PartIII.TheOriginofComplexLife
ChapterNine–LifeShapesProterozoicEarth
NewLandscapesandClimateintheProterozoic
9.1 Atmospheric Oxygen and Global Cooling Changed Proterozoic Earth
9.2 New Land Masses changed the Face and Climate of Proterozoic Earth
9.3 Proterozoic Earth became a “Snowball” on several occasions
ANewDomainofLife
9.4 Free Molecular Oxygen provided new Evolutionary Opportunities for Life
9.5 Endosymbiosis brought together new Innovations in a Single Cell
9.6 The Eukaryotic Cell became established in Proterozoic times as the Progenitor of Complex
Life
TheOriginofMetazoans
9.7 Fossils trace the Rise of Metazoans at the End of the Proterozoic
9.8 Three of Four Key Animal Groups appeared by the End of the Proterozoic
Chapter 9, devo pass, December 2010 1
PartIII.TheOriginofComplexLife
What are the chances that life would evolve to complex or intelligent forms if it began on
some other planet? The appearance of complex life is viewed by some scientists as less likely
than the origin of life itself. Others see it as something inevitable once life takes hold, so long as
a life-supporting environment can persist for billions of years. It was in the Proterozoic Eon that
life on Earth evolved beyond its prokaryotic origins and first began to flourish. Here we look at
the conditions associated with this transition to better understand its likelihood on other Earthlike planets. NewLandscapesandClimateintheProterozoic
Planet Earth experienced a dramatic transformation at the end of its first two billion
years. Changes in atmospheric composition were wrought by life itself and paved the way for the
appearance of increasingly complex life forms. The accumulation of proto-continents was
poised to merge into the first supercontinent. Geologists have acknowledged the magnitude of
these changes by designating their onset with a new period of geological time illustrated in
Figure 1: the Proterozoic Eon.
Chapter 9, devo pass, December 2010 2
Figure 1 – The Proterozoic Eon spans a time period from 2.5 billion to 543 million years ago. It lies
between the Archean and Phanerozoic Eons. [Flip this figure in time and omit the eras of the
Phanerozoic]
The term proterozoic is composed of two Greek roots. “Protero-” means before, and
“zoic” pertains to life. Proterozoic rocks were once thought to represent a time “before life,”
because the fossils typical of younger rocks were not found. Charles Darwin predicted that
fossils would eventually be found in Proterozoic rocks, however, and he was correct. We now
know that, like the Archean Eon before it, the Proterozoic Eon contains significant evidence for
life. Fossil traces of this life were difficult to find in Darwin’s time, because it was almost
entirely microbial.
Key differences between the Archean and Proterozoic Eons did not appear
instantaneously. The rock record from then is also sparse and without abundant identifiable
fossils. Consequently, the beginning of the time period has been set at 2.5 billion years without
reference to any particular rock outcrop. This convention differs from the method for setting
boundaries between more recent periods of geological time. We will discuss that practice in
more detail in the next chapter. For now, you should think of the Archean-Proterozoic time
boundary as an approximate designation for a time of great changes that took place over many
millions of years.
9.1AtmosphericOxygenandGlobalCoolingChangedProterozoicEarth
Chapter 9, devo pass, December 2010 3
In the chapter 8, we noted the presence of banded iron formations (BIFs) as an indicator
of oxygenic photosynthesis in the Archean. But we see no geochemical evidence that oxygen
was accumulating in the atmosphere at that time. The lack of evidence may be because iron and
sulfur was present in the oceans in sufficient quantities to absorb all oxygen that was produced.
Iron is soluble when oxygen concentrations are low. Oxygen first released by cyanobacteria,
however, would have led to the immediate precipitation of iron in the form of mineral layers like
those pictured in Figure xx of Chapter 8. BIFs decrease in the beginning of the Proterozoic, a
signal that oxygen-absorbing iron in the seas was being used up. Other indicators support the
idea that, with the depleting of sinks for oxygen, it began to accumulate in the atmosphere.
a)
b)
Figure 2 – a) Red Beds from the Proterozoic Hammersley Group in Western Australia. Hematite, a
form of oxidized iron, is responsible for the reddish color. b) Pyrite, sometimes referred to as
“fool’s gold” because of its deceptive appearance, is shown here in a conglomerate with rounded
pebbles that indicate sediment transport in an anaerobic environment. Sample is from 2.7 billion
year old Archean sediments found in South Africa. [figure from link at
http://www.humboldt.edu/natmus/lifeThroughTime/PreCam.web/index.htm ]
Red beds like the one pictured in Figure 1a) are composed of sandstones and shales that
have a reddish appearance due to the presence of iron oxide cement that holds the grains
Chapter 9, devo pass, December 2010 4
together. Most of these were deposited on land as river deposits. In order for iron oxides to
precipitate out in such an environment, oxygen would have had to have been present in the
atmosphere. Geologists have taken note that no red beds are found in Archean rocks. They do
not begin to appear until the earlier part of the Proterozoic. The advent of red beds and the
passing of BIFs likely both tell the same story of a Proterozoic buildup in atmospheric oxygen.
More evidence comes from the oxidation state of minerals in Archean and Proterozoic
rocks. Minerals like pyrite (FeS2) and uraninite (UO2) disintegrate rapidly by oxidation when
they are exposed to oxygen in the present-day atmosphere. When these minerals are transported
as sedimentary grains, they tend to break down and be absent from sedimentary rocks. Pyrite, for
example, contains the reduced form of iron and is replaced by hematite (Fe2O3) or magnetite
(Fe3O4) in the presence of oxygen. Nodules of pyrite and uraninite are found in Archean
sedimentary rocks like that shown in Figure 1b. Their presence signals that atmospheric Archean
oxygen abundance was less than 1/1,000th of the current level. These kinds of deposits disappear
in the Proterozoic and are rare in sandstones that are 2.3 billion years old. Their disappearance is
yet one more indication that oxygen rose well above Archean levels at that time.
Isotopic levels of sulfur also reflect biologically important changes at the ArcheanProterozoic boundary. Sulfur has four stable isotopes that occur in nature in the following
proportions: 32S (95.02%), 33S (0.75%), 34S (4.21%), and 36S (0.02%). Ultraviolet radiation in the
upper atmosphere affects the reaction rates of these isotopes differently as they combine to form
gaseous compounds. The resulting fractionation of isotope ratios depends on properties other
than the mass of the isotope. In contrast, the formation of sulfur-bearing minerals like gypsum
will fractionate sulfur in a mass-dependent way. In the absence of atmospheric oxygen, mass
independent fraction (MIF) produced in the upper atmosphere is reflected in the composition of
Chapter 9, devo pass, December 2010 5
minerals that form at the surface and dominates any mass-dependent processes that might occur.
When oxygen is present, however, these atmospheric compounds are quickly re-oxidized and all
traces of MIF erased before they are imported into minerals. This occurs when atmospheric
oxygen is only 1/100,000th of the present level. An MIF signal for sulfur in Archean rocks
indicates that the level of atmospheric oxygen at that time was negligible. This signature
disappears at the Archean-Proterozoic boundary.
Figure 3 - Oxygen levels as at the Archean-Proterozoic transition as implied by
geochemical signatures. [This is from
http://scienceblogs.com/highlyallochthonous/2007/11/how_the_air_we_breathe_became.ph
p and would have to be revised (less terminology)]
A schematic representation of rising Proterozoic oxygen levels is charted in Figure 2
together with the lines of evidence that support it. A brief whiff of oxygen is indicated 2.9
billion years ago at the end of the Archean by sulfur isotopes without MIF. This signature
returns, however, until 2.3 billion years ago. At this time, reduced forms of minerals disappear
Chapter 9, devo pass, December 2010 6
and red beds appear, signaling a much more significant rise in oxygen levels that is sometimes
referred to as the Great Oxidation Event (GOE).
The appearance of atmospheric oxygen in the Proterozoic stands as one of the most
significant events in the history of life on Earth. Its toxic influence led to the extinction of many
organisms. Those that evolved to tolerate it, however, were able to use its reactivity as a superior
energy source. They evolved to become larger and more complex. There were also dramatic
consequences for climate. With addition of oxygen, the balance of atmospheric greenhouse
gases shifted. Global temperature changed dramatically, leading to further extinctions and the
opening up of environmental niches for new organisms.
We have seen that, despite a less-luminous Sun, Archean Earth remained warm through
the action of very efficient greenhouse gases like methane and carbon dioxide. Atmospheric
models indicate that the average molecule of methane remained in the Archean atmosphere for
10,000 years before disappearing in other chemical reactions. At the current rate of biological
methane production, this lifetime would lead to methane concentrations that are hundreds of
times the levels we observe today. Methane levels are reduced today because oxygen destroys it
by reactions that convert it to carbon dioxide and water. Oxygen does this fast enough to reduce
the residence time for atmospheric methane to just a few years. It’s not clear that Archean
methane production levels were as high as today, but they may well have been.
The ability of the Proterozoic atmosphere to retain heat would have suffered a blow with
the loss of methane. Carbon dioxide is also a greenhouse gas but not as efficient, and the Sun
was still far less luminous than today. So were carbon dioxide levels high enough to keep the
Earth from freezing over? Part of the answer lies in the complex interaction of CO2 with Earth’s
land masses and tectonic cycles. All of these factors, too, were changing in the Proterozoic.
Chapter 9, devo pass, December 2010 7
9.2 New Land Masses change the face and climate of Proterozoic Earth
As we saw in Chapter 8, Archean land masses consisted largely of small volcanic island
chains that accreted onto each other in the course of accelerated tectonic motions. Toward the
end of the Archean, the largest of these chains existed as the cratonic origins of proto-continents.
By the end of the Proterozoic, the stable cores of today’s continents had merged into a supercontinent. The presence of increasing land masses had consequences for Earth’s atmospheric
composition and temperature.
The chemical weathering of continental rocks removes carbon dioxide from the
atmosphere. As illustrated in Figure 4, CO2 combines with rainwater to form carbonic acid. This
gentle acid is the same as that in carbonated beverages. One of its products is a bicarbonate ion
(HCO3-) that reacts with silicate minerals like feldspars to form minerals that can easily be
transported away to stream, lake, and eventually ocean environments. The net effect is to remove
carbon dioxide from the atmosphere.
Figure 4 - Chemical weathering of continental rocks removes carbon dioxide from the
atmosphere and silica from continental rocks. These are transported to the sea in streams
and rivers.
Chapter 9, devo pass, December 2010 8
Bicarbonate ions eventually make their way to the sea, where they combine with ions like
calcium and precipitate as a carbonate. Calcium carbonate accumulates on the sea floor and
hardens to form limestone. If this was the end of the process, Earth’s carbon dioxide would be
completely used up. As we’ve seen in Chapter 8, however, tectonic activity ensures that most
sea-floor limestone deposits eventually are melted again at a subduction zone. When that
happens, carbonates are disintegrated and carbon dioxide is released. It is returned to the
atmosphere through volcanic emissions to complete the carbonate-silicate cycle illustrated in
Figure 5.
Figure 5 – The carbonate-silicate cycle. Carbon dioxide and silicate minerals are dissolved in
rainwater to yield bicarbonate ions and other silicate products. These are transported to offshore
(A) or near-shore (B) environments where they are precipitated out as carbonates (limestone, for
example) and sometimes as silica gels (chert, for example). Metamorphism eventually returns the
Chapter 9, devo pass, December 2010 9
carbon dioxide to the atmosphere and reforms silicate minerals, but only after many tens of
millions of years.
The carbonate-silicate cycle works in tandem with an organic carbon cycle in which
carbon is cycled by organisms. Photosynthesizing organisms absorb atmospheric both water and
carbon dioxide and convert these to organic matter and oxygen. When the organic matter is
buried, the carbon is sequestered away from the atmosphere. By means of decay and respiration,
however, it is eventually returned. On very long timescales, this cycle is much less important
than the carbonate-silicate cycle as a determiner of atmospheric carbon-dioxide levels, because it
is a much smaller carbon reservoir than carbonate rocks. On short timescales, however, it may
destabilize climate.
Figure 6 - The carbonate-silicate cycle forms a negative feedback loop that stabilizes climate.
The carbonate-silicate cycle is a good example of a negative feedback loop as illustrated
in Figure 6. As we have seen, carbon dioxide is a greenhouse gas, and its abundance in the
atmosphere has a strong effect on global temperature. As it turns out, silicate weathering rates
Chapter 9, devo pass, December 2010 10
are very sensitive to temperature and proceed more rapidly when temperatures are high. The net
effect is to stabilize Earth’s climate. As illustrated in Figure 6, added carbon dioxide will raise
temperatures and increase the rate of silicate weathering, thereby removing excess carbon
dioxide. A deficit of carbon dioxide, however, will lower temperatures and the silicateweathering removal process will be slowed until they can build up again.
The time period for returning carbon dioxide to the atmosphere through the carbonatesilicate cycle is very long. The average lifetime of seafloor material between creation and
subduction is 60 million years. Additional sources of carbon dioxide include ocean ridge and hot
spot volcanism and may help stabilize CO2 levels on a timescale of 10 million years. This cycle
is an important contributor to the habitability of our planet. But as we shall see in the next
section, variations on shorter timescales can prove catastrophic for many kinds of organisms.
Figure 7 – Terranes of the North American continent that were accreted to the Archean craton
during the Proterozoic Eon. During the end of this period, the North American continent was
attached to others in a super-continent (see Figure 8).
Chapter 9, devo pass, December 2010 11
The carbon-silicate cycle depends on the existence of substantial land masses to stabilize
carbon dioxide levels. In Proterozoic times, land masses were finally attaining continental sizes
for the first time as multiple terranes were accreted to their margins. This is evident in a map of
Archean and Proterozoic rocks in North America shown in Figure 7. The Archean craton of an
ancient continent called Laurentia is centered in Canada and was expanded considerably in
Proterozoic times by the addition of terranes accreted onto its borders.
The continent of Laurentia was partially rifted during the Proterozoic, but stayed intact to
become part of a Proterozoic supercontinent known as Rodinia, pictured in Figure 7. This land
mass was assembled about 1.1 billion years ago as the result of the tectonic suturing of smaller
drifting continental land masses. Rifting of the new super-continent eventually broke it apart,
starting in the late Proterozoic about 750 million years ago. The most prominent rift opened up
the region that is now the Pacific Ocean.
Figure 8 – The configuration of super-continent Rodinia in the late Proterozoic. Laurentia was the
forerunner of today’s North American continent. Antarctica, Australia, and India were joined in
what has come to be called Gondwana
Increased land mass like the continents that merged to make Rodinia helped cool Earth as
the Sun grew brighter. Most importantly, they provided more land for weathering and carbonChapter 9, devo pass, December 2010 12
dioxide removal so that more atmospheric CO2 was transferred to carbonate rocks. They also
reflected sunlight back out to space more efficiently than oceanic surfaces did. From past
chapters, you can recall the definition of albedo as a surface property of reflectivity. A more
reflective planet will be cooler, because it absorbs less heat from its star. Albedo differences for
land and water surfaces are modest. A far more contrasting surface albedo is exhibited by highly
reflecting ice and snow. During Proterozoic times this effect pushed Earth’s climate far from the
stable point maintained by the carbonate-silicate cycle.
9.3 Proterozoic Earth became a “Snowball” on several occasions
The high albedo of frozen water is a key element of a positive feedback effect that works
globally to cool planet Earth. Cold temperatures encourage the growth of ice caps and glaciers.
These are highly reflective and decrease the amount of solar energy absorbed by the planet.
Consequently, their growth causes temperatures to fall even more because of the reduction in
absorbed solar radiation. The lower temperatures lead to further growth of the ice coverings, less
absorbed sunlight, and even lower temperatures. And so on. The fainter Sun of Archean and
Proterozoic times would seem to have made Earth even more vulnerable to this type of runaway
freeze than today. A greater abundance of methane and carbon dioxide greenhouse gases
apparently kept the process in check. Or did it?
Box9.1–TheIce‐AlbedoFeedbackEffect
Chapter 9, devo pass, December 2010 13
The growth or decrease of ice affects the global temperature on Earth as a positive
feedback mechanism. This means that the temperature influence on the ice is affected by the
ice’s response to increase its change in whichever direction it had initially changed. Decreasing
temperature will increase the amount of ice and cause temperature to decrease further. Increasing
temperatures will reduce ice, causing temperatures to increase even more.
To understand how much the temperature changes with a change in ice cover, we can use
the equation from Box 3.2 of Chapter 3 to see how albedo changes effect temperature. From Box
3.2, we have:
(1-A) (R/a)2σT4eff = 4σTp4.
The radiation of the sun at the orbit of the earth is sometimes expressed as the solar constant, S =
1.366 kW/m2. This is as the amount of incoming solar electromagnetic radiation per unit area
that would fall on a plane perpendicular to the rays, at a distance of one astronomical unit (AU).
When we substitute this value into our equation from Box 3.2, we have:
σT4 = (S/4)(1-A)
We can rearrange this equation to solve for the surface temperature of the Earth in the absence of
greenhouse effects as:
T = [(S/4σ)(1-A)]1/4
Here, S is the solar constant, σ is the Stefan-Boltzmann constant, and A is the albedo of Earth’s
surface. To find the ratio of temperatures as a function of albedo, we have:
Tf /Ti = [(S/4σ)(1-Af)]1/4/[(S/4σ)(1-Ai)]1/4 = [(1-Af)/(1-Ai)]1/4
Chapter 9, devo pass, December 2010 14
This gives us the ratio of temperatures before (Ti) and after (Tf ) albedo changes have taken place
from Ai to Af.
The Earth’s average albedo results from a combination of mostly absorbent oceans, highly
reflective clouds and ice, and land surfaces that vary in between these two. We can approximate
the albedo of ice-covered Earth as Aice ≈ 0.8. If the remaining surface has an average albedo of
Anon-ice ≈ 0.25, then the total albedo of Earth can be written as Aearth = F(Aice) + (1-F)(Anon-ice) =
F(0.8) + (1-F)(0.25) = F(0.55) + 0.25, where F is the fraction of Earth that is covered by ice.
Then we have:
Tf /Ti = [(1-Af)/(1-Ai)]1/4 = [(1- (0.55Ff + 0.25))/(1- (0.55Ff + 0.25))]1/4 = [(0.75- 0.55Ff)/(0.750.55Fi)]1/4
Where Fi is the fraction of Earth’s surface initially covered by ice, and Ff is the final fraction.
Example Question: How would Earth’s temperature change in the absence of greenhouse
effects if the current ice cover of 10% (0.1) increased to 50% (0.5)?
Solution: From the above, we have Fi = 0.1 and Ff = 0.5
Tf /Ti = [(0.75- 0.55(0.5)/(0.75- 0.55(0.1))]1/4 = [(0.75- 0.275)/(0.75- 0.055)]1/4 = [0.475/0.695]1/4
= 0.909
This number implies that the temperature would decrease by about 10% if the ice cover
increased to 50%. The Earth’s average surface temperature is about 288K (15o C). A 10 percent
decrease would change it to about 259 K or about -13.8o C. At these temperatures, the Earth
would quickly freeze the rest of the way!
Chapter 9, devo pass, December 2010 15
The Proterozoic rock record contains evidence of extensive ice cover. Glaciers are large masses
of ice on land that accumulate whenever snowfall exceeds the amount of ice that melts or
sublimes in warmer seasons. Mountain glaciers atop permanently snow-capped peaks flow
gradually downhill. More extensive continental glaciers cover the surfaces of Greenland and
Antarctica today. These flow away from their thickest sections and leave a distinctive pattern of
erosion and deposition like the examples shown in Figure 9.
a)
b)
Figure 9 – a) Erosional features from the passage of a glacier at Glacier Bay National Park, Alaska.
A pattern of large striations, grooves and polish is distinctive evidence that these rocks were
covered by a glacier. b) Glacial till deposited in a recent Ice Age on the eastern flank of the Sierra
Nevada mountains in California. Note the irregular boulders with a wide range of sizes.
Continental glaciers erode the rock underneath them in a distinctive way as shown in
figure 9a). Scratches or grooves called striations are carved into the rock and show the direction
that ice was flowing. Gouged-out material may be deposited later as an unlayered and poorly
sorted deposit with clay to boulder-sized fragments. A deposit like this is known as till and is
Chapter 9, devo pass, December 2010 16
shown in Figure 9b). Boulders and rocks carried by water-borne ice may be dropped into
sediments beneath to appear as an out-of-place glacial dropstone.
a)
b)
c)
Figure 10 - a) Glacial striations on a late Proterozoic rock from Mauritania [
http://www.swisseduc.ch/glaciers/earth_icy_planet/glaciers15-en.html?id=3 ] b) Proterozoic tillites
overlain by a cap carbonate. c) A glacial drop stone in sedimentary rock from the Proterozoic
Gowganda Formation in Canada.
Recent erosional and depositional features like those in Figure 9 are also preserved in the
Proterozoic rock record and mark the passing of vastly more ancient glaciers. Glacial striations
from late Proterozoic deposits are shown in Figure 10a. Proterozoic till that has turned to stone,
called tillite, is overlain by an extensive limestone layer in Figure 10b. A glacial dropstone in
early Proterozoic sediments from Canada can be seen in Figure 10c. Multiple indicators like
these show us that glaciers were prevalent during Proterozoic times. If these glaciers were
confined to high latitudes, this may simply mean that ice ages cycled from polar caps in much
the same way that they have for the last few million years. The tectonic drift of the continents
has moved them far from their latitude during the time of the glacial deposits, so we have to have
an independent means of discerning their latitude during the Proterozoic. Fortunately, one is
available.
Chapter 9, devo pass, December 2010 17
Figure 11 - Sedimentary rocks record a signature from the earth's magnetic field.
In Chapter 8, we discussed how igneous rocks retain a record of the orientation and
strength of Earth’s magnetic field at the time they solidified. It turns out that magnetic minerals
in sediments do much the same thing. Studies of of paleomagnetism in sediments reveal the
polarity and orientation of the magnetic field at the time the sediments were deposited. The
process is illustrated in Figure 11.
a)
b)
Figure 12 – a) The Earth’s magnetic field intersects the Earth’s surface at different angles in a way
that depends on latitude. b) Geologists can measure the tilt of the Earth’s magnetic field in
Chapter 9, devo pass, December 2010 18
sedimentary rocks and determine the latitude at which they were deposited. [Figure b needs to be
revised to emphasize the magnetic declination]
As illustrated in Figure 12a), the direction of Earth’s magnetic field intersects its surface
at different angles in a way that depends on latitude. High latitudes near the magnetic poles are
associated with field lines aligned nearly in a vertical direction. In contrast, equatorial latitudes
have nearly horizontal field lines. This property allows geologists to estimate the latitude of
ancient sediments by measuring the angle of their remanent magnetic fields.
Paleomagnetic properties of Proterozoic glacial sediments revealed a great surprise.
Glaciers had deposited them at nearly equatorial latitudes. If glaciers had advanced nearly to the
equator, it is almost certain that the ice-feedback mechanism would have ensured that they
covered Earth entirely. Geologists originally rejected this conclusion, because they imagined that
the ice-albedo effect was irreversible. They could not imagine how Earth would have ever
emerged from such a state.
Caltech geologist Joseph Kirschvink was the first to articulate the idea that a global
freeze could have ended due to buildup of carbon dioxide in the atmosphere. He coined the term
Snowball Earth in the early 1990s to refer to the hypothesis that Earth’s surface became nearly
or entirely frozen one or more times during the Proterozoic Eon. He realized that the final endpoint of the ice-feedback mechanism would be a global ice covering that would interrupt the
mechanisms of carbon-dioxide removal in the carbonate-silicate cycle. Extremely cold
temperatures would reduce rates of rainfall. Rocks covered in ice would be protected from
weathering. The net effect would be to curtail the absorption of carbon dioxide from the
atmosphere. Volcanoes, on the other hand, would still erupt through ice and snow. Their gaseous
emission would still return carbon dioxide to the atmosphere.
Chapter 9, devo pass, December 2010 19
Over time, the greenhouse gas would build up sufficiently to melt the ice and break the
ice-feedback loop. For this to happen, carbon-dioxide abundance in the atmosphere would have
to be several hundred times its current level. If CO2 was continually put into the atmosphere at
rates similar to today, then it would take a few million years to achieve these levels. At the end
of this period, ice would melt rapidly because it would reverse the ice-albedo feedback. The
albedo change would happen much faster than the consumption of excess atmospheric CO2 by
silicate weathering. A temporary greenhouse-induced heat wave would follow the retreat of the
ice. Figure 13 - The Snowball Earth Hypothesis. a) A temperature decrease initiates the ice-feedback
mechanism, and glacial ice advances from the poles toward the equator. b) Eventually, the entire
Earth is covered in ice. Removal of atmospheric carbon dioxide by silicate weathering is
interrupted, but volcanic contributions continue. c) Atmospheric carbon dioxide and greenhouseelevated temperatures build up until the global ice melts. [Note – this was the best figure I could
find for this, but it should be modified substantially – I’d like to get in the information that is in the
figure on the right hand side. But I’d also like graphic images that show volcanism with and
without glacier coverings as on the LHS.]
Chapter 9, devo pass, December 2010 20
Kirschvink proposed three tests for the Snowball Earth hypothesis. Evidence for glacial
activity should show up in all rocks deposited at the time of Snowball Earth, no matter where
they are found (global synchroneity). Marine sediments should show that the ocean was
depleted in oxygen at the same time (ocean anoxia), because organisms would have experienced
a massive die off. Finally, the huge buildup of carbon dioxide should have produced pronounced
greenhouse effects at the conclusion of the snowball episode (ultra-greenhouse aftermath). Very
strong evidence for the latter first convinced most geologists that the Snowball Earth effect had
actually occurred.
Figure 10b) shows a Proterozoic tillite that is overlain by a thick cap carbonate.
Rocks like these are frequently found directly on top of Proterozoic glacial deposits. They
indicate a rapid switch back to a very warm climate under ultra-greenhouse conditions. Most cap
carbonates are formed of magnesium carbonate (dolomite) rather than calcium carbonate (calcite
or typical limestone). They have unusual sedimentary features with geochemical signatures and
isotope ratios that are not typically found in other carbonates. They also often occur in sequences
of strata that have no other carbonates at all. Their properties match those expected by rapid
formation in a post-Snowball environment, where temperatures were so high and carbon dioxide
levels were so extreme that carbonate formation was catastrophically rapid.
The ratio of 13C to 12C shifts to lower levels in cap carbonates. To interpret the possible
implications, recall that living organisms preferentially take in 12C, so that 13C is relatively more
abundant in the remaining sediments. A gradual return to isotopically lighter carbon would be
expected to occur during the few million years of the glacial period during which organic
productivity is reduced. Also, high weathering rates during the greenhouse period would mostly
involve inorganic carbon from the atmosphere that built up from volcanic emissions and has a
Chapter 9, devo pass, December 2010 21
relatively small fraction influenced by biological organisms. In short, the Snowball Earth
hypothesis explains glacial tills, cap carbonates, and anomalies in the carbon isotope ratio like
those plotted in Figure 14. Figure 14 – Late Proterozoic glaciations that have been identified as episodes of Snowball Earth.
Tillite deposits that are underlain by cap carbonates are correlated with drops in the ratio of 13C to
12
C. The Snowball Earth hypothesis accounts for these as discussed in the text. [Perhaps orient as
per lower figure]
Chapter 9, devo pass, December 2010 22
Another related piece of evidence is the brief formation of BIFs in the late Proterozoic.
Recall that BIFs are formed from reduced iron that could only have been dissolved in the sea if
oxygen was depleted. BIFs disappear from the geologic record soon after the rise of oxygen early
in the Proterozoic. They briefly reappear, however, at times associated with Proterozoic
Snowball Earth events. This could have been due to glacial die-off of photosynthesizing
organisms and an accompanying reduction in oxygen levels. High temperatures and plentiful
access to sunlight in the ensuing greenhouse phase would have led to a prolific resurgence of
cyanobacteria and quick restoration of oxygen.
Evidence for Snowball Earth is substantial and continues to mount. The idea of lowlatitude glaciations in the Proterozoic is now generally accepted by geologists, and the Snowball
Earth hypothesis is accepted in some form by most. As evidence accumulates and details are
worked out, the Snowball Earth hypothesis may become a fully established theory about climate
evolution and environmental influences on the development of life in Proterozoic times.
Meanwhile, scientists are still trying to determine exactly how a cooler period got started initially
and whether or not life had anything to do with it.
Chapter 9, devo pass, December 2010 23
Figure 15 - Oxygen levels throughout geologic time. Oxygen first dramatically increased at the
beginning of the Proterozoic in the Great Oxidation Event. It increased about 20-fold at the end of
the Proterozoic Eon at a time when complex multi-cellular life proliferated. This later change was
prologue to the Phanerozoic Eon, a time in which complex life is found throughout the fossil record.
The rise of oxygen during the Great Oxidation Event during early Proterozoic times
(Paleoproterozoic Era) is a plausible mechanism for the initiation of the first Snowball Earth
event. It would have starkly reduced atmospheric methane and its considerable contribution to a
greenhouse effect. Nearly 2 billion years later, oxygen levels increased another 20 times as
shown in Figure 14. Snowball Earth events that occurred then (Neoproterozoic Era) left the
strongest record of Proterozoic global glaciation. As we will see, this time period also coincided
with the dramatic appearance of new multicellular life forms.
The evolution of complex non-prokaryotic life was perhaps the most significant
development in Proterozoic biology. To understand both its influence on and reaction to
Proterozoic climate change, we need to examine the biological innovations that first appeared in
Proterozoic times.
Chapter 9, devo pass, December 2010 24
A New Domain of Life
9.4FreeMolecularOxygenprovidednewEvolutionaryOpportunitiesfor
Life
Carbon isotopes tell us that the initial rise in oxygen was accompanied by a proliferation
in microbial life. The ability to produce and use oxygen clearly afforded life with an improved
capacity for flourishing on planet Earth. To see why, we will take a closer look at the advantages
of oxygenic respiration and photosynthesis over other metabolic pathways.
Metabolic extraction of energy in catabolic and anabolic processes is fundamentally a
process of charge flow between chemical compounds with varying degrees of electrical
potential. In this respect, it is rather like the flow of electrons between the terminals of a battery.
Just as batteries can be used to power all kinds of human-made devices, the flow of electrons in
biochemical networks provides the energy for the function of living organisms. In metabolic
oxidation-reduction processes, electrons flow from an electron donor (reducer) to an electron
acceptor (oxidizer). Organisms oxidize or “eat” electron donors and reduce or “breathe” electron
acceptors. The first organisms had to use the compounds that were easily available from
inorganic sources. These did not necessarily yield the largest energy-producing reactions and
were insufficient to sustain the evolution of complex life.
Among the metabolic substances used by organisms today, oxygen has the strongest
affinity for electrons. The greatest amount of energy is gained by a flow of electrons to oxygen
from organic molecules that have been consumed or synthesized. This is immediately evident
from the quantitative comparison of the energy released in different catabolic pathways that is
presented in Table 9.1.
Chapter 9, devo pass, December 2010 25
Table 9.1 - Energy yield for some Common Catabolic Pathways
Catabolic
Process
Aerobic
Respiration
Methanogenic
Respiration
Sulfate
Respiration
Lactate
Fermentation
Electron
Donor
Glucose
Hydrogen
Hydrogen
Lactose
Oxygen
Carbon Dioxide
Sulfate
Internal
Compounds
2870
131
152
196
(What is “eaten”)
Electron
Acceptor
(What is “breathed”)
Energy Yield
(in units of
Kilo-Joules)
Carbon dioxide, sulfate, and hydrogen were certainly available to the earliest organisms.
As listed in table 9.1, respiration processes that use these substances yield between 100 and 200
kJoules of energy. Fermentation processes “eat” sugars like lactose (a sugar found in milk), but
do not take their electron acceptors from the environment. Instead, they use an internal metabolic
intermediate. Although this process is seemingly more advanced, it is free from the constraint
imposed by the use of an environmental electron acceptor. Lactate fermentation consumes
lactose and yields slightly more energy than methanogenic (methane producing) or sulfate
(sulfate consuming) respiration processes.
The advantages of oxygenic respiration are immediately apparent in Table 9.1. The
energy yield is 15, 19, and 22 times that for lactate fermentation, sulfate respiration, and
methanogenic (methane producing) respiration, respectively. Each of these processes stores
energy in ATP that becomes available for cellular activity like growth and replication. The extra
energy provided by aerobic respiration is available to make organisms of larger size and greater
complexity. To all appearances, the availability of oxygen was a fundamental condition for the
evolution of complex life.
Chapter 9, devo pass, December 2010 26
The oxygen used in aerobic respiration is produced by aerobic photosynthesis. As we
have seen, photosynthesis entails the reverse of aerobic respiration. Carbon dioxide and water are
consumed, and glucose and oxygen are produced. This process requires that an amount of energy
like that yielded by aerobic respiration be consumed. As we have learned, the energy is supplied
by the capture of photons from the Sun. Table 9.1 shows us that the ability to harness solar
radiation to make free molecular oxygen and use it in respiration gave Proterozoic life new
energy resources that were about 20 times greater than those used by its predecessors. This set
the stage for new developments in the evolution of life.
As you may recall from Chapter 8, purple and green sulfur bacteria carry out forms of
photosynthesis that are based on sulfur. These bacteria can often carry out a second reaction in
which oxygen is extracted from water to produce the sulfate (SO4) that is used in sulfate
respiration. This reaction yields no free molecular oxygen, however.
Many cyanobacteria are also capable of using water in a photosynthetic pathway to
produce sulfate. They are perhaps better known as the first organisms to use a similar process to
produce free molecular oxygen. Their key innovation was the development of chlorophyll as
well as a chemical photosystem. These probably evolved from simpler microorganisms that were
restricted to oxygen-free habitats where only sulfur was available as a source of electrons.
Molecular clock data like those discussed in Chapter 8 indicate that the emergence of
cyanobacteria coincides with the beginning of the Proterozoic. Organic-chemical-rich marine
sediments that are 2.5 billion years old also contain abundant evidence for specific
cyanobacterial biomarkers known as 2-methylhopanoids. These facts suggest that the Great
Oxidation Event was the result of the first global proliferation of cyanobacteria.
Chapter 9, devo pass, December 2010 27
The first accumulation of oxygen produced by cyanobacteria was almost certainly
catastrophic for most life forms. As we have seen, accumulation of oxygen led to a great
decrease in the greenhouse gas, methane, and an associated plunge in global temperatures.
Oxygen was also highly toxic to organisms present at that time. It is likely that aerobic
respiration is a descendant of mechanisms that evolved in anaerobic organisms to neutralize
these toxic effects. A few of these organisms developed the ability to fend off the poisonous
influence of oxygen and take advantage of it in respiration. In the modern biosphere, this
capability is carried out by many bacteria and archaea. In early Proterozoic times, it was used to
take cellular life in a new direction.
Chapter 9, devo pass, December 2010 28
9.5EndosymbiosisbroughttogethernewInnovationsinaSingleCell
In chapter 7, we discussed natural selection in rather simple terms. Organisms that have
an advantage in a contest for limited resources will outlast their less capable competitors. This
description is so simple that it may obscure a tactic that has been used time and time again to
secure an advantage for evolving organisms: collaboration. Organisms that perform mutually
beneficial actions may have an advantage over those who do not.
Symbiosis is a close ecological relationship between different kinds of organisms. Most
plants, for example, harbor fungal symbionts that extract phosphorus and nitrogen from the soil
and release them in the roots of the plant. Ruminant mammals like cows, sheep, and goats cannot
digest cellulose in the plants they consume but depend on a vast population anaerobic bacteria
and protozoa that lives in their digestive tract. Even you depend on intestinal microbes to digest
your food. They outnumber your own cells by a large margin.
Modern-day stromatolites like the ones shown in Figure 22a of Chapter 8 contain
hundreds of symbiotic microbes that carry out dozens of specialized chores. One type produces
a chemical that allows others to survive in very salty water. Another provides a sunscreen that
protects many of the others from ultra-violet radiation. [Would like a figure for this but have not
found a good example] The resulting mounds are identical to fossil stromatolites dating back
billions of years and suggest that symbiotic relationships were present at the very beginning of
life. We can especially imagine that, in the early Proterozoic, aerobic respiring bacteria found it
very useful to be associated with cyanobacteria.
Chapter 9, devo pass, December 2010 29
Figure 16 – Phylogenetic tree of life showing horizontal gene transfers. The transfer of genetic
material is not only upwards on the tree, but may occur sideways or “horizontally.”
The interrelatedness of microbial organisms is especially evident in the case of
horizontal gene transfer of genetic material. In this process, individual genes are not passed
simply from parent to offspring but directly into another species. This transfer can occur by
direct uptake of foreign genetic material or by viral transport of DNA from one cell to another..
As you might imagine, this process interferes with assumptions that gene transfer is only from
ancestor to descendant. A phylogenetic tree with pathways of horizontal gene transfer is shown
in Figure 16.
The mechanism of horizontal gene transfer provides an important opportunity for the
evolution of life. It implies that genes optimized by natural selection for one type of microbe are
not confined to that particular organism and its descendants but can be imported into others as
well. This adds a new twist to evolution by mutation and selection from among best-fit
individuals. It has been heralded by some as a new paradigm in evolutionary biology and
described as a “non-Darwinian” mechanism for evolution.
Chapter 9, devo pass, December 2010 30
Symbiotic relationships and horizontal gene transfer are credited with taking biological
collaboration to new levels to produce Eukarya, a third domain of life that flourished in the
Proterozoic. Microbiologist Lynn Margulis proposed that mutually beneficial microbes exceeded
the level of cooperation implied in simple symbiosis to become an entirely new single organism.
In the 1960s, she proposed a theory of endosymbiosis to account for the origin of the eukaryotic
cell.
Endosymbiosis postulates that the complexity of the eukaryotic cell (see Fig. 17) arises
from its origin as a small colony of symbiotic prokaryotes. This idea was originally rejected by
many scientists at the time it was put forward, but it was later corroborated by genetic studies. It
is now widely accepted as the mechanism by which key features of the eukaryotic cell have
arisen. The underlying theme of Margulis’s theory emphasized the interdependence and
cooperative existence of different kinds of prokaryotic organisms. To understand the theory in
detail, we must first acquaint ourselves with the properties of the eukaryotic cell shown in Figure
17.
Chapter 9, devo pass, December 2010 31
Figure 17 - Structure of the Eukaryotic Cell, contrasting an animal cell and a plant cell.
The term eukarya is also the product of Greek roots, in this case meaning “good nut” or
“kernel,” a reference to the membrane-enclosed nucleus that distinguishes eukaryotic cells from
prokaryotic ones. The eukaryotic cell houses several subunits called organelles, identified in
Figure 16. These entities carry out specialized tasks within the cell. The nucleus contains the
primary DNA in the cell. Nuclear DNA is surrounded by protein complexes that wrap it up into
several units called chromosomes. The work of aerobic respiration is carried out mainly in
organelles called mitochondria. The cell ingests food and other materials through the process of
endocytosis. Substances are engulfed in this process when the outer membrane surrounds then,
folds inwards, and pinches off to form a compartment or vacuole around the material. This
process is referred to as invagination. It is likely that many of the membrane-bound organelles
were formed in this way (see Figure 17).
Chapter 9, devo pass, December 2010 32
Extensions of the double nuclear membrane aid in protein transport within the cell and
are referred to as the endoplasmic reticulum (ER). Ribosomes are attached to the rough ER and
deliver newly synthesized proteins to its interior. These typically enter vesicles that have budded
off from the smooth ER (see Fig. 16). Other organelles include the Golgi apparatus and
lysosomes that carry out specialized tasks of protein processing and enzymatic digestion of
nutrients.
Eukaryotic cells can be divided into animal and plant cells, each with distinctive
characteristics as shown in Fig.16. Most importantly, plant cells possess chloroplasts where the
work of photosynthesis is carried out. They also have a structurally firm cell wall (as distinct
from a more pliable cell membrane) and are often centered with a large vacuole. Animal cells are
largely distinguished by the absence of these features and the inclusion of centrioles.
The division of cellular labor in the eukaryotic cell provides it with an advanced and
powerful infrastructure for growth and replication. To house all these specialized structures,
eukaryotic cells are typically ten times larger than prokaryotic cells. Complex multicellular life is
composed of cells like these. Clearly, they are a giant evolutionary step away from the
prokaryotic cell pictured in Fig. 26 of Chapter 8.
We would naturally like to understand the steps by which this kind of cell evolved. We
have already mentioned invagination as a process that could have been responsible for some of
the eukaryotic organelles. The critically important mitochondria, chloroplasts, and, perhaps, even
the cell nucleus may have originated by the process of endosymbiosis.
Chapter 9, devo pass, December 2010 33
Figure 18 - Endosymbiosis and invagination are important processes in the origin of organelles in
the eukaryotic cell.
Endosymbiosis proceeds by steps in which one organism engulfs another (endocytosis),
yet both survive and evolve in a symbiotic relationship as illustrated in Figure 18. This process is
implied by the prefix “endo-” which pertains to something internal. Endosymbiosis is a
symbiotic relationship where one symbiont has been incorporated within another. The theory
gained strong support in the 1980s from genetic analyses. These revealed that mitochondria and
chloroplasts have their own DNA independent from the DNA in the eukaryotic nucleus.
Subsequent intracellular gene transfer led to some swapping of genetic material between
mitochondria and nuclear DNA. These results have led to a model for the evolution of eukaryotic
cells illustrated in Figure 18.
Chapter 9, devo pass, December 2010 34
Figure 19 – Evolutionary steps to the Eukaryotic Cell by means of Endosymbiosis In Figure 19, we see steps that could have led to the origin of the third and final domain
on our phylogenetic tree of life. In one model, bacterial and Archean prokaryotes fused to form a
eukaryotic cell without mitochondria. It is widely accepted that the next steps included the
endosymbiotic ingestion of an aerobic respiring bacterium as the ancestor of mitochondria and a
photosynthesizing cyanobacterium as the predecessor of chloroplasts.
With the advent of oxygenic photosynthesis and respiration, and the endosymbiotic
fusion of organisms into the eukaryotic cell, the stage was set for the leap to complex life forms
like us. The subsequent advance is also a model of the evolutionary power of collaborative
networking between cells. A glimpse of the evolutionary pathway by which it advanced is
recorded in the late Proterozoic fossil record and in genetic comparisons between modern
descendants of organisms that diverged from common ancestry about a billion years ago.
Chapter 9, devo pass, December 2010 35
9.6TheEukaryoticCellbecameestablishedinProterozoictimesasthe
ProgenitorofComplexLife
The earliest biomarkers for eukaryotic cells are found in 2.7 billion-year-old sedimentary
rocks in Western Australia. This is a surprisingly early time, because it predates the full
oxygenation of the atmosphere. Microfossil acritarchs have been identified in rocks that are 2.1
billion years old. The name acritarch has been given to organic-walled microfossils that occur in
Proterozoic shales and sandstones. They come in a variety of shapes and sizes like those imaged
in Figure 20. They are usually considered to be eukaryotic cysts or spores from microscopic
algae that lived in the well-lit surface layers of oceans, seas, or lakes. Their size (greater than 25
μm) implies that they are eukaryotes. Large spheroidal acritarchs are reported in Archean tidal
sediments from 3.2 billion years ago. If these are eukaryotic, they push the time of origin much
further into the past than was previously thought. It was not until the middle to late Proterozoic,
however, that acritarchs became the most abundant eukaryotic fossil in the geologic record.
Figure 20 - Acritarchs from late Proterozoic sedimentary rocks. "Dictyodium" is pictured at left. It
has a thick, complex wall that is unknown in prokaryotic cells. A different acritarch of similar size
is pictured at right.
Chapter 9, devo pass, December 2010 36
Evidence for multicellular eukaryotes first appears as carbonaceous impressions of algal
ribbons wound into loose coils (see Figure 21). They are classified as eukaryotes because of their
relatively large sizes and distinctive shapes. These include disks, ellipses, ribbons, sausage-like
shapes, leaf-like shapes, and irregular shapes. The diversity and complexity of these fossils
increase in the late Proterozoic. More than 100 kinds have been classified.
Figure 21 - Grypania Spiralis, a fossil algal ribbon that is visible to the naked eye. Earliest examples
are found in 2.1 billion-year-old rocks. It is thought to be Eukaryotic in origin, partly because of its
relatively large size.
Animal-like fossils of unicellular eukaryotes are pictured in Figure 22 appear in 750-
million-year-old rocks of the late Proterozoic. They resemble the skeletons of modern amoebas.
These are the first unambiguous fossils of animal eukaryotic cells. Their proliferation coincides
with the final rise of Proterozoic oxygen depicted in Figure 15. Multicellular animals, called
metazoans, probably existed at this time as well. As we will see in the next section, fossils
appear in the late Proterozoic record that look startlingly like primitive animals.
Chapter 9, devo pass, December 2010 37
Figure 22 - Fossil skeletons of animal eukaryotes from 750 million years ago. They resemble those
of modern skeletonized amoebae.
The rise of oxygen at the end of the Proterozoic is marked by an increased proliferation
of fossil organisms. The aptitude of eukaryotes to develop into multicellular life seems to have
accelerated. An increase in the diversity of acritarchs proceeded apace but was temporarily
halted during the Snowball Earth episodes of the last few hundred million years. In contrast, the
diversity of stromatolite communities of prokaryotes began to decline about one billion years
ago. This may have been due to the rise of grazing metazoans. These trends marked the
replacement of prokaryotic dominance in the fossil record with a new eukaryotic regime.
The Origin of Metazoans
9.7 Fossils trace the Rise of Metazoans at the end of the Proterozoic Eon
The fossil record of the earliest metazoans is stymied because early animals were softbodied and difficult to preserve. Many ancient organisms may have had shapes with no modern
counterpart. These may also be difficult to distinguish from unidentified sedimentary structures
with no biological origin. Proterozoic metazoans appear to have left trace fossils, however.
Sometimes called ichnofossils, these are geological records of biological activity. They often
Chapter 9, devo pass, December 2010 38
consist of impressions made on sediments, such as burrows, borings, footprints, or other
indicators of passing motion such as the dragging of a tail. Bioturbation is a prominent example
in which sediment was disturbed by digging organisms. The earliest claims of Proterozoic trace
fossils are also controversial. It is often difficult to tell whether a sedimentary structure was left
by some unknown organism or is the result of some unknown inorganic process.
The earliest animal fossils with widespread acceptance are members of the Ediacaran
Fauna. A fauna is a collection of associated animals, while a flora refers to a similar collection
of plants. The place name refers to the Ediacara Hills in Australia where spectacular examples
have been discovered. The name is also given to a geological period (the Ediacaran Period) that
marks the end of the Proterozoic Eon, from 635 to 542 million years ago. These fossils represent
a wide variety of soft-bodied marine animals with forms that resemble jellyfish, worms, and sea
pens. Others have no resemblance to any living creature. Examples are shown in Figure 23.
Chapter 9, devo pass, December 2010 39
Figure 23 - Fossils of the Ediacaran Fauna from the late Proterozoic that may be forerunners of
modern life or that may have been evolutionary dead ends. Tentative interpretations that have been
put forward are as follows: A) Charnia - probably stood upright on the sea floor. B) Disks that may
represent holdfasts that attached the bases of organisms to the sea floor. C) Dickinsonia - a flat
creature that may have been an ancestor to polychaete worms. D) Tribrachidium - may be related to
echinoderms like sea stars and sea urchins. E) Mawsonites - perhaps a sea anemone, F) Spriggina an arthropod ancestor, G) Kimberella – a trace fossil of a foot imprint made by what may have been
an early mollusk.
The mobility of Ediacaran organisms is recorded by ichnofossils from the same rocks.
Figure 24 shows a representative example of horizontal burrows made by an Ediacaran
organism. We don’t know what organism it was, but its traces signal the flourishing of
metazoans that wriggled beneath the floor of a shallow ocean.
Features are found in rocks older than a billion years that have been interpreted by some
as pre-Ediacaran trace fossils. This would signal a much earlier date for the origin of metazoans.
Molecular-clock data do not rule out the possibility, because they provide a range of dates from
Chapter 9, devo pass, December 2010 40
1.5 to 0.7 billion years ago for the first divergence of early metazoans. However, the biological
origin of these features is not universally accepted.
Figure 24 – Horizontal Burrows are widely accepted trace fossils showing the motions of Ediacaran
Metazoans
As we will discuss more fully in Chapter 11, modern metazoans begin life as a single
fertilized cell that divides and organizes itself as a developing embryo. The earliest cell divisions
produce configurations of cells that are remarkably similar for metazoans that may differ widely
as adults. Fossil embryos are found in rocks from China that date to about 580 million years ago
and are pictured in Figure 25. These show the earliest evidence for bilateral symmetry, a
property of organisms with a left and right side that are mirror images of each other. Bilaterally
symmetrical organisms tend to be relatively fast moving and skilled at finding food and escaping
predators. Their body shape contrasts with animals with radial symmetry pictured in Figure 22d)
and e). These animals tend to be slower moving or even free-floating.
Chapter 9, devo pass, December 2010 41
Figure 25 – Fossil embryos in Chinese rocks from 580 million years ago predate the Ediacaran
Fauna. A) Small groups of embryonic cells that probably belong to animals with bilateral
symmetry. B) Vernanimalcula – A small multicellular organism with undisputed bilateral
symmetry. They appear to have an inner, middle, and outer cell layer as well as a tube through
which food was passed for digestion.
The Ediacaran Fauna and fossils that immediately predated them share some
characteristics of modern animals. It would appear that modern metazoans got their start at this
time. But were Ediacaran Fauna truly our ancestors or were they a false start, a kind of
evolutionary dead end? To answer this, we need to make a closer comparison between Ediacaran
features and those of the modern organisms they most resemble.
9.8 Three of Four Key Animal Groups appeared by the End of the Proterozoic
Well over a million kinds of animals have been identified on Earth today. Yet these can
be broadly grouped into a small number of categories of evolutionary relatedness. The simplest
metazoans are aggregations of similar cells with a few cell types but no specialized tissues and
very little coordination of cellular activity. Sponges like the one illustrated in Figure 26 are a
familiar example and may represent the earliest metazoan for which we have some geological
Chapter 9, devo pass, December 2010 42
evidence. Biomarkers for sponges are found before the Ediacaran period. Sponges have
calcareous or silica spines called spicules that have been reported in rocks as old as 750 million
years. Well-preserved fossil sponges are found in 580-million-year-old rocks. This simple
metazoan probably preceded the more complex groups discussed below.
a)
b)
Figure 26 – a) Body plan of a simple sponge. Sponges strain bacteria and small algae floating in the
water as their main food. Some secrete calcium carbonate or silica to form small spicules that have
been preserved in the fossil record. b) The Venus' Flower Basket, (Euplectella aspergillum), is a
modern sponge that is anatomically similar to sponges that lived during the late Proterozoic Eon.
Most of the animals with which we are familiar differ a great deal from sponges. For one
thing, they are made up of many specialized cell types that develop from the same embryo and
reside in groupings that carry out a unique function. These tissues make up different kinds of
organs like skin, muscles, and nerves. Animals with well-defined tissues and specialized cells
are known as eumetazoans. They begin life as an embryo like the fossils displayed in Figure 25.
There is an important distinction between eumetazoans with radial symmetry (Radiata) and those
with bilateral symmetry (Bilateria). We see in Figure 23 that both had appeared by the end of the
Proterozoic.
Chapter 9, devo pass, December 2010 43
Bilaterally symmetrical animals have complete digestive tracts with mouth, anus, and an
internal body cavity. They are divided further into two groups, depending on whether the gut
develops front to back or from back to front. In one (the protostomes), the mouth side develops
first. The mouth develops last in the other (deuterostomes). Even though this property can only
be observed during embryonic development, it is a fundamental division in evolutionary
relatedness. Molecular clock studies suggest that these two broad lineages diverged at about 630
million years ago, well before the formation of Ediacaran fossils. A hypothetical reconstruction
of the common ancestor (Urbilateria) is pictured in Figure 27. Perhaps an animal like this
created the fossil burrows in Figure 24.
Figure 27 – “Urbilateria” is a hypothetical ancestor of bilateral protostomes and deuterostomes
that has been reconstructed from common anatomical features and dated to the late Proterozoic
with molecular-clock techniques.
What are the general lessons of the Proterozoic Eon for life in the universe? During this
time, Earth’s atmosphere was transformed by life itself. The ability to produce, tolerate, and
eventually use oxygen was critical for the evolution of larger cells and multi-cellular forms.
Chapter 9, devo pass, December 2010 44
The central role of oxygen is perhaps an explaining factor for complex life, but it also
raises a question. If oxygenic photosynthesis and respiration are necessary, how likely are they to
arise elsewhere under similar conditions? Fortunately, oxygen leaves its spectroscopic signature
in the atmosphere. The answer to our question may be forthcoming with advances that let us
observe the spectra of Earth-like planets around other stars.
End of Chapter Materials
KeyWords: All terms listed in bold. Definitions and citation page numbers could follow here as well
as in an end-of-book glossary. Glossary will also include italicized terms of importance that are secondary
to “key words.”
Bioturbation
Carbonate-Silicate Cycle
Cell Nucleus
Cell Wall
Chloroplasts
Chormosomes
Ediacaran Fauna
Embryo
Endosymbiosis
Eukarya
Eukaryotic Cell
Glaciers
Great Oxidation Event (GOE)
Horizontal Gene Transfer
Invagination
Metazoan
Mitochondria
Negative Feedback Loop
Organelles
Organic Carbon Cycle
Positive Feedback Loop
Ribosomes
Snowball Earth
Chapter 9, devo pass, December 2010 45
Symbiosis
The Ice-Albedo Feedback Effect
Trace Fossils
Vacuole
Key Ideas:
Proterozoic Oxygen – Oxygen became a permanent constituent of Earth’s atmosphere in the Proterozoic
Eon. Evidence includes the cessation of BIFs, deposition of red beds, the disappearance of massindependent fractionation, and the disappearance of reduced minerals from sedimentary rocks formed
through transport mechanisms that operated in contact with the atmosphere.
Supercontinent Rodinia – The precursors to today’s continents merged at the end of the Proterozoic to
become a large supercontinent called Rodinia. It was fully assembled about 1.1 billion years ago, and
began to rift apart by 750 million years ago.
Carbonate-Silicate Cycle – This negative feedback cycle stabilizes greenhouse control of global
temperature in a cycle that alternates between the weathering removal of atmospheric carbon dioxide to
eventually form carbonate rocks, and return of carbon dioxide to the atmosphere through volcanoes when
these same rocks are metamorphosed at tectonic plate boundaries.
Snowball Earth – The hypothesis that Earth was completely covered with ice several times in the
Proterozoic Eon. Evidence that supports this hypothesis includes glacial deposits that were formed at
near-equatorial latitudes, cap carbonates, changes in carbon-isotope ratios, and theoretical expectations
based on the ice-albedo feedback mechanism.
Eukaryotic Cell – A cell distinguished by the presence of a nucleus. Eukaryotic cells also possess
organelles such as mitochondria, an endoplasmic reticulum and, in the case of plants, chloroplasts. These
cells are about 10 times larger than those of prokaryotes and form the basis of multicellular life.
Endosymbiosis – A process in which microbes engulfed by endocytosis remain alive and functioning
within their hosts to become part of a newly evolved organism. The evolution of ingested cyanobacteria
to become chloroplasts is one example.
Horizontal Gene Transfer – A process of DNA or RNA sharing that occurs between organisms that are
not in parent-descendant relationship. Horizontal gene transfer happens, for example, when genetic
material is inserted into target cells from viral entities.
Discussion Questions
1. Evaluate the evidence for a complete global freeze during the Proterozoic. Do you think the
Earth froze over completely or just partially? Why or why not?
Chapter 9, devo pass, December 2010 46
2. What kind of scientific tests have been used or could be used to test the endosymbiotic theory
of the origin of the eukaryotic cell?
3. What complications are introduced by horizontal gene transfer for the construction of
phylogenetic trees?
4. What factors might encourage or inhibit a Great Oxidation Event on another Earth-like
planet?
5. If life exists on other extra-solar planets, is it likely to be simple, complex, or even
intelligent? Consider the timescales for life’s transition from prokaryotes to eukaryotes to
metazoans and humans.
Review Questions:
1.
Which of the following is not an observation that supports the idea that oxygen levels
rose in the Proterozoic?
a. The disappearance of BIFs
b. The disappearance of MIF
c. The disappearance of stromatolites
d. The appearance of red beds
e. The disappearance of reduced minerals in sedimentary deposits
2.
What is a term used to describe the following general process? “A produces more of B
which in turn produces more of A.”
a. Negative feedback loop
b. Induction
c. Positive feedback loop
d. Enhanced causation
e. Autocatalytic deduction
3.
Which of the following is not an element of the carbonate-silicate cycle?
a. Carbon dioxide dissolved in water.
b. Silica weathering yields dissolved bicarbonate and silica ions.
c. Volcanoes replace carbon dioxide in the atmosphere.
d. Carbonate rocks form in the sea.
e. Ice and snow halts the process of silicate weathering.
4.
Which of the following is the name of a supercontinent that formed during the
Proterozoic Eon?
a. Pangaea
b. Laurentia
c. Gondwana
d. Rodinia
e. Panthalassa
Chapter 9, devo pass, December 2010 47
5.
Which of the following is a subset of the class of bilateral animals?
a. Deuteronomy
b. Stomata
c. Radiolaria
d. Deuterostomes
6.
Which of the following best refers to the Ice-Albedo Feedback Effect?
a. The lower that a planet’s albedo is, the more likely that ice will form in order to
compensate
b. The higher a planet’s albedo is, the more likely that ice will melt in order to
compensate.
c. The greater the fraction of Earth’s surface that is covered by ice, the more sunlight
will be absorbed, causing the temperature of the Earth to rise and melt some of the
ice, thereby reducing the effect.
d. The smaller the fraction of Earth’s surface that is covered by ice, the more sunlight
will be reflected, causing the temperature of the Earth to decrease, leading to the reformation of ice and thereby reducing the effect.
e. The greater the fraction of Earth’s surface that is covered by ice, the more sunlight
will be reflected, causing the temperature of the Earth to decrease so that even more
ice will form.
7.
Which of the following is NOT a sign of glaciation in ancient rocks?
a. Striations
b. Dropstones
c. Crossbedding
d. Till
8.
Which best refers to a property of remnant magnetic fields that is used by geologists to
infer paleolatitudes from paleomagnetic measurements?
a. Magnetic field lines with switched polarity
b. Magnetic field lines are nearly vertical with respect to the bedding plane
c. Magnetic field lines are nearly horizontal with respect to the bedding plane
d. The polarity of remnant magnetism on the seafloor shows alternate polarities in
stripes that run parallel to ancient spreading centers
e. Magnetic fields lines point to a north pole that is located where the current north pole
is.
9.
Which best describes the sequence of oxygen levels throughout the Proterozoic Eon?
a. A decrease in oxygen levels at the beginning followed by a steady rise throughout
b. The onset of low oxygen levels at the beginning and a sharp rise in levels at the very
end.
c. A sharp increase in oxygen levels at the beginning followed by a steady decrease
throughout
d. Levels that regularly alternate above and below an average level of 1% of today’s
values.
e. Onset of high levels at the beginning followed by near-disappearance of oxygen for
the entire period of glaciations (snowball earth) during late Proterozoic
10.
How would you interpret the presence of excess 13C in carbonate sediments, given the
discussion centering around Figure 13?
Chapter 9, devo pass, December 2010 48
a. A proliferation of life preferentially took in heavy, and its fossilized remains
dominated the signature of carbon in the sediments
b. A proliferation of life preferentially took in light, leaving an excess of heavy carbon
in the material that made up the remaining sediment.
c. A great extinction removed light carbon from surrounding sediments
d. A great extinction added heavy carbon to the surrounding sediments.
11.
Which of the following is the most efficient process at extracting chemical energy for use
by the cell?
a. Fermentation
b. Sulfate Respiration
c. Aerobic Respiration
d. Methanogenic Respiration
e. Photosynthesis
12.
Which best describes “endosymbiosis?”
a. An ecological relationship between different kinds of organisms
b. The transfer of genetic material between organisms that are not closely related
c. The formation of a cell wall
d. A process in which one organism engulfs another, yet both survive and evolve in a
symbiotic relationship
e. A double nuclear membrane that aids in protein transport within the cell
13.
Which of the following is the domain of life to which metazoans belong?
a. Prokarya
b. Bacteria
c. Archea
d. Eukarya
e. Grypania
14.
Which of the following refers to fossil impressions made on sediments, such as burrows,
borings, footprints, or other indicators of passing motion such as the dragging of a tail?
a. Dubiofossil
b. Ichnofossil
c. Ediacaran Fauna
d. Deuterostomata
e. Acritarch
15.
Which of the following is NOT an example of Late Proterozoic metazoan life?
a. Charnia
b. Mawsonites
c. Dickinsonia
d. Spriggina
e. Grypina Spiralis
Chapter 9, devo pass, December 2010 49
AdvancedQuestions:
1. How much would Earth’s temperature change in the absence of greenhouse effects if the
current ice cover of 10% disappeared?
2. If metazoans first appeared less than 1 billion years ago, what fraction of Earth’s history
was occupied by only single-celled forms of life?
Chapter 9, devo pass, December 2010 50