The evolution of CO₂ on Earth

To begin at the beginning, in the famous words of poet Dylan Thomas, we must go back 4.5 billion years and reach into a past and a darkness that is beyond our field of vision and knowledge.

In cosmology, the Nebular hypothesis (1) is the most widely accepted model for the formation and evolution of our solar system. Scientists believe that, over time, gravity began its unwavering, relentless work and pulled together a molecular cloud, known as a solar nebula, made of gas and dust — possibly deriving from an exploded star. A young and bright burning new star, now called the Sun, was born from the death of an old star. As the Sun began its infant life, the flat disc of gaseous and dusty material surrounding it slowly condensed and eventually formed eight planets, amongst them our home, the Earth.

In proximity to the sun, the first four planets are Mercury, Venus, Earth, and Mars, which are also known as the terrestrial planets. The four outer planets in our solar system are two gas giants, Jupiter and Saturn, and two ice giants Uranus and Neptune.

Many will recognize how, particularly Mars and Venus, feature heavily in the public imagination regarding our fascination with extraterrestrial life. Think only of cultural representations, from HG Wells’ “War of the Worlds” from 1897 to David Bowie’s 1971 hit “Life on Mars?”.

This fascination is by no means coincidental. Their shared definition as terrestrial planets refer to Mars, Venus, and Earth, composed of either silicate rocks or metal. Venus and Mars are therefore also called Earths’ sibling planets. They all have volcanoes, weathered surfaces, and their atmospheres contain clouds and respond to the thermal forces of the Sun. The similarities between the original atmospheres on Mars, Venus, and Earth are striking. Earth’s atmosphere was rich in methane, ammonia, carbon dioxide, neon, and water vapor. At the same time, today, these gases make up only about 0.1 percent of Earth’s atmosphere, consisting of 78 percent nitrogen, 21 percent oxygen, and 0.9 percent argon.

As we now know from extensive research, Mars and Venus contain no life as their atmospheres are made almost entirely of CO₂. In many ways, studying them is to gaze into our distant past and the early stages of Earth’s history, where we catch a glimpse of our original atmosphere and the absence of all planetary life.

Earth Years and Human Years

And this is where it all begins — in the vast and unknown shadow lands before life on Earth was born. To understand our Earth, we must, in turn, begin with birth by initiated gravity some 4.5 billion years ago. This journey of the mind reaches into a time beyond comprehension for most of us. When looking at the future of Earth, we expect that the Sun will absorb Earth in approximately 7.5 billion years. Long before that, however, life on Earth will have ceased to exist, some 3.5 to 4 billion years from now. In other words, our living Earth dies after an expected 8 billion years of life (2).

If you imagine those 8 billion years as the expected lifespan of an individual human being — about 80 years — then our Earth is now a middle-aged person of 45.

We may, with greater ease, understand an almost unfathomable time span by translating the years of planet Earth into a different time scale, where 100 million years equal 1 ‘Earth Year’ (E.Y.) in the life of the Earth.

By the simple trick of changing the order of magnitude from 100 million to Years to Earth Years, Earth’s 4.5-billion-year life span so far can be transformed into 45 Earth Years. In other words, one billion years is equivalent to ten Earth Years, and we can abbreviate 4.5 billion years into 45 E.Y.

As such, Earth is 45 Earth Years old, and we will, for future reference, define Earth’s age being one E.Y., two E.Y., and so forth.

In reverse, when applying this E.Y. system, 100 human years are equivalent to 30 seconds in Earth’s age, and dinosaurs and humans were born within the same year in Earth’s 45-year-old life. Moreover, only a bit more than one full day in the life of Earth would equate to the time passed since the dating of the oldest skeletal remains of our species, the homo sapiens (3).

Early development of Earth’s atmosphere

Returning our focus to Earth’s infancy, scientists believe its atmosphere to have been much like that of Mars and Venus. For the critical change to occur, and for our planet to become a living habitat and conducive to life, water, Oxygen, and specific temperatures were required. The absence of such conditions meant that no life could form, and the eventual development was extremely slow. Gradually, though, and for reasons we are still unable to find answers to, a change took hold of the Earth and altered the course of our climatic evolution, enabling life to emerge.

Three sibling planets, born from the same matter and almost the same atmospheric composition, are today separated in identity by the crucial matter of life. Today, the atmospheres on both Mars and Venus are composed almost entirely of CO₂, while on Earth it is, in relative terms, inexistent. Why this enormous difference between these three almost identical planets? The answer is that unlike on Mars and Venus, particular conditions on Earth made it possible for life to originate.

Around the age of two Earth Years, water arrived on our planet, and three Earth Years later, the first definition of life on Earth emerged. The first life existed in the form of single-celled bacteria, known as Cyanobacteria.

While relatively simple and by no means what we today intuitively understand as life, these bacteria — microscopic and invisible to the naked eye — began a process that irreversibly changed the course of Earth’s life from those of its two sibling planets.

While simple, these blue organisms — hence their other name, blue-green algae — changed the entire composition of Earth’s atmosphere. They obtained their energy through photosynthesis — harnessing energy from sunlight to build up their structure. Much in the same way we use carbohydrates to run our bodies. One highly consequential by-product of the photosynthesis employed by the Cyanobacteria was Oxygen.

According to widely accepted scientific theory, Oxygen first appeared when Earth was 10 E.Y. old. The Cyanobacteria gradually released Oxygen into the ocean. In the beginning, dissolved iron captured this Oxygen. Upon contact with Oxygen, soluble iron turns into insoluble ferric iron compounds that sink to the seabed, and this created iron formations.

Go to Pilbara, a region in Western Australia and home to many of its Aboriginal people. You can stand in the dry air, your feet placed on the deep red Earth and run your hands across huge sedimentary rocks made of alternating oxides and iron-poor chert. These red-banded formations record the first process of photosynthesis which is still essential to life on our planet, and they are the physical manifestations of the first simple life on Earth.

Over approximately 10 E.Y., the ocean became oxidized, it eventually became saturated, and Oxygen started to build up in the atmosphere. This release of vast quantities of Oxygen into the atmosphere is also called the Great Oxidation Event (4). It is seen as one of the most significant climate events in Earth’s history. It marks a turning point in the evolutionary life of planet Earth, a changing of the guard — a shift in dominance from CO₂ consuming organisms to Oxygen consuming organisms, which impacted the environment fundamentally.

The increase of Oxygen significantly altered Earth’s atmosphere. While Oxygen is a source of life for us, it proved poisonous to the dominant CO₂-consuming organisms, and most Cyanobacteria became extinct. In the shallow water at Western Australia’s Hamelin Pool Marine Nature Reserve, traces of the first life on Earth can still be found in sediments, known as stromatolites. They exhibit various forms and structures — conical, domed, branching — but they all witness our beginnings and the first life on Earth. These early life forms were the first crucial steps in the atmospheric change now imperative to our very existence and survival.

As the stromatolites record, the Great Oxidation Event was a catastrophe to most life forms on Earth. Yet with one’s disaster in evolution comes another’s opportunity. The dying out of most Cyanobacteria opened up ecological ‘niches’ and enabled more complicated and, crucially, oxygen-consuming life forms to flourish.

We may intuitively still consider this new dominant species an extremely simple life. However, the change of dominance from CO₂-consuming to oxygen-consuming bacteria has enormous implications. An oxygen-consuming organism can form a much larger cell — almost 1,000 times the size of the early cells — so its potential is vast compared to that of the first cells known to us. However, all potential forms of life require conducive conditions to develop and thrive, and this atmospheric tipping of scales meant that a new kind of world was coming.

Snowball Earth

“A cold coming, we had of it […] the very dead of winter” are the opening words to one of T.S. Eliot’s poems (5). While Eliot doesn’t refer to geological changes, his poem speaks of the death of one world and the birth of another. After the Great Oxidation Event and the consequent dominance of Oxygen consuming life forms, another dramatic change to Earth was approaching at around 21 E.Y. of age. It has become known as the Huronian Glaciation, an event that initiated a shift from one climate to another, resulting in Earth becoming unrecognizable and hostile to the life that had taken hold here.

The Huronian Glaciation (6) occurred after all continents on the planet had collated around the Equator. Landmass reflects more sunlight than water, as the oceans more effectively absorb this energy source. All continents had gathered around the Equator, the area where most sunlight hits the surface of the Earth. This continental geometry resulted in increased sun reflection, and a gradual cooling down of the Earth had begun. The lowering of temperatures coincided with an oncoming ice age. The gathering of two such powerful phenomena resulted in the ice caps being spread much further around the globe, reflecting even more sun. A self-reinforcing cycle gained traction, and the growing ice further cooled the planet, increasing ice coverage and so forth.

A cold coming, indeed. The entire Earth was covered in kilometer thick ice and had become a bright white planet reflecting all light, with barely inhabitable conditions of approximately -50 degrees Celsius (-58 Fahrenheit).

The Huronian glaciation lasted for about 300 million years (3 E.Y.). Due to a slow stirring underneath its white exterior, intense pressure was building up from Earth’s core. Volcanoes, long silenced under the tremendous white glacial blanket, eventually broke the ice and its long cold grip on Earth.

This surging magma released enormous masses of lava and CO₂ into the atmosphere, raising the CO₂ levels to thousands of times of today’s content. These volcanic eruptions broke the cycle that once turned our Earth into a bright white planet and set the course for other extreme conditions, with temperatures changing from -50 to +50 degrees Celsius (-58 Fahrenheit to 122 Fahrenheit) (7).

During this time, nearly all bacteria died out — life could not sustain itself under such conditions and changes, and only a few organisms could adapt to these new conditions.

The few organisms that had adapted to the cold now faced such extreme changes in temperature that meant they could not sustain themselves. Sediments on Earth tell us a story of how this dramatic change caused the Earth to once again be almost entirely without a trace of life.

Modern life

Yet the forces of nature don’t only show themselves in strong and kilometer thick ice or through lava-spewing volcanoes. Quietly, resiliently, incredibly, some existence persevered and survived the unimaginable. Once again, our planet stood at the threshold of life. The eukaryotic cells contain the cores that structure and are the basis of the animals and plants of today. These cells have their beginnings here and were the dominant force on Earth for about 5 of its 45 Earth Years. These are the same cells we carry within us — with cores where our genetic material is stored and from which our entire being stems.

28 Earth Years after the birth of the Earth was the beginning of life as we know it.

Eukaryotic cells paved the way for multicellular life, such as fungi. To the untrained eye, fungi are quiet cohabitants on our Earth, but their emergence and continued life on our planet is a kingdom, with a dizzying diversity of color and shape. Fungi may be considered fragile, yet they have made the entire Earth their habitat.

They can be found everywhere, from deserts through to deep seas. They have lived on the Earth for its last 15 Earth Years — compared to us humans who, in turn, have existed here for about one day on the E.Y. scale.

A 10 Earth Years period of multicellular activity existed on Earth, where fungi and plant life increased in variety and complexity. We may have difficulty imagining a planet without animals, let alone humans, — yet the very basis for our existence, sexual reproduction, took place here in its earliest form, in fungi some 1.2 billion years ago. In the following 10 Earth Years, life as we know it today exploded.

Earth has at numerous points been formed and transformed by explosions. Scientists believe that our solar system was born from the after-effects of an exploding star; erupting volcanoes have shaped landscapes; they have even broken the Huronian 300-million-year glacial grip on the planet. But this time, the explosion shaping the Earth was evolutionary, in the form of the Cambrian Explosion.

When Earth was about 40 Earth Years of age, the gradual, slow evolution of multicellular life accelerated with such force that a mere five years in Earth’s life has seen the evolution of all life on the planet as we know it. Five Earth Years ago, the mollusks, from which the earliest fossils derive, arrived on Earth. Today, among these are as diverse a range of species such as cuttlefish, squid, snails, and slugs; followed by the dominant dinosaurs — amongst them the regal Pterosaurs — died out, 66 million years ago — or seven months ago in Earth’s 45 years of existence. Next arrived the earliest mammals, warm-blooded vertebrates that have counted over 6,400 species, and today includes diverse a range from bats through to blue whales — and us humans too.

From the early mammals also came the birds, and our ancestral line of primates began here too around 85 million years ago — some ten months ago in Earth’s life. Finally, yesterday in Earth’s E.Y. life, we were born as the wise man, Homo Sapiens.

The history of Earth (8)

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This is the second chapter of my book “Atmosphere, CO₂ on my mind”. You can find more information and references on my website.

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References

1. (1755) Immanuel Kant, “Universal Natural History and Theory of the Heavens”, (1796) Pierre Laplace
2. Peter D. Ward and Donald Brownlee, “The Life and Death of Planet Earth”
3. Hublin, J. et al. Nature 546, 289–292 (2017).
4. Heinrich D. Holland, “The oxygenation of the atmosphere and oceans”
5. T.S. Eliot, ”Journey of the Magi”
6. Coleman AP (1907) A lower Huronian ice age. Am J Sci 23:187–192; Bekker A. (2014) Huronian Glaciation. In: Amils R. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg.
7. Kopp, Robert & Kirschvink, Joseph. (2004). The Earth’s Worst Climate Disaster.
8. https://commons.wikimedia.org/wiki/File:Geologic_Clock_with_events_and_periods.svg