
layers 3 and 4 are bisected by an eroded valley. The Principle of Lateral Continuity says that
these layers were originally continuous across the valley. You can also see this principle at work
in the geological cross-section at the bottom of Figure 5. Layers exposed at the top of the Grand
Canyon, for example, are exposed in Zion Canyon as well. The cross-section displays the lateral
continuity between layers at the locations of Grand Canyon, Zion Canyon, and Bryce Canyon.
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The explanation for an exception to the Principle of Lateral Continuity should be evident
from Figures 5 and 6. Depositional environments change laterally and will result in gradual and
continuous facies changes over a wide extent. For instance, coarse-graded sandstone from a
high-energy transport environment may change continuously with lateral distance to a finegraded mudstone that settled out where waters slowed.
Figure 7 –Walther‟s Law illustrated. Lateral changes in facies can be identified at the surface of the
sediments. From left to right are carbonates from a deep-water environment, an offshore clay, a
nearshor silt and beach sand. As the shoreline moved inward, vertical facies changed in the same
order.
Changes in lateral facies are limited to those wrought by adjacent depositional
environments. Deep marine deposits may grade into shallower off-shore deposits but not into
wind-blown sand dune deposits. Because sea levels may rise or fall with time, the kind of lateral
change may also occur with time and result in a gradual facies change between rocks above and
rocks below. This relationship is illustrated in Figure 7 and is known as Walther‟s Law: In a
continuously deposited sequence of sedimentary rock, only facies that occur next to one another
laterally can appear vertically adjacent in the rock record.
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a)
b)
c)
Figure 8 - a)The Principle of Cross-Cutting Relationships says that a geologic feature that
cuts across another is younger than the material it cuts across. Here, material labeled as 4
cuts across layers 1, 2, and 3 and is younger than those layers. It is overlain by layer 5,
however, which is younger still by the principle of superposition. b) An example of a
volcanic intrusion that cuts across sedimentary rock in the Grand Canyon. The volcanic
material is younger than the sediment it cuts through. c) An earthquake fault has left a
crack in sedimentary layers that are offset by the quake. The quake event was later than
the deposition of the layers crosscut by the large crack.
Another stratigraphic principle makes it possible to attach an absolute timescale to the
relative ordering of events in sedimentary rock. The Principle of Cross-cutting Relationships
was put forward at the turn of the18th to the 19th century by James Hutton and championed by
Charles Lyell. Simply put, it says that any geological feature that cuts across another is younger
than the feature it has intersected. Examples are illustrated in Figure 8 and include structural
cracks, earthquake faults, or intrusions of magma. In the latter case, the igneous rock can be
dated with radioisotope methods and used to put limits on the ages of the adjacent material.
The Laws of Stratigraphy and many more detailed principles are critical tools with which to
unravel the historical sequence of events for ancient Earth. We will see, for example, that
Archean rocks lie beneath rocks from every subsequent period whenever these have not been
eroded away. Radioisotope dating of cross-cutting igneous materials helps us place the timing of
the Archean Eon before 2.5 billion years ago and stretching back to the age of Earth’s oldest
known rocks at about 4 billion years.
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8.3 A Dynamic Earth left its signature on Archean Rocks
Earthquakes and volcanoes are a direct consequence of heat that flows from Earth’s interior
to its surface. Today, most of that heat is generated by the decay of radioactive elements. During
Archean times, parent radioisotopes had not had much time to decay and were more abundant.
Heat was also contributed from frequent meteor impacts and the formation of an inner core.
These combined sources yielded a heat flow at the beginning of the Archean that was three times
larger than today. As a consequence, Archean Earth was far more geologically active.
The geologic activity on the surface of a planet or moon is determined not only by the
amount of heat released but by the process that governs the release of its internal heat.
Convection is an important mechanism for the release of Earth’s heat, just as it is for the interiors
of dwarf stars. Although the interior of planet Earth is not in a gaseous state, hot mantle material
in Earth’s interior expands as it is heated and becomes buoyant and pliable. It is able to rise
toward the surface where its heat is radiated away. On today’s Earth, this action drives the
motion of cool hard shells on the surface of Earth’s crust called tectonic plates. Their boundaries
are often sites of earthquake and volcanic activity as shown in Figure 9.
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Figure 9 - The Earth's surface is covered by tectonic plates that are set in motion by convective
blobs of rising mantle material. Their boundaries are marked by the presence of earthquakes and
volcanoes. The epicenters of large earthquakes are shown above as yellow dots. Volcanoes are
shown in red. Both are aligned with tectonic plate boundaries shown in blue.
There is plentiful evidence for volcanic activity in the Archean, but it is not clear when plate
tectonics started. The driving energy of plate tectonics is most evident at divergent plate
boundaries like the one illustrated in Figure 10. As mantle material rises, it pushes tectonic
plates apart and adds new material to the crust by spreading at the center of the boundary.
Volcanism and earthquake activity are typically present. Divergent plate boundaries that lie
under the ocean are referred to as undersea ridges. A prominent undersea ridge was discovered
when the first mapping studies of the Atlantic Ocean floor were carried out. It is visible in
Figures 9 and 10 as the plate boundary between North America and Europe, and between South
America and Africa.
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Figure 10 – Tectonic plates move apart from one another at a divergent plate boundary. New
material is added to the crust from the convecting mantle beneath. Beneath oceans, divergent plate
boundaries form undersea ridges.
The scientific theory of plate tectonics was debated by geologists in the first half of the
20th century, but it then became fully established by observations of the magnetic properties of
seafloor rocks near spreading centers. When basaltic rocks cool and solidify from molten lava,
they retain an imprint of natural remanent magnetization due to the alignment of their ironbearing minerals with the Earth’s magnetic field. The remanent magnetism of ancient rocks can
be observed today and used to infer properties of Earth’s magnetic field from millions of years
ago.
Observations like these are fundamental to the field of paleomagnetism. Earth’s magnetic
field is approximately a dipole with a north and south magnetic pole as illustrated in Figure 11.
The polarity of this field is determined by which magnetic pole is north and which is south.
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Figure 11 –The approximate configuration of Earth‟s magnetic field is shown as a dipole magnet
with a north and south magnetic pole that is alternately aligned (left) and then oppositely aligned
(right) with Earth‟s rotational poles.
Paleomagnetic observations of seafloor rocks show that the polarity of Earth’s magnetic field
completely changes direction on time scales that range from many thousands to millions of
years. Regions of seafloor that formed at a particular time show the same polarity. As shown in
Figure 12, these regions are arranged as a system of large stripes parallel to the divergent plate
boundaries from which they originated. Radioisotope ages confirm that these stripes are
progressively older farther from the spreading center. This system of stripes with alternating
polarity is thus a distinctive signature of plate tectonics.
a)
b)
Figure 12 – a) Material formed at an undersea ridge bears the magnetic imprint of the magnetic
field polarity. As the material cools and moves away from the ridge, lava on both sides that was
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formed at the same time will have the same geomagnetic signature. b) The resulting polarity of
remanent magnetism at appears as a system of stripes parallel to the undersea ridge as crust
solidifies on either side of the ridge and moves away from it.
Newly forming divergent boundaries may begin their lives on continents, where they form a
continental rift zone. As the continents spread apart from a rift zone, the newly formed heavy
basaltic material will subside to elevations below sea level and evolve to become an undersea
ridge. Seafloor material that has been extruded at undersea ridges will continue to move away
from the ridge as new material takes its place. The new material continues to get denser as it
cools and will eventually sink back into the mantle when it collides with another tectonic plate.
a)
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b)
c)
Figure 13 - Convergent Plate Boundaries a) Ocean-ocean boundary, b) Ocean-continent boundary,
c) Continent-continent boundary
A region where tectonic plates collide with each other is known as a convergent plate
boundary and is illustrated in Figure 13. The sinking edge of a plate of oceanic crust dives
underneath the edge of another plate at a subduction zone. As it heads back into the mantle, it
forms a large trench in front of the oncoming plate. The lowest elevations on the planet consist of
deep sea trenches depressed as subducting plates sink back into the mantle. The Marianas trench
depicted in Figure 13a) contains the lowest elevation on planet Earth today. Friction between the
two plates results in partial melting of plate material which then rises to form a chain of
volcanoes parallel to the trench. If both plates are composed of dense basaltic material, these
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volcanoes emerge on the overlying plate as an arc of volcanic islands shown in Figure 13a). If
one of the plates is carrying a continent, the volcanoes will emerge as a large mountain range
parallel to the convergent plate boundary. The Andes Mountains depicted in Figure 13b) are a
good example. For converging continental plates, material is pushed to higher and higher
elevations. The Himalayans are the highest mountains in the world. As illustrated in Figure 13c)
they are the results of converging continental plates.
Figure 14 – The San Andreas Fault in California is a good example of a transform plate boundary
where two plates are sliding past one another.
Transform plate boundaries occur where plates slide past one another without sinking
back into the mantle. These sites experience regular earthquake activity as the plates slip past one
another in brief episodes. The San Andreas Fault in California is a well-known example that
affects heavily populated urban areas and is depicted in Figure 14.
The partial melting of plate material to form volcanoes at convergent plate boundaries results
in chemical fractionation of the rock. Lighter material rises as magma to form islands that are
less dense than the sinking basalt. As these materials form sediments, they lose their heavier
minerals and become lighter and more felsic. The large continents of today are the result of
continued episodes of this kind of crustal fractionation as small regions of light crust, sometimes
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called terranes, collided with each other to form the stable cores of continents. Cores older than
a billion years have survived many cycles of merging and rifting and are called cratons. Their
central Archean portion accreted terranes in ancient collisions between plates. Continental rocks
were continually added in this way to make the continents we see today.
Uplifted rock is eroded and deposited where it may become sedimentary rock. These rocks
may, in turn, be cycled through regions of heat and pressure at tectonic boundaries. The resulting
metamorphic rocks will have distinctive minerals that betray the temperature and pressure
conditions they have endured. By studying these characteristics together with the conditions
associated with particular tectonic environments, geologists can discover the tectonic
environments of the past. This cycle of uplift, sedimentary transport, burial, and tectonic uplift
provides a big-picture setting for geology that is depicted in Figure 15.
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Figure 15 – The cycle of erosion and tectonic activity sometimes referred to as the “Rock Cycle.”
Felsic sedimentary rocks form as the result of erosion, transport, and burial of uplifted continental
rock. This material may experience metamorphosis at subduction plate boundaries and adjacent to
associated volcanism. Complete melting may also occur, in which case the sediments are recycled as
more silica-rich igneous rock. Mafic igneous rock is formed as seafloor spreading materials from
mantle material.
The cycle of crustal materials shown in Figure 15 explains many properties of Earth’s
crustal rocks. For example, we find no oceanic crust older than about 250 million years even
though Earth is 18 times older! Their absence is explained as the result of the disappearance of
oceanic crust at subduction zones. In addition, the amount of rock that we can find to represent a
period of time decreases rapidly as we look at older and older rocks. This rarity of oldest rocks is
due to the fact that there is a greater chance for destruction and recycling of older rock. There is
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also a greater chance that the oldest rocks have endured episodes of metamorphism, making it
difficult for traces of life in them to endure. All these factors come to bear on our reconstruction
of key event for life during Archean times.
Evidence for Archean Life on Earth and Mars
8.4 The Origin of Protocontinents is revealed by Archean Rocks
Archean rock environments are found embedded in the cratonic foundations of the
world’s continents. Their locations are mapped in Figure 16. They are sometimes referred to as
shields where they are exposed at the surface and platforms where they lie at the bottom of
young sedimentary rock layers.
Figure 16 – Locations for Archean Rocks are shown in dark brown. Most of these underlie
sedimentary rock layers as “platforms”. A few are exposed on the surface as “shield” outcrops.
Most of these rocks were formed in the late Archean (2-5-3.5 billion years ago). Archean rocks
older than 3.5 billion years make up a minority of the regions mapped above [NEED A FIGURE
WHICH DISTINGUISHES OLD AND YOUNG ARCHEAN ROCKS].
The rocks of the Archean typically show varying degrees of metamorphism. We first
encountered this idea in Chapter 6 with a discussion of meteorites and the concept of
metamorphic grade for Earth rocks. The degree to which meteorites had been altered was an
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important indicator of the formation environment, whether deep within a parent body, on its
surface, or somewhere in between. Similarly, the metamorphic grade of earth rocks depends on
constituent minerals and indicates the temperature and pressure of the locale in which they were
altered. Figures 11 and 12 of Chapter 6 showed examples of metamorphic rocks organized
according to the amounts of heat and pressure they have endured.
Archean cratons are formed mainly of metamorphic assemblages that are uniquely
associated with early tectonic environments. The composition of Archean Greenstone belts is
graphically illustrated in Figure 17. These complexes are made up largely of variably
metamorphosed volcanic rocks with associated sedimentary rocks that have endured only lowgrade metamorphism. They typically lie adjacent to granitic rocks that resemble those formed in
the unextruded remains of magma chambers below island arc volcanoes. The sediments are like
those found today in oceanic trenches above subduction zones.
a)
b)
Figure 17 – a) Origin of Greenstone Belts. Convergent oceanic plates subject sediments weathered
off their island-arc volcanoes to low-grade metamorphism. These are adjacent to metamorphosed
volcanic rocks and granites associated with the magma chambers beneath the island arc volcanoes.
b) Greenstone Belts after repeated subduction events. Granites and volcanic materials are lighter
than the oceanic crust and remain after the latter has descended into the mantle. These are
separated by bands of metamorphosed sediments. [“B” from the left figure has the same info a “b”,
but I like the “b” presentation better. Left figure is from Earth System History, right from Understanding
Earth]
Archean greenstone belts are believed to represent early subduction zones associated with
the formation of Earth’s first proto-continents. With its increased heat flow, the Archean Eon
saw a greater subduction rate than today, one that was probably associated with many smaller
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plates undergoing more rapid tectonic drift. It is within the lightly metamorphosed sediments of
Archean greenbelts that we can look for the earliest geological evidence for life on Earth.
The Isua Greenstone Belt in southwestern Greenland dates from 3.8 to 3.7 billion years
ago. Most of the belt is composed of basaltic pillow lavas like those shown in Figure 18. These
rocks take their name from the pillow-like shapes that lava naturally takes on when it erupts
underwater. The pillow lavas of Isua are intruded by a kind of rock that no longer forms today.
Banded Iron Formations (BIF) are shown in Figure 18c) adjacent to pillow lavas.
a)
b)
c)
Figure 18 – a) Pillow lavas from recent undersea volcanic eruptions. b) Ancient pillow lavas
unearthed in the Isua Greenstone Belt. c) Light reddish-colored Band Iron Formations (BIF)
adjacent to dark basaltic pillow lavas in the Isua Greenstone Belt.
Banded Iron Formations are made of iron oxide minerals in thin layers that alternate with
iron-poor layers of sedimentary shale and chert. The iron oxide is present as either magnetite
(Fe3O4) or hematite (Fe2O3) and imparts a reddish hue to the beds. These minerals are
sufficiently abundant to render BIFs an important source of commercially mined iron ore. Chert
is a chemical sedimentary rock composed of microcrystalline quartz that is highly resistant to
erosion and serves as a unique preservative of microfossils.
The layering of BIFs indicates alternating processes in which oxygen was intermittently
available to combine with iron dissolved in the ancient ocean. Magnetite and hematite are not
soluble in water, so they would have immediately been deposited on the sea floor. BIFs are
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found in rocks as old as 3.7 billion years and were quite abundant by 2.4 billion years ago. They
are rarely found in rocks younger than 1.8 billion years. The total amount of oxygen in BIF rocks
has been estimated to exceed the volume of oxygen in the modern atmosphere by a factor of
twenty.
In summary, the picture of Archean Earth given by its remaining rocks is one in which
small proto-continents were merging under the influence of plate tectonics. Oxygen was
periodically available to combine with iron and magnesium in shallow seas. We next turn to
examine what kinds of life evolved in this environment. Before we do, however, it is worth
asking how unique this environment was. We have already seen, for example, that early Mars
had abundant water. But did it have a similar tectonic environment? Recent evidence from Mars
Global Survey indicates that the answer is yes!
Figure 19 - A map of Mars obtained with the magnetometer onboard Mars Global Surveyor. The
map is made up from many thousands of orbits and uses colors to represent the strength and
direction of the field caused by crustal magnetization. The data show a "striping" of the magnetic
that resembles stripes adjacent to undersea ridges on Earth. The map also shows evidence of
transform faults that are a signature of plate tectonics on Earth."
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Figure 19 displays a map of Mars obtained with the Mars Global Surveyor. A
magnetometer was used to map the magnetic field in Martian rocks. The results for field strength
and polarity are color-coded in Figure 19. They reveal a telltale striping very like that seen for
oceanic crustal rocks in Figure 12. Mars appears to have had active plate tectonics early in its
history, even though it does no longer.
We may well ask why plate tectonics is no longer active on Mars. The answer most
probably lies with its size. Smaller planets lose their internal heat more quickly, and it appears
that Mars has lost sufficient heat to drive plate tectonics and maintain a protective magnetic field.
Geologists estimate that Earth will follow suit on timescales of hundreds of millions of years.
Magnetic evidence for plate tectonics on early Mars includes the implication that Mars once had
a protective magnetic field. Add to this the evidence for liquid water on early Mars, and we
begin to see that the planet was once a far more habitable environment.
8.5 Archean Rocks have Geochemical Properties that may be the result of
Early Life
Since BIFs are not forming today, their origin presents a puzzle. Oxygen is present today,
but the oceans are now depleted in iron. Earth’s ancient oceans probably had a higher abundance
of iron. Oxygen, however, was probably not present in the early atmosphere in sufficient
quantities. It is a highly reactive element and would not likely have been available to make BIFs
unless it was replenished somehow. But how? The oxygen in our atmosphere today is provided
almost entirely by photosynthetic organisms. We have seen in Chapter 5 that oxygen in an
exoplanet atmosphere might signal the presence of life on that planet. Many scientists have
concluded that the oxygen in BIF iron oxides came from ancient photosynthesizing organisms.
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The presence of a chemical substance that is due entirely to the activity of living
organisms is sometimes referred to as a biosignature. As we will see, considerable scientific
debate surrounds the question as to whether or not a chemical signature is truly the result of
biological activity or results from an unidentified inorganic process. Oxygen in today’s
atmosphere is produced by photosynthesizing bacteria and plants as outlined in Box 8.1. Earliest
life had not developed photosynthesis, however, and the earliest photosynthetic systems did not
produce oxygen. Debate continues as to whether or not living organisms are responsible for the
oxidizing of iron minerals in Isua BIFs dating to the early Archean.
Box 8.1 The Chemistry of Photosynthesis
Oxygenic photosynthesis is a chemical process that uses light energy to synthesize sugars
from CO2 and H2O. The net chemical equation for what is a set of reactions is given as:
Light Energy + 6H2O + 6CO2
6O2 + C6H12O6.
Water molecules are split in the process, resulting in the extraction of electrons and the release of
free molecular oxygen into the atmosphere.
In bacteria, photons are captured by light-harvesting complexes containing molecules of
the green pigments chlorophyll a and chlorophyll b. The complexes are referred to as
photosystems and use the energy to split a molecule that provides electrons ,which are then
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passed along in a chain of reactions that result in the storage of chemical energy. Photosystem I
is a such a complex and is shown in Figure 20. It operates together with Photosystem II in
oxygenic photosynthesis. Photosynthetic bacteria that cannot produce oxygen possess a single
photosystem and may use pigments other than chlorophyll. The absorption response of
chlorophylls is shown in Figure 20b and explains why chlorophyll lends a greenish color to
plants and algae.
a)
b)
Figure 20 – a) Light-harvesting protein complex found in bacteria. It contains many subunits,
including the pigments chlorophyll a and b. b) The absorbing efficiency of the chlorophyll a and b
as a function of wavelength. It absorbs green light the least, so the human eye detects it as a green
pigment.
Early photosynthesizing organisms avoided the toxic side effect of oxygen’s high
reactivity by relying on a different source of protons and electrons. For example, the reaction
6CO2 + 6H2S (+ light energy)
C6H12O6 + 12S
yields the far less toxic and more easily disposed elemental sulfur. Some modern bacteria use a
single photosystem to obtain electrons and protons from sulfides (H2S or FeS) instead of water.
Molecular comparisons strongly indicate that the Photosystem I used in oxygenic photosynthesis
evolved from a precursor system like this one.
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The biological origin of rising oxygen levels as indicated by late Archean BIFs would be
a more convincing hypothesis if independent biosignatures were found in rocks from the same
time period. The earliest evidence for which claims have been made is in the form of Carbon
isotopes from the Isua Greenstone Belt.
Isotopic forms of the same element are chemically alike, but there are processes that can
lead to fractionation of their relative abundances. For example, CO2 made with 13C is a bit
heavier than that made with the more common 12C. Carbon-13 moves more slowly and is less
likely to be taken up by photosynthetic systems. Consequently, living organisms generally have a
higher 12C/13C ratio than is found in nature. Photosynthetic carbon in the upper ocean, for
example, shows just such a preference for the lighter isotope of carbon. Rocks in the Isua
Greenstone Belt contain carbon that is enriched in 12C as would be expected if the carbon came
from biological sources. Similar examples of possible biosignatures exist in both the Carbon and
Sulfur isotopes of all the major Archean Greenstone Belts.
8.6 Fossil Evidence for Life emerges in Archean Rocks
Fossils provide strong evidence for ancient life on Earth by preserving direct remnants or
traces of past organisms. Paleontology encompasses the study of organisms with time and is
assisted the study of how fossilization occurs,. From such studies, we know that fossilization is a
rare event. Most organisms decay and leave no trace by the time sediments have hardened into
rock. Complex, multicellular organisms with hard body parts like shell and bone are easiest to
preserve, because these structures lend themselves to replacement by hard minerals. Softer and
simpler organisms do not preserve nearly so well. This is especially true for microfossils of the
earliest bacterial cells. Add to this the billions of years of erosion and metamorphism undergone
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by Archean rock, and it becomes apparent that fossil representation of the earliest life should be
scant.
The earliest possible evidence for fossil bacteria comes from cherts in Australian Archean
rocks. As we mentioned earlier, cherts are sedimentary rocks that accompany Greenstone belts
and are especially resistant to erosion and metamorphic processes. The black cherts of the 3.5billion-year-old Warrawoona Group in Western Australia contain filaments of cell-like units
shown in Figure 21. These have been interpreted as the earliest microfossil evidence of life, but
neither non-biologic means of formation nor contamination at a later date has been wholly ruled
out. The first characteristic, biological origin, is sometimes referred to as biogenicity. If the
alleged micro-fossils were not produced by organisms, they are not biogenic and tell us nothing
about Archean life. The second criterion, lack of contamination, requires that the features were
made at the same time as the rock, a condition that is termed syngenicity. Bacteria that entered
the rock much later, even if it was millions of years ago, could have produced fossils that are not
datable and tell us nothing certain about Archean life.
a)
b)
Figure 21 – a) Microfossils from black cherts of the Archean Warrawoona Group. The rocks date
to 3.47 Billion years ago, so these features may represent early Archean life. Their origin is still the
subject of debate. b) Microfossils from the 2.55 billion year old Transvaal Supergroup in Africa.
Dividing and filamentous cells are clearly of biological origin and syngenetic with their late
Archean sediments.
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As we proceed forward in time in the rock record, the microfossil evidence for life
expands and becomes more compelling. Archean microfossils from the Transvaal Supergroup in
South Africa date to the very end of the Archean Eon. These fossils are associated with cherty
carbonate rocks and show cell division and organization in filaments. They are also accompanied
by a mixture of organic chemical compounds known as kerogen. Taken together, these
properties make it clear that the microfossils are biogenic. Moreover, their syngenicity is
establish by the fact that they are randomly distributed but aligned with sedimentary grains in the
rock, proving that they were deposited with the original sediments.
The organization of bacteria into colonial filaments begins to look remarkably like a
multicellular life-form with greater potential for fossilization. This is especially true for large
colonies that grow in mounds of sediment. Stromatolites are layered sedimentary structures that
are produced by the growth and metabolism of microbes. As illustrated in Figure 22a), they are
still found today in rare localities like Shark Bay, Australia.
Fossil stromatolites are found in many rocks older than 2.5 billion years. In some cases,
disagreement persists about their biological origin. Paleontologists are strongly convinced,
however, that structures like the one in Figure 22b) are biological in origin. Stromatolites from
the 2.72-billion-year-old Tumbiana formation were deposited in carbonate sediments within
evaporating lakes and show internal details that establish their biological character.
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a)
b)
Figure 22 – a) Present-day stromatolites in Shark Bay, Australia. b) Fossilized stromatolites are
layered on top of each other after long periods of growth and burial.
Biosignatures and fossils attest that life was well established 2.5 billion years ago, by the
end of the Archean Eon. Less definitive evidence suggests that life began as much as a billion
years earlier, at the beginning of the Archean Eon. These facts suggest a Hadean origin for life.
As we’ve seen in Chapter 6, however, heavy impacts may have complicated this picture. Is it
possible life arose somewhere else and was transported to Earth? The planet Mars had a thick
atmosphere and ocean early on. It may have been more favorable to life’s origin in other respects
as well.
8.7 A Meteorite from Mars contains possible evidence for Martian Life
during the Archean Eon
The Allen Hills 84001 meteorite (ALH84001) was the first meteorite collected off the
Antarctic ice in the vicinity of Allen Hills in a 1984 expedition funded by the National Science
Foundation. Meteoriticists routinely collect in the Antarctic ice fields, because of the ease of
spotting rocks that came from above rather than from below. ALH84001 as shown in Figure 23
a) was archived in a collection for several years before analysis revealed that it was from Mars.
A number of Martian meteorites have been identified by isotope ratios trapped in gas bubbles.
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These ratios show unique values measured for Mars by the Viking Lander spacecraft. Similar
values have not been found for other meteorites or on other planets.
a)
b)
Figure 23 – a) Martian meteorite ALH84001 was collected from Antarctica in 1984 and is the target
of investigations by multiple scientific investigators. It contains possible evidence for early life on
Mars that has been hotly debated. b) The announcement of “possible” life on Mars was amplified
by media claims as indicated by this editorial cartoon.
ALH84001 gained worldwide attention when, in 1996, NASA announced that the
meteorite contained possible evidence for ancient life on Mars. A press conference was held to
present 4 main lines of evidence for ancient life in the meteorite. Although NASA scientists
conceded that each of these could be individually taken into question, they argued that the most
probable explanation for their combined appearance lay in a biological origin. This moderately
sober presentation was given a more dramatic interpretation in the media, as indicated by the
editorial cartoon shown in Figure 23b).
The story of ALH84001 begins with a narrative about its origin and how it managed to
travel from Mars to Earth. The rock itself has been dated using the Lu-Hf radioisotope system,
yielding an age of 4.091 ± 0.030 billion years. Dating indicates that it formed before the Late
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Heavy Bombardment in the Earth-Moon system, making it more than 2 billion years older than
any other Martian meteorite analyzed to date. As we discussed in Chapter 6, this period
corresponds to an epoch when Mars was warmer and wetter than today. Soon after its formation,
the rock was shocked and slightly metamorphosed. Carbonates were deposited in the cracks. RbSr dating of the carbonates yields an age of 3.90 ± 0.04 billion years, still within the time frame
for a warm, wet Mars.
ALH84001 remained on Mars until about 15-16 million years ago, when an impact
blasted it off the surface and into orbit around the Sun. During this time, it was exposed to
cosmic rays that left their cumulative imprint in its minerals. From this imprint, scientists have
estimated the length of time that the meteorite was in space. Its orbit brought it near enough to
Earth that it eventually landed here. Carbon-14 measurements were used to estimate that
ALH84001 has been on Earth for 13,000 years.
The Martian origin and subsequent history of ALH84001 are largely accepted by
scientists who study meteors. The dates of formation and impact metamorphism are more
uncertain than indicated in the above ages, however, since the metamorphism may have
interfered with some of the isotopic systems. Nevertheless, the picture of formation and
secondary mineralization by carbonates during an early warm, wet Mars is not in doubt.
NASA researchers at first identified four lines of evidence for ancient life in ALH
84001. These four included microfossils, chemical biosignatures in the carbonates, the presence
of organic material in the form of Polycyclic Aromatic Hydrocarbons (PAHs), and the detection
of chains of magnetite crystals the resemble those produced by terrestrial bacteria.
Our brief discussion of evidence for life in Archean rocks should alert you to possible
questions that could be raised by these lines of evidence. Were the microfossils actually of
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biological origin? Were they formed with rock or are they the result of contamination from
terrestrial bacteria? ALH84001 has endured well over a decade of intense scrutiny with questions
just like these. The biological origin of these features has not been disproven, but many scientists
are not yet persuaded.
a)
b)
Figure 24 – a) Alleged “microfossils” in ALH 84001. The segmented filament in the center of the
image resembles a chain of very small bacteria. b) Carbonate globules in ALH 84001 are the result
of secondary mineralization. They were likely deposited by water and contain side-by-side elements
that are out of equilibrium with each other.
NASA researchers used a scanning electron microscope to image structures like the one
shown in Figure 24a). This and similar structures in ALH 84001 ranged from 20 to 100
nanometers (nm, or –one-billionth of a meter) in size, much smaller than typical terrestrial
bacteria. It is also below a 200-nm limit for a viable bacterial cell that was hypothesized at the
time of the NASA announcement. This size range is smaller than some viruses and has continued
to be a source of criticism for the microfossil evidence. Evidence for terrestrial nanobacteria has
confirmed that structures this size do exist and may be implicated in some health issues. Recent
studies have failed to find DNA or RNA in them, however, so that some scientists have
concluded that they are non-living crystalline entities that multiply by inorganic means.
Consequently, many scientists still consider the microscopic evidence for microfossils in
ALH84001 to be inconclusive.
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Within the rims of the Carbonate globules shown in Figure 24b) are minerals that appear
to be out of equilibrium. Early estimates of past temperatures for the carbonates assumed the
carbonate minerals were in equilibrium and generated values for temperatures that were too high
to support life. More recent work, however, has confirmed that carbonates are in separate regions
or zones and yield lower estimates of past temperatures that do not rule out life.
a)
b)
Figure 25 - a) The double-ring structure of naphthalene, one of the simplest of the Polycyclic
Aromatic Hydrocarbons found in ALH84001. b) Upper figure – a chain of magnetite crystals
assembled by a modern bacterium. Lower figure: Magnetite crystals and chains of magnetite
crystals in the Martian meteorite ALH84001
A third line of evidence for life in ALH 84001 is in the form of PAHs found in the
carbonate globules. As shown in Figure 25a), PAHs are composed of multiple hydrocarbon
rings. PAHs form in the decomposition of bacteria, but they form in other ways as well. As we
saw in Chapter 4, they even form in space! The PAHs in ALH 84001 are unlike ones found in
meteorites, yet they represent a small fraction of those that result from the decomposition of
terrestrial life. Consequently, this line of evidence remains circumstantial but inconclusive
evidence for life on Mars.
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NASA researchers also found chains of microcrystals that resemble those assembled by
terrestrial bacteria. On earth, bacteria assemble chains of magnetite crystals that aid their
navigation with respect to Earth’s magnetic field. Recent studies of the magnetite crystals in
ALH 84001 are consistent with a biological origin and argue against an inorganic origin. Some
scientists still maintain, however, that an inorganic origin has not been strictly eliminated.
As we can see, no single line of evidence for life in ALH 84001 is thoroughly
convincing. What the original researchers argued, however, is that the most probable explanation
for the combined evidence is biological origin. Certainly, it is true that no alternative hypothesis
has been accepted, so that interpretation would seem appropriate. Some would argue, though,
that an unknown process is more likely to have been at work. At issue is whether or not one
thinks, a priori (before making observations), that life is likely or unlikely. For life’s skeptics,
even the most improbable inorganic scenario is more likely. Consequently, an interpretation of
ALH 84001 will probably not garner widespread agreement until we learn more about Mars.
Modern Analogues of Archean Life
In our discussion of ancient Archean life, we have repeatedly made comparisons with
modern primitive organisms. Modern organisms are highly evolved and unlikely to be identical
with their ancestors from billions of years ago. In addition, atmospheric conditions are far
different today from what they were during the Archean. However, evolution is not progressive
but adaptive, and remnant environments like Archean ones still exist in some places.
Consequently, features of ancient organisms that are adapted to environments like those in the
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Archean are likely to persist in similar environments for long periods of time. Moreover, the
genetic code of modern organisms bears the signature of past evolution.
8.8 Prokaryotic Cells provide clues to the nature of Archean Life
When we talk about Archean life, we typically think about the modern prokaryotic cell.
Bacteria are a good example of prokaryotes. A bacterial prokaryotic cell is certainly far removed
from the simplest protocellular life that we discussed in Chapter 7. However, it is likely to
resemble the various types of organisms that came to prominence in the Archean Eon and
eventually transformed its atmosphere.
Figure 26 - The Prokaryotic Cell. It contains a plasma membrane that encloses DNA, ribosomes,
and cytoplasm. Additional structures include a cell wall, hair-like projections called pili, and a
whip-like flagellum that helps in movement.
The term prokaryote comes from Greek for “before the kernel” and signifies that
prokaryotic cells do not have a nucleus. Instead, DNA resides openly in the middle of the cell.
All prokaryotic cells exist in single-cell form, so they are not visible to the unaided eye. Their
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basic components are illustrated in Figure 26 and include a plasma membrane that encloses a
jelly-like cytoplasm with RNA ribosomes scattered throughout and a resident genome that exists
as one or more circular or linear strands of DNA. Many prokaryotes also possess a more rigid
cell wall, a movable tail-like flagellum and hair-like projections known as pili.
As we discussed in Chapter 7, all living things must carry out catabolic metabolism to
release energy for homeostasis, growth, and reproduction. Prokaryotes use a diverse set of
catabolic pathways that make them suitable to various environments, some of which resemble
conditions in the early Archean. Photosynthesis discussed in Box 8.1 is one example of
catabolic metabolism in which light energy is used to move electrons from reduced to oxidized
forms of chlorophyll and generate energy-storing organic compounds from carbon dioxide.
Purple and green sulfur bacteria are photosynthesizing prokaryotes that use hydrogen sulfide
(H2S) as a source of electrons. Purple non-sulfur bacteria use molecular hydrogen (H2). These
microbes are imaged in Figure 26 and are restricted to habitats without oxygen. Therefore, they
live in environments that most resemble the early Archean.
a)
b)
c)
Figure 27 – a) Purple nonsulfur bacteria Rhodobacter ferrooxidans. The rod-shaped prokaryotes
are surrounded by crystalline iron hydroxide b) Chlorobium tepidum bacteria are green sulfur
prokaryotes that grow in hot springs like those found in Yellowstone National Park, and in other
sulfide-rich waters, mud, and sediments. c) Cyanobacteria like these are important contributors to
free Oxygen in the atmosphere.
Cyanobacteria like those pictured in Figure 27c) are the most abundant of the
prokaryotes that carry out oxygenic photosynthesis today. They are currently responsible for 20Chapter 8, devo pass
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30% of Earth’s photosynthetic activity, using water as a source of electrons and liberating
Oxygen in the process. Cyanobacteria are the primary residents of modern-day stromatolites.
They have evolved the ability to dispose of excess oxygen with an antioxidant enzyme called
superoxide dismutase. This enzyme packages dangerous elemental oxygen into more benign
molecules of O2 and delivers them outside the cell wall. As we will see in the next chapter, their
rise at the end of the Archean produced large changes in Earth’s environment that were
important to the evolution of multicellular life.
Figure 28 – Energy stored in chemical bonds in sugars is released in the process of aerobic
respiration and stored in ATP molecules.
Respiration is the process used by most non-photosynthesizing organisms in catabolic
metabolism. In aerobic respiration, illustrated in Figure 28, organisms use oxygen as an electron
acceptor to break down simple sugars and store the released energy in ATP. A lot of energy is
released in aerobic respiration, but it was obviously not of much use before oxygen was
available. The rise of oxygenic photosynthesis and aerobic respiration signaled the acceleration
of change in organisms at the end of the Archean. Earlier anaerobic respiring prokaryotes used
substances like carbon dioxide, sulfur compounds, iron, and nitrates in place of oxygen as
electron acceptors.
Methanogens use carbon dioxide and release water and methane according to the
equation:
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4H2 + CO2
CH4 + 2H2O.
These primitive respirers may have played an important role in keeping Earth warm during the
period of low solar luminosity, since methane is a very efficient greenhouse gas. They are
currently found in oxygen-deficient marshy wetlands as producers of marsh gas, as well as in the
guts of ruminants and humans. They lend a methane component to belching of ruminants and
flatulence in humans. Some methanogens live in environmental conditions that we would
consider extreme. The term extremophile is given to organisms like the methanogens that live in
hot springs, hydrothermal vents, and kilometers below the Earth’s surface in “solid” rock.
Figure 29 - Examples of extreme environments inhabited by prokaryotic life. Hyperthermophiles
live in extreme heat near undersea vents and in hot springs. Other extremophiles endure extreme
cold in ice-covered Antarctic lakes and the arid conditions of the Atacama Desert. The sub-ice
environment of Jupiter‟s moon, Europa may pose no greater difficulty than these environments.
Perhaps prokaryotes live there as well.
In recent times, prokaryotic extremophiles have been discovered living in many
environments that were formerly considered to be uninhabitable. A few of these environments
are illustrated in Figure 29. Thermophiles and hyperthermophiles are “heat-loving”
extremophiles that may live in temperatures above 120o centigrade where high pressures keep
water in the liquid state. Prokaryotes also occupy niches that are highly saline (halophile), acidic
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(acidophile), alkaline (alkaliphile), extremely cold (psychrophile), dry (xerophile), and
embedded within the pore spaces in rocks (lithophile). Deinococcus radiodurans, a radiotolerant
extremophile, has even been found thriving in nuclear reactors!
Over and over again, prokaryotes have revealed their hardiness and adaptability to
conditions previously thought hostile to any and all forms of life. Needless to say, these
properties have expanded our ideas about life on the early Hadean and Archean Earth. They have
also changed our ideas about where life might be hiding on other planets.
8.9 Genomes of Modern Organisms Map the Evolutionary Pathways of the
Past
We have seen that modern prokaryotes have adapted to a wide diversity of
environmental conditions. Some of these may be similar to the Archean environment. But
which ones? And is there an independent way to tell which modern prokaryotes are most
like the earliest forerunners of life? The genomes of modern organisms represent the
accumulation of billions of years of evolutionary steps. Hidden within the code is
information about which traits appeared earliest and which appeared recently. All we need
do is learn how to extract the information.
As we will see in a later chapter, biologists first classified organisms by their
observable physical traits. Characteristics like wings, fir, fins, or leaves were used to
classify animals and plants according to their common features. These groupings were
eventually understood within an evolutionary framework. Animals with similar traits were
interpreted as having descended from a more recent common ancestor than animals that
were dissimilar.
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Figure 30 – The ancestral history of life can be visualized as a tree. It reveals points of common
ancestry between organisms that evolved to become separate populations that no longer interbreed.
The relatedness of organisms is often portrayed in terms of a phylogeny. This
evolutionary history of individuals is often displayed as an evolutionary tree like the one
shown in Figure 30. It is similar to a genealogy or “family tree” with which you may be
familiar. The key difference, however, is that it shows the evolutionary history of
populations of organisms, rather than the history of individuals. Branching points of the
tree represent the last common ancestral population between two groups of organisms.
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Figure 31 –Ancestor-descendant relationships between populations of organisms can be inferred by
comparing their DNA.
The use of physical characteristics is not always a precise measure of relatedness for
several reasons. Similar traits may evolve independently and produce a false appearance
of relatedness. Certainly, physical traits are less useful for classifying prokaryotes that
seem to have relatively few identifiable characteristics to begin with! Our search would go
much better if we could directly compare the underlying genome of organisms. Evolution
proceeds largely by mutations in this genome, so a mathematical measure of the difference
between genomes should yield a direct and independent estimate of how distantly they are
related to a common ancestor. Fortunately, technological advances of the last few decades
have given us the ability to make these kinds of observations. By directly reading out the
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order of nucleotide bases in the DNA of different organisms, we can compare how closely
related they are. The underlying principles are illustrated in Figure 31 and Box 8.2.
Box 8.2 Calculating Genetic Distance
As we discussed in Chapter 7, DNA codes for the formation of life’s enzymes with the use of
nucleotide base pairs abbreviated, A, G, C, and T. RNA uses a similar sequence to read and
implement the code with A, G, C, and U. The sequence of nucleotides for an individual organism can
now be read out with the use of a DNA sequencer. Entire genomes have been sequenced for many
organisms, including human beings. Genomes can be compared and the number of differences
measured in a way that shows how many changes have occurred between the organisms since they
evolved from a common ancestor. These differences are quantified by using a measure of genetic
distance. This measure should give a value of zero for identical genomes and be largest for
genomes with nucleotide sequences that differ greatly. Comparison can be done using either RNA
or DNA sequences.
Consider two RNA sequences that differ by only a few bases:
I.
G C U A A U C AG U G C G U
II.
G C U A C U C AG U G C U U
A careful look reveals that these two sequences differ in only two places. Can you spot them? The
Hamming Distance between these strings is a mathematical construct that counts the number of positions
for which different bases occur. In this case, the Hamming Distance is 2. Consider now a third sequence:
III.
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G C U A U U C AG U G C G U
46
This sequence differs from sequence I by only one character, but it differs from sequence II by 2
characters. Therefore, the Hamming Distance for I-III is 1, and the distance for II-III is 2. The natural
interpretation of these distances is that organisms I and III are more closely related than are organisms II
and III. This information can be graphically illustrated in the form of a phylogenetic tree as follows:
II
I
III
Here, the length of segments is determined by the Hamming Distance, and we can easily see that
organisms I and III are more closely related to each other than to organism II. The tree implies that I and
III shared a common ancestor more recently than did I and II or III and II.
Molecular biologists use distance measures other than the Hamming Distance to compare
nucleotide sequences where deletions and insertions also occur and where sequences must be aligned in
an optimal way. The principle of distance is the same.
Genetic comparison between prokaryotes and more complex organisms has yielded
startling results. We now understand that bacteria are separated from a differing group of
prokaryotes that has been given the name Archaea. This group includes methanogens and
anaerobic non-photosynthesizing prokaryotes like those that could have endured conditions in
the early Archean Eon! The results are illustrated in the phylogenetic “Tree of Life” illustrated in
Figure 31. A third group, Eukarya, includes organisms like ourselves with cells that are more
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complex than those of prokaryotes. We will discuss the evolution of this group in the next
chapter. These three groups have been designated as a new taxonomic category: the domain.
Figure 32 –A phylogenetic tree for the three domains of life. Prokaryotes include both domains
Dacteria and Archea. Eukarya include animals like ourselves with large and complex cells that
evolved much later.
The phylogenetic Tree of Life in Figure 32 shows the relatedness of large categories
of organisms in a model where they have all descended from a common ancestor. With the
aid of reasonable assumptions about the rate at which nucleotides have mutated,
researchers can use genetic distances to get a rough estimate of the length of time that has
passed since the organisms descended from a common ancestor.
Using an estimate based on this kind of molecular clock, it is believed that the Last
Universal Common Ancestor (LUCA) of life thrived during the early Archean Eon, roughly
3.5 to 3.8 billion years ago. Modern organisms that lie near the root of this tree frequently
are thermophilic, suggesting that our last common ancestor thrived in a hot environment.
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In summary, geologic, paleontologic, and genetic evidence paints a picture of the
Archean as a time in which microbes eked out an existence on small continents, during
much geologic activity, in an oxygen-poor atmosphere, and under warm conditions. With
the advent of oxygenic photosynthesis at the end of the Archean, this picture was set to
change dramatically.
End of Chapter Materials
Key Words:
All terms listed in Bold would go here. Definitions could follow here as well or, as in “Universe”, page
references could be given for where they are discussed and/or for definitions supplied in an end-of-book
glossary
Key Ideas:
Depositional Environments – The environment in which sediments accumulated to form
sedimentary rocks is revealed by the properties of the sediments and gives a clue to early
conditions in the Archean
Stratigraphy – The study of sedimentary rock layers reveals the order of events in geologic
history. The Law of Original Horizontality holds that most sedimentary layers were originally
laid down horizontally. The Law of Superposition says that, in undisturbted strata, the youngest
layers are on top. The Law of Lateral Continuity says that sedimentary layers initially extended
laterally (“to the side”) in all directions.
Plate Tectonics – Geologic activity on earth drives the motion of tectonic plates. Archean rocks
show evidence of formation at subduction plate boundaries associated with volcanic island arcs.
Biosignatures – Chemical signs of life are found in Archean rocks. To be convincing, however,
these must be unambiguously of organic origin.
Microfossils and Stromatolites from Archean Times – The earliest possible examples of this
occur in the early Archean. Conclusive examples with indisputable biogenic origin that is
Syngenic with the formation of the original rocks is found near the end of the Archean.
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Modern Prokaryotes and Archean Life – Simple forms of life that are present today and that
live in extreme environments like those of the Archean provide insight into what early life was
like. These include anaerobic and non-photosynthetic members of the domain “Archea” as well
as photosynthetic Cyanobacteria from the domain “Bacteria.”
Genome sequencing points to Archean evolutionary pathways – A phylogenetic “tree of life”
has revealed separate prokaryotic domains (Bacteria and Archaea) that last had a common
ancestor in early Archean times.
Discussion Questions
1. Scientists still debate the interpretation of some of the earliest evidence for Archean
life. What are the most important factors to consider in the interpretation of chemical
biosignatures of early life? What about alleged microfossils?
2. Do you think the evidence for ancient life in the Martian meteorite, ALH 84001, is
strong or weak? Why or why not?
3. What are the advantages and disadvantages of using modern prokaryotic organisms to
interpret life in the Archean?
4. How would you go about determining the environment of deposition for a
sedimentary rock?
Review Questions
1.
Which of the following is NOT a piece of evidence from ALH 84001 that was used by
NASA scientists to infer the presence of ancient Martian life?
a. Microfossils
b. PAHs
c. RNA
d. Crystals of Magnetite
2.
a.
b.
c.
d.
3.
Which of the following does not represent a boundary between tectonic plates
Subduction Zone
Undersea Ridge
Transform Boundary
Depositional Environment
Which of the following is not a law of stratigraphy?
a. The Law of Superposition
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50
b. The Law of Crosscutting relationships
c. The Law of Original Horizontality
d. The Law of Depositional Environments
4.
a.
b.
c.
d.
Which of the following is not a feature of all prokaryotic cells?
Cytoplasm
Cell membrane
DNA
Flagellum
a.
b.
c.
d.
Which of the following is a non-photosynthetic respiring prokaryote?
Cyanobacteria
Green Sulfur Bacteria
Methanogens
Purple Sulfur Bacteria
5.
6.
Metamorphosed Archean rocks that were formed in subduction zones with Island- Arc
Volcanoes are known as which of the following?
a. Sandstones
b. High-grade metamorphic rocks
c. Greenstone Belts
d. Ultra-mafic rocks
e. Limestone
7.
rocks?
a.
b.
c.
d.
Which is not an important criterion for recognizing the validity of micro-fossils in ancient
Syngenicity
Biogenicity
Species identification
Lack of contamination
8.
In photosynthesis, organisms release the energy stored in sugars by splitting the molecule
and making ATP
a. True
b. False
9.
In aerobic respiration, organisms harness the energy of sunlight to synthesize sugars that
store chemical energy in their molecule bonds for later use.
a. True
b. False
10.
Which of the following is not a factor that links seafloor magnetic striping to the presence
of a divergent tectonic boundary?
a. Volcanic rocks imprint a record of the strength and polarity Earth’s magnetic field at the
time of their formation
b. The Earth’s magnetic field flips polarity on timescales of many thousands of years
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51
c. Tectonic plates move away from a divergent plate boundary with time
d. Oceanic plates disappear into the mantle at subduction zones
11.
Which of the following is most applicable to a genetic phylogeny?
a. Physical traits are compared to determine which organisms are most closely related
b. Physical traits of many organisms are compared to determine how long ago they had a
common ancestor
c. DNA sequence data is compared to determine which organisms are most closely related
d. The relatedness of organisms is determined on the basis of their complexity
12.
a.
b.
c.
d.
Which of the following is not one of the three “domains” of life?
Eukarya
Prokarya
Archea
Bacteria
13.
Which of the following is typically never a component of the layers in a Banded Iron
Formation?
a. Chert
b. Magnetite
c. Shale
d. Granite
e. Hematite
14.
The 12C/13C ratio in photosynthesizing systems is most likely to be which of the
following?
a. Higher than in the surrounding environment
b. The same as in the surrounding environment
c. Lower than in the surrounding environment
d. Equal to unity
15.
a.
b.
c.
d.
e.
The stable interior of a continent is known as which of the following?
Continental Platform
Craton
Spreading Center
Greenstone Belt
Continental Shelf
Advanced Questions:
1. Calculate the Hamming distance and construct a phylogenetic sequence for organisms
with the following nucleotide sequences.
AUG CUC UGU CUA GAG
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AGG CGC UGU CUA GAG
ACG CAC UCU CUA GAG
AAG CCC UAU CAA GAG
2. If all life was destroyed during the Late Heavy Bombardment, how much time did life
have to arise and evolve before it left its signature in the Isua Greenbelt rocks?
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