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). Geologicaland molecular evidence suggests that cyanobacteria evolved after various other types of bacteria,including anoxygenic photoautotrophs (organisms performing photosynthesis which do notproduce oxygen).

Figure 1: 3.5 Ga old microfossils (A) and stromatolites (B) from Western Australia (sources:A. Schopf, 1993; B. Van Krakendonk, 2006)

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 led to the formation of thick accumulations of red stratified sedimentaryrocks rich in iron oxide called Banded Iron Formations (BIFs, Fig. 2A). Once the oxidation
process was completed, oxygen started to accumulate in the atmosphere and gave rise to theGreat Oxygenation Event (GOE) 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 underoxidizing conditions confirms 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 bacteria, 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 of62

oxygen is probably the mechanism behind the emergence of eukaryotes.

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. The eukaryotic cell may have evolved from the ingestion bylarge anaerobic bacteria of small aerobic bacteria which later became a structural part of the larger
cells. The small aerobic bacteria may have survived inside the larger cells by using organic
molecules produced by their host. The larger anaerobic bacteria on the other hand may havebenefited from the ability of aerobic bacteria to use O2 and produce energy through aerobic
respiration. These aerobic bacteria may have evolved into the mitochondria of eukaryotic cells,organelles used for cellular respiration. In a similar manner, photosynthetic bacteria may havebeen ingested by larger cells and evolved into chloroplasts, organelles of plant and algal
eukaryotic cells which carry out photosynthesis. This scenario, which implies the evolution from a
predator-prey to a host-symbiont relationship, is called the endosymbiont hypothesis (proposedby 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.

thFigure 3: The Endosymbiont Hypothesis (source: Biology, Life on Earth, 9 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)12.3. Multicellularity
Once predation evolved, larger single-celled organisms 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 alsoless chance to be ingested by others. However, larger single-celled organisms require more63

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 asingle-celled organism (Fig. 5). Beyond such limit, the surface of the cell is not large enough topermit enough exchange to sustain cellular metabolism. The solution to grow larger withoutslowing down metabolic reactions is to adopt a multicellular body. Besides greater predation
efficiency and better protection against predator cells, multicellularity enables the evolution ofspecialized cells and body parts. For example, a multicellular alga can have roots to anchor itself
on the seafloor and leaves to harvest the Sun’s energy for photosynthesis.

thFigure 5: Surface area and volume relationships (source: Biology, Life on Earth, 9 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 US.
Multicellular organisms evolved shortly after the GOE once aerobic respiration provided the energy
needed to sustain more complex forms of life.

Figure 6: 1.2 Ga multicellular red algae Bangiomorpha pubescens from arctic Canada (A) and 1.6 Ga problematicGrypania spiralis from India (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. The first phase of animal diversification produced what is known as theEdiacara biota (Fig. 7). Ediacaran animals were the first large and complex forms of life. They had
a bilateral or radial symmetry and were mostly soft-bodied. Their taxonomic relationship withextant groups is still debated. They disappeared at the onset of the Cambrian period. Ediacara
marks the transition between two distinct biospheres: the Precambrian biosphere dominated bysingle-celled organisms (“the age of bacteria”) and the Cambrian biosphere comprising all known
animal phyla.

64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. All known animal
phyla were already present at the beginning of the Cambrian. This explosion of life forms is known
as the Cambrian radiation of life. The diversification of life and the emergence of modern phyla
had already begun in the Precambrian. For example, the first traces of Porifera (sponges) andCnidaria (anemones, corals…) are found in Precambrian rocks. The explosion of diversity aroundthe Precambrian-Cambrian boundary gave rise to an astonishing variety of body plans andarchitectures, many of which seem to have no equivalent today and may represent evolutionary“dead-ends” (Fig. 8).

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,
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).

12.5. The colonization of land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  6CO2 + 6H2O). Less O2 consumed by the decomposition of organic matter means
more O2 left in the atmosphere.
Pelagic organisms live in the water column. Benthic organisms live on or near the seafloor.

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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 of
predators 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 fishes. A good 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.

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 were followed bya period of diversification. Although the biosphere has been hit by 5 major mass extinction events,
the global species richness has increased through time.
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 years
later. 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 haveplayed 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 shows66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 covers
approximately 2 million km2.
What would be the impact of a catastrophic volcanic eruption 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).
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 which
may 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.
The Cretaceous-Tertiary boundary (known as the K-T or K-Pg boundary) is characterized by adistinctive layer that can be followed worldwide and which contains a high concentration of Iridium(rare in earth’s crust but abundant in meteorites) and components indicative of a meteoritic impact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.

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(shocked quartz, tektite, nanodiamonds). The crater associated with the K-T boundary has been
located near the Yucatan Peninsula in Mexico (Chicxulub Crater). The meteorite that produced theChicxulub Crater must have been at least 10 km in diameter. The consequences of such a large
meteorite impact on the environment are multiple and take place at various time and spatial scales(table 1).

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

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

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