­Four Billion Years of Life on Earth

Author: Marc Siepman, marcsiepman.nl

When a dying red giant expelled its atmosphere, the Ring Nebula was formed. In the middle you can just make out the white dwarf that it became. Its approximately 2000 light years from the Earth so what you see on the photo is what it looked like in Roman times. The colors are relatively true Dark blue is helium, light blue is hydrogen and oxygen, orange is nitrogen and sulphur.
Photo: NASA/Hubble Space Telescope.

During the 13,7 billion years the universe exists, a lot has happened. First, the building blocks of life came into existence and 4,6 billion years ago the Earth emerged. Life usually flourished, but sometimes there were large scale extinctions. In this article I want to take you on a trip to see some ordinary miracles, to show you that life on Earth is not self-evident.

The emergence of the building blocks

The young universe contained light elements: hydrogen and helium. The hydrogen served as fuel for stars in which lighter elements are fused to heavier elements, such as carbon and oxygen. As soon as the stock of hydrogen is spent, a heavy star might explode. The resulting supernova expels a cloud consisting of gases and dust into the universe, out of which new stars and planets can be born. In such a way the Earth with all her biodiversity came to be. Three elements are instrumental to life: carbon, hydrogen and oxygen. With these three atoms the well-known and crucial-to-all-life molecules are formed: water (H2O), oxygen (O2) and carbon dioxide (CO2).


Carbon (C on the periodic system) is everywhere: in rocks, the oceans, the atmosphere, in organisms – and that carbon is always on the move between these places. Incredible amounts of carbon were needed for the emergence of the biosphere, that thin layer of life covering our blue planet. Your body contains, if you weigh 70 kilos, some (or 7·1027) carbon atoms. That’s enough to make the graphite of nine thousand pencils, or 50 to 100 diamonds.

What makes carbon special

Carbon can bond with many different elements. But carbon is different in that it can also bond with other carbon atoms. Together, they form allotropes, like graphite or diamond. Graphite is extremely soft, while diamond is the hardest mineral we know. The atom’s core possesses a cloud around it that contains six electrons with a negative charge. By sharing these with other atoms, each carbon atom can bond with no less than four other atoms. Carbon atoms are the smallest capable of that feat. The bond may be single, but double and triple bonds are also possible.

A compound of two or more atoms is called a molecule. In the case of carbon, these molecules are rarely two-dimensional chains or flat surfaces, but complex structures found in an enormous variety: to date, over ten million molecules are known. Because the bonds are relatively strong, they are quite stable. But they aren’t too stable; the bonds can be severed so that time upon time new compounds are formed with the same atoms.


Hydrogen is the most common element in the universe. It is also the lightest and simplest element on the periodic table, where it’s denominated with the letter H and atomic number 1. The most common isotope has just one proton as a nucleus and no neutrons at all. Hydrogen has just one electron.

At room temperature, hydrogen atoms bind to other hydrogen atoms to form hydrogen gas (dihydrogen), which is very flammable. During combustion (just add oxygen) water (H2O or dihydrogenoxide) is formed. They used hydrogen and oxygen to launch the Space Shuttle into space, which illustrates how much energy is released.

Expressed in atoms, hydrogen is the most common element in our body, but expressed in weight it’s number three (after oxygen and carbon).


Oxygen is O on the periodic table, but the oxygen we breathe is a diatomic gas: O2. Oxygen is the most reactive non-metal and the third most common element in the universe (after hydrogen and helium).

When oxygen binds to an element, carbon for instance, it’s called oxidation: C becomes CO2 for example. Wood, which consists of 50% carbon, oxidizes in a timespan of years or decades because it is consumed by fungi and numerous other organisms that oxydize carbon. When you burn the wood, it also oxydizes. But instead of taking years, it takes no more than a few hours for the carbon to be released back into the atmosphere.

So, oxygen is very reactive and its nothing short of a miracle that there is free oxygen in the atmosphere. We owe this fact to single-celled organisms.


Nearly all atoms on Earth are billions of years old, and the stable forms will still be around when the universe comes to its end – unless they become involved in a fusion or fission process. The same atoms are used over and over again: your body contains the same carbon that was part of nearly all humans, animals and plants that lived up till now. They were also part of rocks and spent a great deal of their time in the atmosphere.

Photosynthesis and metabolism

Photosynthesis and metabolism are two processes that complement each other perfectly.


A plant (or tree) only gets about 4% of its biomass from the soil nutrients; a staggering 96% of a plant consists of carbon, hydrogen and oxygen. It gets these three elements, using light energy, from water and carbon dioxide.

This process is called photosynthesis, which means ‘putting together with light’. It plays a crucial role for nearly all life on Earth: it’s almost the only way carbon is made available to non-photosynthesizing organisms. Plants, trees, algae and cyanobacteria fix more than 100 million tons of carbon dioxide into biomass. These organisms are called primary producers.

All other organisms, including humans, are called consumers. They need to, directly or indirectly, eat primary producers to obtain the energy to live.


Humans are obligate aerobic organisms: we need to breathe, otherwise we’ll die rather quickly. The air you breathe in contains about 0,04% carbon dioxide and 21% diatomic oxygen. The air you breathe out contains about 5% carbon dioxide and just 15% oxygen. By breathing you burn (oxidize) the carbon from your food (carbohydrates, fats, proteins): the carbon bonds to oxygen, releasing energy. Carbon dioxide and water are returned to their original forms. Thanks to this metabolism you’re able to maintain your body temperature and so on. This holds true for all aerobic life on Earth.

Metabolism is the exact opposite of photosynthesis. That’s no coincidence!

And then there was life

Thanks to the abundance of carbon, hydrogen and oxygen and a handful of other building blocks, life on Earth could emerge. Apparently, the conditions were just right: life emerged almost immediately after water first appeared as a liquid. This life was by definition autotrophic, which means these organisms can feed themselves. They don’t need to eat other organisms to get their energy (like humans; were heterotrophs).


The first life on Earth was anaerobic: the bacteria that emerged 4280 million years ago (mya) used anorganic energy sources like iron of sulfur and carbon dioxide to create organic compounds. They had little choice, since there was almost no free oxygen in the atmosphere and a whole lot of carbon dioxide. We call call them chemoautotrophs because they feed themselves with chemical compounds. They turned out quite successful: for 1700 million years, they ruled the world and they still exist.

The first photosynthesis

About 3400 million years ago the first bacteria that used sunlight as source of energy appeared. They absorbed near-infrared light and produced sulfuric compounds as a waste product of their metabolism. They were the first photoautotrophs: organisms that, using light energy, combine carbon dioxide and water are able to feed themselves. They were obligate anaerobic: they could not survive the presence of oxygen.

The Great Oxygenation Event

About 2500 million years ago, an unprecedented environmental catastrophe occurred. Single-celled organisms, called cyanobacteria, started harvesting visible light from the sun on a large scale. While doing that, they produced a toxic waste product, which meant the massive die-off of the anaerobic organisms: oxygen. During 200 million years so much free oxygen became available that most of the anaerobic organisms were driven away to places where no oxygen could come – like deep underground. There is some indirect proof that this type of bacteria developed almost 3000 milion years ago. But maybe they’re even older than that.

13.700 mya: the universe came into existence
4540 mya: the Earth came into existence
4400 mya: liquid water
4280 mya: first life
2700 mya: cyanobacteria
1200 mya: red and brown algae
750 mya: green algae
650 mya: multi-cellular life
475 mya: mosses and liverworts
430 mya: vascular plants
390 mya: trees
230 mya: the first dinosaurs
65 mya: dinosaurs go extinct
2 mya: first humans


Nowadays, we can’t live without oxygen. Therefore, we are completely dependent on photoautotrophs: despite the fact that 21% of our atmosphere consists of free oxygen, it would bind rather quickly with other elements if it wasn’t replenished continuously. About 650 million years ago the algae took over from the cyanobacteria in the oceans; since then organisms became increasingly complex. The first land plants developed over 400 million years ago and worked together with fungi to get nutrients. Trees did not develop before plants developed their own roots and vascular tissues. This started some 390 mya. Chlorophyll in plants is most likely a remnant of an endosymbiosis: cyanobacteria (that exist for at least 2700 million years) came to live inside plants and became part of the plant itself. This is not that strange: there exists a sea slug that practices photosynthesis via endosymbiosis.

Phytoplankton (a group of organisms consisting of algae and cyanobacteria) still produces more than half of all oxygen on Earth. The very existence of algae cleaners is a telltale sign that humans don’t feel themselves to be part of nature. Apparently, we think that that the production of oxygen is not tidy.


Without the greenhouse effect our planet would a big ball of ice. Because carbon, oxygen and hydrogen are constantly immobilized and freed up again, the climate on Earth finds a new dynamic equilibrium. We humans have no control over these complex exchanges, but we do affect them. We are continuously disrupting the balance by our incessant emission of water vapour, carbon dioxide, methane, nitrous oxide and other greenhouse gases. But as that weren’t bad enough, we’re also disrupting the natural processes that might restore the balance.

Life on Earth

Only if start seeing we are interdependent with all other organisms, from microbe to mega-fauna, we can we build a civilization with a future. We live in the illusion of separation – we deny the connection that makes us tic. This is the true reason behind all problems we encounter all over the world. We need to evolve to a society that embraces biodiversity as the life that creates the conditions conducive to life.

Whether we’re talking about social or ecological problems, to solve we’ll need to reconnect with each other and be an intrinsic part of are natural environment. Physically, but also mentally.