12. Origin and evolution of life (part II)
The earliest form of life was probably a prokaryote. Unlike the eukaryotic cell, the prokaryotic cell
has a more simple structure with no nucleus and no membrane-bound organelle (organelle =specialized subunit of the cell). This early prokaryote must have been able to survive without
oxygen (anaerobic prokaryote). Anaerobic metabolism is carried out today by some species ofbacteria and archaea.
In the text below, I review some of the main stages of the evolution of life and the evidence fromthe geological record.

12.1. The Great Oxygenation Event (GOE)The free oxygen (O2) present in the atmosphere today was probably produced initially byphotosynthetic prokaryotes similar to modern cyanobacteria (the same microorganisms behind theformation of stromatolites, see chapter 7). The first unambiguous occurrence of cyanobacteria in
the geological record is in 2.15 Ga old rocks from Canada. Fossilized stromatolites andmicrofossils with an age of 3.5 Ga suggests an earlier origin (but controversial, Fig. 1B). Geologicand molecular evidence suggests that cyanobacteria evolved after various other types of bacteria,including anoxygenic photoautotrophs (organisms performing photosynthesis but which do not
produce oxygen).

Figure 1: 3.5 Ga old microfossils (A) and stromatolites (B) from Western Australia (sources:A. Schopf, 1993; B. Van Krakendonk, 2006)
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The oxygen produced by photosynthesis did not accumulate directly in the atmosphere. Oxygen, apowerful oxidizing agent, first reacted with various elements. Massive amounts of iron oxide(Fe2O3) precipitated in the oceans when Fe2+ (ferrous ions)-rich water mixed with O2. These“mass-rusting” events are believed to be responsible for the formation of thick accumulations of red
stratified sedimentary rocks rich in iron oxide that are now referred to as Banded Iron Formations(BIFs, Fig. 2A). Once the oxidation process was completed, oxygen started to accumulate in theatmosphere and gave rise to the Great Oxygenation Event (GOE), which is a sharp increase inatmospheric oxygen at around 2.4-2.3 Ga ago. The occurrence of fluvial deposits older than 2.4
Ga containing sediments composed of mineral grains that would not be stable under oxidizing
conditions is one evidence of the lack of free oxygen in the atmosphere prior to the GOE (Fig. 2B).
The GOE had a major impact on life. First, oxygen being toxic for anaerobic prokaryotes, the GOE
probably caused the first major mass extinction event. Second, the GOE led to a major innovation:
the aerobic metabolism. Cellular respiration using O2 as an electron acceptor produces much
more energy than anaerobic respiration and was an essential step to sustain the energetic needsof more complex forms of life. Third, the adaptation of large prokaryotic cells to the toxicity ofoxygen may be the mechanism behind the emergence of eukaryotes (see next section).

Figure 2: Banded Iron Formation (A) and 2.7 Ga old Witwatersrand conglomerate (South Africa) containing Pyrite
flakes and uraninite (B) (sources: A. Encyclopedia Britannica, B. Natural Museum of Humbolt State Univiversity)

12.2. The eukaryotic cell
Another important step in the evolution of life is the development of a new way to obtain carbon by
feeding directly on biological matter: predation. This innovation requires the cell to have a flexible,
non-rigid membrane to engulf its prey. This mechanism may have led to the evolution of eukaryotic
cells. Indeed, one hypothesis suggests that the eukaryotic cell may have evolved from theingestion by large anaerobic bacteria of small aerobic bacteria which later became a structural partof the larger cells. The small aerobic prokaryotes may have survived inside the larger cells byusing organic molecules produced by their host. The larger anaerobic prokaryotes, on the otherhand, may have benefited from the ability of aerobic prokaryotes to use O2 and to produce energy
2through aerobic respiration. These aerobic prokaryotes may have evolved into the mitochondria ofeukaryotic cells, organelles used for cellular respiration. In a similar manner, photosynthetic
prokaryotes may have been ingested by larger cells and evolved into chloroplasts, organelles ofplant and algal eukaryotic cells which carry out photosynthesis. This scenario, which implies theevolution from a predator-prey to a host-symbiont relationship, is called the endosymbiont
hypothesis (proposed by Lynn Margulis in 1966, Fig. 3).
The oldest fossils of eukaryotes are found in a 1.8 Ga old rock formation in China (Fig. 4).
Biomarkers indicate the existence of eukaryotes as early as 2.7 Ga. A recent molecular study
suggests that the oldest ancestors of extant eukaryotes may have emerged between 2.5 and 1.6
Ga ago, and then quickly diversified.

Figure 3: The Endosymbiont Hypothesis (source: Biology, Life on Earth, 9th edition)

Figure 4: 1.4 Gyr eukaryote from Canada (A) and 1.8 Gyr eukaryote from China (B)(sources: A. Butterfield, 2005, B. Lamb et al., 2009)3

12.3. Multicellularity
Once predation became established as a new strategy to obtain carbon, larger single-celledorganisms may have had an advantage over smaller ones. For a cell, it is easier to engulf a prey
which is comparatively smaller. Larger cells have also less chance to be ingested by others.
However, larger single-celled organisms require more exchange of matter through the cell
membrane (intake of O2 and nutrients, waste removal). Since the surface/volume ratio decreases
as the cell becomes larger, there is a limit to the size of a single-celled organism (Fig. 5). Beyond
such limit, the surface of the cell is not large enough to permit enough exchange to sustain cellular
metabolism. The solution to grow larger without slowing down metabolic reactions is to adopt a
multicellular body. Besides greater predation efficiency and better protection against predator
cells, multicellularity enables the evolution of specialized cells and body parts. For example, amulticellular alga can have roots to anchor itself on the seafloor and leaves to harvest the Sun’senergy for photosynthesis.

Figure 5: Surface area and volume relationships (source: Biology, Life on Earth, 9th edition)

In the fossil record, the oldest undisputed occurrence of a multicellular organism with a clear
affinity with an extant group is the red algae Bangiomorpha pubescens found in a 1.2 Ga old rockformation in Canada (Fig. 6A). An older fossils of unknown affinity and possibly derived from amulticellular organism is Grypania spiralis (Fig. 6B). The oldest specimen of G. spiralis is from a1.85 Ga old rock formation in the USA.
Multicellular organisms evolved shortly after the GOE once aerobic respiration provided the energy
needed to sustain more complex forms of life.
4Figure 6: 1.2 Ga multicellular red algae Bangiomorpha pubescens from arctic Canada (A) and 1.6 Ga problematic
Grypania spiralis from India (B) (sources: A. Butterfield, 2000, B. Butterfield, 2009)12.4. The Cambrian radiation of life
A sharp increase in animal diversity is recorded in the fossil record at the end of the Precambrian
600-550 million years ago. This is the first phase of animal diversification and the resulting group oforganisms is referred to as the Ediacara biota (Fig. 7). Ediacaran animals were the first large andcomplex forms of life. They had a bilateral or radial symmetry and were mostly soft-bodied. Theirtaxonomic relationship with extant groups is still debated. They disappeared at the onset of theCambrian period. Ediacara marks the transition between two distinct biospheres: the Precambrianbiosphere dominated by single-celled organisms (“the age of bacteria”) and the Cambrianbiosphere comprising all known animal phyla.

5Figure 7: Examples of Ediacaran fossils ― note the trackways on the bottom left corner (source: Narbonne, 2005)

Cambrian life is characterized by a major innovation: the shell or carapace, and a rapid andprodigious diversification of complex life forms. This explosion of life forms is known as theCambrian radiation of life. All known animal phyla were already present during the early
Cambrian. However, the diversification of life and the emergence of modern phyla had alreadybegun in the Precambrian. For example, the first traces of Porifera (sponges) and Cnidaria(anemones, corals…) are found in Precambrian rocks. The explosion of diversity around thePrecambrian-Cambrian boundary gave rise to an astonishing variety of body plans andarchitectures, some of which are difficult to link with today’s life forms, and have even beeninterpreted by some researchers as potential evolutionary “dead-ends” (Fig. 8).

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Figure 8: Examples of early Cambrian animals (source: Steven J. Gould, 1989)

A combination of factors is probably responsible for the Cambrian radiation of life. Here are several
factors which may have played an important role:➀ Environmental factors
 End of the Proterozoic glaciations
 Rise in atmospheric O2 providing an energetic advantage
 Change in ocean chemistry facilitating biomineralization② Genetic factors
 Emergence of key developmental genes resulting in new evolutionary possibilities③ Ecological factors
 Adaptations related to a predator-prey relationship. For example, preys evolved traits to avoid
being caught by predators, such as a shell or a carapace, a pelagic mode of life, or body parts
which improved their mobility. Predators on the other hand evolved traits to help them catch preys, Some scientists suggest that this second “oxygenation event” may have been triggered by the break-up of the supercontinent

Rodinia. The break-up of Rodinia created more extensive continental margins on which large amounts of organic matter could beburied and escaped decomposition (C6H12O6 + 6 O2 → 6CO2 + 6H2O). Less O2 consumed by the decomposition of organic mattermeant more O2 left in the atmosphere.
 Pelagic organisms live in the water column. Benthic organisms live on or near the seafloor.

7such as improved sensory organs (e.g., the eye). The evolutionary interdependence between
different groups of organisms is referred to as coevolution (distinct groups of organisms caninfluence each other’s evolution). This may have led to rapid biological diversification.

12.5. The colonization of land
The earliest multicellular fossils of land plants are found in rocks of Ordovician age approximately
475 million years old (Fig. 9A). The advantage of a terrestrial mode of life for the earliest land
plants was threefold: (1) abundant sunlight, (2) abundant nutrients, and (3) absence of predators.
The arthropods (crabs, spiders, scorpions, centipedes…) were the first animals to colonize theland. The earliest fossil evidence is based on trackways preserved in Late Cambrian-Early
Ordovician eolian sandstone in Canada (Fig. 9B). Optimal environmental conditions and the lack ofpredators enabled some arthropods to reach a gigantic size during the Carboniferous (e.g., 70 cm
large dragonflies, 2 m long millipedes). The first terrestrial vertebrates (tetrapods) evolved from agroup of Silurian fish. An example of an early tetrapod is Ichtyostega which lived during theDevonian period (Fig. 9C). Reptiles appeared during the Carboniferous. In this group, thedinosaurs became extremely diverse and some grew larger than any other land vertebrates during
the Jurassic and Cretaceous. They flourished for a hundred million years before vanishing 65
million years ago (see section 12.6.2.). Their only living descendants are the birds. The last 65
million years is known as the Cenozoic and is marked by the diversification of mammals. This
group includes the genus homo (us and our ancestors), the earliest representative of whichappeared around 2.3 million years ago.

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Figure 9: 475 million years old spores of an early land plant (A), Late Cambrian or Early Ordovician trackways ofarthropods, and a 3D image of a skeleton of Ichtyostega (sources: A. Wellman et al., 2003, B. McNaughton et al.,2002, C. Pierce et al., 2012).

12.6. Mass extinctions
The history of life is characterized by five major extinction events (Fig. 10). Each mass extinction
was followed by a period of diversification.
We will now focus on two of these mass extinction events: (1) the most severe of all: theend-Permian mass extinction; and (2) the most famous of all: the end-Cretaceous mass extinction.
12.6.1. End-Permian mass extinction
The end-Permian mass extinction occurred 250 million years ago. Roughly 90% of all species wentextinct. The crisis affected both marine and terrestrial ecosystems.
The Permian is a geologic period which began 300 million years ago and ended 50 million yearslater. The geography of the Permian is characterized by the presence of a single supercontinent
called Pangaea. The reduction of shallow marine environments enhanced the competition forresources and may have contributed to species extinction. However, other factors must have9

played a role to cause a mass extinction of such a magnitude. One of the most likely factors is a
catastrophic volcanic eruption which disrupted global climate. Such an intense and long-lasting
eruption has never been witnessed in human history. However, the geological record shows
evidence that gigantic eruptions have occurred several times in the past. These eruptions haveproduced tremendous amounts of basalt called flood basalt. Flood basalts cover extensive
regions of the crust called flood basalt provinces. One such province, the largest one on acontinent, is found in Siberia and its age coincides with the end-Permian extinction: the Siberian
flood basalts (or Siberian Traps) which have accumulated during a million years and cover
approximately 2 million km2.
What was the impact of such catastrophic volcanic eruptions on global climate?
➀ On the short term: massive amounts of volcanic ashes released in the upper atmosphere would
reflect some of the incoming sunlight and cause a short-lived global cooling.
② Acid rains would result from the reaction between volcanic gases and condensing water
droplets (H2SO4, HCl, HF) present in the atmosphere.
③ On the long term: the release of massive amounts of CO2 in the atmosphere would trigger aglobal warming. The consequences of global warming are complex.
For example, global warming may disrupt the ocean circulation and prevent the renewal of O2 inthe deep ocean. The warming of seawater also decreases the solubility of O2 in the oceans.
Moreover, the stability of methane hydrate present in deep sea sediments and in high-latitude
permafrost may be affected by global warming. Methane hydrate is composed of water ice withmolecules of methane trapped in the crystal lattice of the ice. Methane is mainly produced bymethanogen archaea through anaerobic respiration involving the byproducts of the decompositionof organic matter. During global warming, there is risk of destabilization of methane hydrate whichmay lead to a massive input of methane in the atmosphere. Methane is a very potent greenhousegas which would further enhance global warming and amplify the destabilization of methane
hydrate, creating a positive feedback loop.
12.6.2. End-Cretaceous mass extinction
The end-Cretaceous mass extinction 65 million years ago is the most famous mass extinction
event because it caused the extinction of dinosaurs. Unlike the other mass extinction events, thecause of the end-Cretaceous mass extinction is relatively well constrained. There are several
convincing lines of evidence supporting the hypothesis of a large meteorite impact as the maincause.
 One of the main source of deep ocean water is in the North Atlantic where cold and salty (dense) water sinks to the bottom of

the ocean and begins its journey to the south hemisphere (= thermohaline circulation). If the ocean surface warms, thedownwelling of O2-rich surface water in the North Atlantic and elsewhere may be disrupted and prevent the oxygenation of thedeep ocean.

10The Cretaceous-Tertiary boundary (known as the K-T or K-Pg boundary; Pg stands for thePaleogene period) is characterized by a distinctive sedimentary layer that can be followed
worldwide and which contains a high concentration of Iridium (rare in earth’s crust but abundant inmeteorites) and components indicative of a meteoritic impact (shocked quartz, tektite,nano-diamonds). The crater associated with the K-T boundary has been located near the Yucatan
Peninsula in Mexico (Chicxulub Crater). The meteorite that produced the Chicxulub Crater musthave been at least 10 km in diameter. The consequences of such a large meteorite impact on theenvironment are multiple and take place at various time and spatial scales (Table 1).

Figure 10: Evolution of biological diversity over the past 600 million years (source: Understanding Earth 6th edition)

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Table 1: Environmental effects of the meteorite impact at the K-T boundary (source: Toon et al., 1997)12