When did life arise on Earth?

Radiometric dating showed that this zircon crystal was 4.1 Gyr old and the carbon inclusion exhibited a larger ratio of  ¹²C to ¹³C. Was this the chemical stain of primordial life, dating back to 4.1 Gya? On Earth today, this would be a good indicator of organic material. But, interpreting this result from so long ago is more controversial.

Stromatolites today are composed of layers of microbial mats of photosynthesizing cyanobacteria, as shown in the YouTube video below. Photosynthesis depletes carbon dioxide in the water, precipitating calcium carbonate deposits, which along with other sandy sediments, is trapped in the sticky bacterial film. The bacterial colonies grow upward towards better exposure to the sun, and over time the layers of bacterial film and mineral precipitates build up to form distinctive layered stromatolites. Similar stromatolite structures are formed today.

The Age of Life

Evolution and Extinction

On geological time scales, time is organized into eons, eras, and periods. There are four main eons subdivided into different eras. The eras are in turn split into different periods. The current eon, the Phanerozoic Eon (541 Mya through today), is known as “The Age of Life.” The Cambrian Period is the earliest period of the Phanerozoic eon, lasting from 540 to 500 Mya. The events leading up to this period set the stage for an explosion of complex life. Changing atmospheric oxygen concentration allowed for more efficient aerobic metabolism and increasingly complex life forms.

The transitions between different geologic periods are defined by changes in the fossil record with many of the most dramatic changes occurring during mass extinction events, defined as a period of time when the rate of species going extinct is increasing with respect to the rate at which new species arise. Throughout the Phanerozoic era, there were 15 documented major extinction events. Five of these were major mass extinction events where more than half of all species on Earth at the time were lost. While mass extinctions are devastating, they do serve to encourage diversity. Had the dinosaurs not been eliminated 65 Mya, there may never have been such a successful emergence of mammals. The frequency of life in the Universe is a function of how often it arises, how often it survives, and how long it endures after evolving. In this way, the frequency and intensity of mass extinctions control the frequency of life.

The Proterozoic Eon began 2.5 billion years ago and set the stage for Phanerozoic life. From the geologic record, we know that oxygen levels were low before the Proterozoic. Therefore, microbes from the start of this era would have primarily been anaerobic chemotrophs, producing energy through chemical pathways.

The Proterozoic Eon was likely a time of rapid diversification of life. Today, there are systems of proteins that run checks during the replication of DNA to minimize transcription errors. However, these types of proteins are less common in prokaryotic microbes, which were common during the Proterozoic Era. A higher rate of mutations would have accelerated natural selection and driven a greater diversification of organisms. One of those mutations enabled photosynthesis, a more efficient process of respiration that produces oxygen as a byproduct.

The Rise of Oxygen

The rise of oxygen likely began with organisms known as cyanobacteria, an early type of photosynthetic bacteria thought to be the first organism to produce oxygen as a byproduct. Photosynthesis is the process by which organisms can harness the energy of the sun to generate energy for their own use. Cyanobacteria are the only known prokaryotes to produce oxygen, and this adaptation brought about the destruction of many other organisms.

The rise in atmospheric oxygen was far from smooth and steady (see Figure 6 below). Cyanobacteria slowly became more abundant, but there was still a significant delay in the build up of oxygen. There are several processes that would have hindered the rise in oxygen. Oxygen would have reacted with various chemicals, mainly iron, in the oceans, and these reactions would trap oxygen, prevent it from building up the atmosphere. Once it was able to escape the ocean, oxygen also reacted with rocks. The result is that it took about two billion years for atmospheric oxygen to rise to a level similar to today.

Oxygen may also have been taken up by microbes in metabolic pathways that generate energy. Organisms that used the oxygen to oxidize ammonia appear to have been plentiful at the time and could have been effective in the reduction of free-floating oxygen. Other organisms produced methane as a byproduct that could have acted as a sink for atmospheric oxygen. However, organisms that are known to produce methane require nickel to carry out the necessary reactions, and concentrations of nickel were dropping. The decrease of nickel would mean less excreted methane, and would allow oxygen to begin accumulating.

The increase of oxygen in the atmosphere was poisonous for anaerobic organisms. Most life was anaerobic, so the rise of atmospheric oxygen surely resulted in one of the most significant extinction events in Earth’s history. The fossil record shows a mass extinction of anaerobic life around 2.4-1.6 billion years ago, coincident with the rise of aerobic life.

Anaerobic vs Aerobic Life

The rise of atmospheric oxygen coincided with the appearance of far more complicated life forms. There is every reason to believe that the rise in oxygen would have been responsible since aerobic metabolisms are more efficient.

Respiration, in biological terms, describes the process by which organisms convert nutrients into usable energy by forming ATP bonds. Respiration begins with glycolysis, wherein glucose, a sugar, is broken down to form two molecules of a compound called pyruvate along with two molecules of ATP. Without oxygen, organisms have no choice but to undergo anaerobic respiration, or fermentation. Anaerobic respiration of yeast is what makes bread rise and beer bubbly. In this scenario, pyruvate is shuttled down a pathway which produces just two molecules of ATP.

In the presence of oxygen, aerobic respiration becomes possible. With the help of oxygen, pyruvate can be broken down and enters a more complicated pathway known as the Krebs cycle or the citric acid cycle (note that organisms exhibit an enormous variety of metabolic pathways; the citric acid cycle merely represents one of the more common and well understood pathways). Through the citric acid cycle, organisms can produce from 30–36 ATP from just one molecule of glucose. Though more complicated to assemble, and therefore likely taking longer to evolve, this process can be up to 18-fold more efficient than anaerobic respiration. With more energy, it is possible to carry out more biochemical processes. This may have allowed cells to become increasingly complex and trend towards the more varied, multicellular life we see today. The rise of oxygen therefore enabled an explosion of complex multi-cellular organisms that appeared at the start of the Cambrian geologic era.

The Cambrian Explosion

At the time of the Cambrian explosion, there was an exponential increase in the number and complexity of organisms. Complex organisms likely preferred living in the oceans at first, where there was easy mobility and easy access to nutrients. Even very shallow levels of water provide protection from damaging UV radiation.

The earliest evidence of complex, land-based life comes from early land plants that begin to appear in the fossil record around 475 Mya. DNA evidence suggests that these plants evolved from algae, a general term for aquatic, photosynthetic organisms. It is a likely transition given that many breeds of algae prefer growing in shallow water to begin with. Algae adapted protections, such as thicker cell wells in order to survive dry spells, that would have been beneficial in the transition from water to land.

With other organisms still constrained to the water, land plants flourished and grew large in size. The lack of competition further allowed for increasing complexity: the development of root systems and tubes to transport water and nutrients throughout the plant. Photosynthesizing land-based plants had abundant energy from the Sun.

The Carboniferous Period

Forests began to cover the lands and dead organic matter began to pile up and decompose among the plants. Millions of years later, this organic matter would be compressed into coal, earning this period the name “carboniferous,” which means “coal carrying” in Latin.

The Carboniferous period began roughly 360 Mya and lasted about 60 million years. It was marked by an era of giantism. The fossil record from early in this period is rich in ocean and fresh water organisms. Land-based organisms appeared in the middle to later part of the period. The first terrestrial organisms were supersized insects and amphibians. By the middle of the Carboniferous period, adult amphibians were up to 6 meters long and developed the scaly skin typical of lizards by the end of the period. The Arthropleura, which resembles a 2.6-meter long millipede, proliferated during the late Carboniferous period and remains the largest-known land invertebrate ever discovered.

The Carboniferous period also gave rise to the largest-known flying insect: the Bolsover dragonfly. The Bolsover dragonfly looks nearly identical to the dragonflies common today, but with a wingspan of 30 cm – six times larger than a modern dragonfly. With a wingspan this large, the Bolsover dragonfly would not be able to fly in today’s atmosphere, with 21% oxygen. This has been used to suggest that atmospheric oxygen levels may have been as high as 30% during the Carboniferous period. Increased levels of oxygen as well as a generally moister environment would explain the large size of organisms at the time. Respiration was likely more efficient so that organisms could produce more energy and sustain their larger sizes.

As life continued to evolve and diversify, ecosystems began to resemble the types of settings we are familiar with today. This penchant for giantism continued into the Mesozoic era, which spanned from 252 to 66 Mya and is known as “the Age of the Dinosaurs.” The Cenozoic era that followed saw the emergence of mammals. The end of each period during the Phanerozoic Eon was marked by mass extinction events that completely upended the biological landscape of the time.

Mass Extinctions

Though organisms change and evolve, the most dramatic changes occur during mass extinction events, defined as a period of time when the rate of species going extinct is increasing with respect to the rate at which new species arise. Throughout the Phanerozoic era, there were 15 documented major extinction events. Five of these were major mass extinction events where more than half of all species on Earth at the time were lost.

Mass extinctions can be either gradual or sudden. The process of extinction follows five different phases:

1. First is the extinction phase, which features a rapid decrease in biotic diversity.
2. The second phase is the survival phase. Diversity is at a minimum here, but there is little further extinction. Organisms that survive past the second phase are known as the holdover taxa.
3. Third is the rebound phase. Diversity begins to slowly increase again as ecosystems recover from the extinction event. The term progenitor taxa is used to describe species that have survived and now provide the evolutionary seeds for future organisms.
4. The fourth phase, the expansion phase, is marked by a rapid increase in diversity due to the evolution of new species. The increase in diversity is helped in part by the opportunities and voids left by the many species lost during the first phase.
5. The last phase, a recovery interval, is marked by a long period of environmental stability as diversity continues to rise. Disaster taxa describe species that proliferate most successfully following an extinction event. They are typically small and simple, hardy rather than complex and therefore capable of surviving extinction events.

Evidence for extinction events come from sweeping changes in the fossil records at different geological changes. However, fossils provide an incomplete and often confused record of past life. Roughly one third of organisms feature calcified parts that could be turned into fossils. The erosion of sedimentary rock may also obscure how old a fossil really is. Our understanding of mass extinctions is therefore inversely proportional to their age (older events have less certain evidence), and is proportional to the extent of the extinction. Extinction events that caused a greater loss in diversity are easier to categorize.

While mass extinction events threaten the very presence of life on Earth, they also lead to enhanced evolutionary diversity. It is important to understand these ubiquitous pressures on life, as similar events are likely to occur on every planet. If life is easily and regularly eliminated through frequent extinction events, then it may make finding surviving life in the Universe much harder.

The Ordovician-Silurian (O-S) Extinction

The Ordovician-Silurian (O-S) extinction, which occurred 450-440 Mya, describes two events that together resulted in the loss of nearly 70% of the worlds species at the time, making this the second-largest mass extinction event. The extinction event had a global effect and particularly affected marine life, where it is estimated that nearly 85% of species were lost.

The event was most likely caused through sudden climate changes. The timeline matches up with a shift in the then super-continent, Gondwana, into the south pole. As Gondwana passed over the south pole, it began to form ice caps across its surface. This shift also exposed more land, causing a drop in CO₂ levels through weathering and cooling the planet.The cooling of the Earth led to the formation of glaciers, which locked up waters from the ocean. This, in turn, caused sea levels to drop, exposing and destroying shallow-water habitats along continental shelves. Evidence for glaciation during this time period has been found in the Sahara Desert. The combination of glaciation and cooling is thought to be the main causes of the O-S extinction.

The event ended when volcanic out-gassing of greenhouse gases increased the atmospheric temperatures enough to melt the glaciers and stabilize sea levels. The rebound phase of the O-S extinction resulted in increased biodiversity on the re-flooded continental shelves. However, recovery species in the ocean and land plants of the time were initially less complex.

The Late Devonian Extinction

At least 70% of all species were also lost during the Late Devonian extinction, which lasted from 375-360 Mya. There is some evidence that this extinction event was actually a series of seven,distinct extinction pulses. As with the O-S extinction, marine life was particularly hard hit.

The Late Devonian extinction is thought to have been caused by changes in the sea level, triggered by global cooling combined with acidification and oxygen depletion in the oceans similar to the O-S extinction, changing sea levels, or oceanic volcanism.

The rapid evolution of more complex plants from 30 cm up to 30 m during this period likely contributed to the oxygen depletion in the ocean. The increase in plant mass required the development of extended root and vascular systems in plants. These roots would have stabilized increasingly deeper layers of soil, causing a change in the chemical composition of which nutrients were run off into rivers, lakes, and eventually the ocean. This effect could have decreased the amount of oxygen in the oceans as well as reduced CO₂ levels, which would have contributed to global cooling.

Less oxygen in oceans also caused organic matter to be better preserved, preventing organisms from decomposing and recycling their nutrients. Instead, many organisms formed into oil, which was absorbed by the porous reef rocks common in the area. These oil deposits remain a major source of oil in America today.

The End Permian Extinction

The End Permian extinction (or Permian-Triassic or P-T extinction) occurred about 252 mya and is the single largest mass extinction event ever recorded. It is nicknamed the “The Great Dying.” Approximately 96% of all marine life was lost along with over 70% of land species, including everyone’s favorite prehistoric creature, the trilobite.

 

 

The cause of such a large extinction was most likely the final stages of the break up of Pangea. The shifting of continents caused molten rock to be exposed and increased volcanic activity, releasing volatile carbon, methane, and SO₂. The effect is actually quite similar to that of burning fossil fuels today. The Earth began to heat up under this new blanket of greenhouse gasses; it is thought that the temperature of the entire ocean increased by 2°C.

 

Warm ocean water transported heat to the poles, which in turn began to melt and release more greenhouse gases that had been frozen before. As the amount of CO₂ increased in the atmosphere, it likewise began to dissolve and accumulate in the oceans, causing oceans to acidify. Furthermore, the warmer ocean water was unable to hold as much oxygen. The combination of increasing acidity and decreasing oxygen likely gave rise to the grievous destruction of marine life during the P-T extinction.

The End Triassic Extinction

The End Triassic extinction occurred 200 Mya, with 70-75% of all species becoming extinct. Most of the large amphibians common at the time were eliminated, and dinosaurs were left with little competition, allowing them to flourish in the following Jurassic period.

The end Triassic extinction event was sudden, lasting less than 10,000 years. Many causes have been proposed, but none are definitive. Gradual climate change can explain some of the observed properties of the extinction. An asteroid impact could also explain some of the properties of the extinction, but a crater of the appropriate size and age has yet to be found. The extinction may also have been caused by an unlucky sequence of volcanic eruptions increasing the release of carbon dioxide and sulfur and triggering climate change. Increased amounts of volcanic compounds can be found in the rock layers from this period.

The Cretaceous-Paleogene (K-Pg) Extinction

The Cretaceous-Paleogene (K-Pg) extinction occurred 66 Mya, and 75% of species became extinct, including all land-bound dinosaurs. Dinosaur fossils are only ever present before this time in the rock records. The extinction had a surprisingly variable impact, with dinosaurs being greatly affected while mammals and birds survived to ultimately become the predominate life forms on the planet.

The rock record from this time exhibits an unusual amount of iridium with gold, osmium, and platinum. These elements are relatively rare on Earth, but are commonly found in meteorites. The amounts of these rare metals could have been contained in an asteroid that was 10-15 km in diameter. The energy from such an asteroid would be equivalent to the force of 100 million hydrogen bombs. Other evidence includes shocked quartz and spherical rock droplets, which require both high temperatures and great pressure to form. Widespread soot deposits also suggest enormous fires across land masses.

The Chicxulub crater, buried beneath the Yucatán Peninsula in Mexico has both the appropriate age and size that are expected of the asteroid that could have caused the devastation seen during the P-T extinction event. The crater stretches 150 km across and is approximately 20 km deep.

Extinctions and Life

While mass extinctions are devastating, they do serve to encourage diversity. Had the dinosaurs not been eliminated 65 Mya, there may never have been such a successful emergence of mammals. The frequency of life in the Universe is a function of how often it arises, how often it survives, and how long it endures after evolving. In this way, the frequency and intensity of mass extinctions control the frequency of life. On Earth, there have been 15 major mass extinctions in the last 500 million years. Five of those 15 eliminated more than half of all species on the planet.