Origin of
Planet Earth
Look at your hand. Every atom of carbon in it was forged inside a star that exploded long before the Sun existed. The story of how an empty, hot universe became a water-covered planet covered in life is wilder than any science fiction — and we have rocks, telescopes, and Moon samples that help us tell it.
Earth has oceans, an atmosphere, and a Moon. None of these were here at the start — and the universe itself didn't have any of the elements we're made of. So how did we get from nothing to this?
The Moon is the wreckage of a planetary collision. Earth's first atmosphere blew away into space. Water came from rocks and comets. The oxygen we breathe was made by tiny ocean bacteria over billions of years. And every atom heavier than helium was cooked up inside a star that died long before our Sun was born.
This topic walks through that story — from the Big Bang 13.7 billion years ago to the moment, somewhere in a shallow sunlit sea, when the first cyanobacteria figured out photosynthesis and started terraforming the planet for everything that came next. Nobody knows where that happened. But South Africa holds some of the best evidence that it did: the 3.5-billion-year-old stromatolites of the Barberton Greenstone Belt in Mpumalanga are among the oldest traces of life on Earth.
Three Threads in a 13.7-Billion-Year Story
The ocean and atmosphere are leftovers from the planet cooking itself.
First thread: where the atoms came from. The Big Bang produced hydrogen and helium. Everything else — carbon, oxygen, iron, gold — was forged inside stars and scattered into space when those stars exploded. Earth and the Sun condensed from the leftover dust.
Second thread: how Earth got its layers. Bombardment heated the proto-Earth. Heavier metals (iron, nickel) sank to the centre. Lighter elements rose to the surface. Volcanic outgassing released water vapour, carbon dioxide and ammonia — which condensed into the early ocean and atmosphere as the planet cooled.
Third thread: how life made the air breathable. The first atmosphere had no free oxygen. Around 3.5 billion years ago, simple photosynthesising bacteria — cyanobacteria — started releasing O₂ as a waste product. Over hundreds of millions of years they transformed the atmosphere into something animals could eventually breathe. Our atmosphere is, biologically, made.
From Singularity to Stromatolites
Seven turning points in the story of how Earth got here. Tap each milestone to see what happened, what came out of it, and how scientists figured it out. The dates are not to scale — 13.7 billion years doesn't fit on one page.
Local connection — South Africa is a window into deep time
The Barberton Greenstone Belt, on the Mpumalanga–Eswatini border, contains some of the oldest rocks on Earth — around 3.5 billion years old. Inside them are fossil stromatolites: the layered remains of cyanobacteria mats. These are among the oldest direct evidence of life anywhere on the planet. And the Vredefort impact crater in the Free State, ~2 billion years old and 300 km across, is the largest verified meteorite impact site on Earth — a UNESCO World Heritage Site, formed by an asteroid roughly 10–15 km wide. The deep-time story isn't somewhere else. It's under your feet.
The Numbers, the Air, the Sweet Spot
Earth's three atmospheres
The air you breathe is the third atmosphere our planet has had. Each one was different — and the third one was made by life.
Hydrogen + Helium
Earth was hot and small. Light gases like H₂ and He couldn't be held by gravity and drifted off into space.
Steam, CO₂ & ammonia
Heating melted Earth's interior. Volcanoes outgassed water vapour, carbon dioxide and ammonia. Comets and meteorites added more water and nitrogen-bearing ice.
Nitrogen + oxygen
As Earth cooled, water vapour condensed → ocean. Cyanobacteria evolved photosynthesis → released O₂. Sunlight broke down ammonia → free N₂.
The eight planets
All eight planets formed from the same disc of dust around the Sun. The four closest are warm and rocky — too hot for ice to stick around. The four furthest are cold gas/ice giants, where it was cold enough to keep huge atmospheres of hydrogen, helium, methane and ammonia.
Pluto is no longer counted as a planet — it's a dwarf planet, possibly a captured moon of Neptune.
Why Earth, and only Earth, has oceans
Earth orbits in what scientists call the Goldilocks Zone — not too hot, not too cold. Just right for water to exist as a liquid. Mercury and Venus are too close to the Sun: any water vapour blew off long ago. Mars is too far and too small: water that's still there is locked up as ice or in the soil. The gas giants are far too cold. Only Earth has all three states of water — solid, liquid and gas — naturally present at the same time. That single fact is why we exist.
Where the Air and the Oceans Came From
CAPS spends five of its fourteen key concepts on this one question, and it ends with a line worth memorising: "on planet earth, the ocean and atmosphere are by-products of heating and cooling processes." Neither was here at the start. Both had to be manufactured.
Locked in rock → released by heat → condensed into oceans → and only much later, breathable.
The early Earth was too small for an atmosphere to develop — its gravity could not hold gas, and the solar wind stripped away whatever was there. Everything that later became air and sea arrived locked inside rock and ice, delivered by the same bombardment that heated the planet, and was then squeezed out of it.
Step by step
1 · Delivered locked in minerals
Water did not arrive as water. It came chemically bound inside minerals in the rocks and ice that bombarded the young Earth — hydrogen and oxygen held in the crystal structure rather than as free gas.
~ CAPS concept 9. Comets and meteorites were the delivery vehicles.2 · Released by heating — outgassing
As the Earth heated and melted, those minerals broke down and released their gases. Volcanic outgassing vented water vapour, carbon dioxide and nitrogen into a second atmosphere.
~ CAPS concept 11: the atmosphere is a by-product of heating and cooling.3 · Cooled and condensed — the oceans
Once the surface cooled below 100 °C the water vapour condensed. Rain fell for what may have been millions of years, pooled in the basins, and became the ocean.
~ the ocean is literally condensed volcanic steam.4 · Nitrogen arrived frozen
Nitrogen is thought to have been delivered as nitrogen-bearing ice in meteorites and comets. It is chemically unreactive, so it accumulated rather than being consumed — and it is now the dominant gas in our atmosphere (78%).
~ CAPS concept 10.The oxygen problem — and why it took two billion years
Here is the part CAPS builds to, and the part most learners miss. Making oxygen was not enough. CAPS concept 7 puts it exactly: "Oxygen gas (O₂) could not accumulate in the atmosphere until a surplus O₂ was produced than was absorbed into rocks and other compounds."
Early Earth was full of oxygen sinks — things that grabbed free oxygen the instant it appeared. Dissolved iron in the oceans was the biggest: it reacted with O₂ and rusted out of the water, settling as the vast banded iron formations we mine today. Volcanic gases, and the rock of the crust itself, soaked up more.
⛔ While the sinks were winning
Cyanobacteria photosynthesise; oxygen is released; iron and rock absorb every molecule of it. Atmospheric oxygen stays at essentially zero for hundreds of millions of years, even though production has already started.
The banded iron formations are the receipt for all that consumed oxygen.
✅ When the sinks were full
Eventually the iron ran out. Production finally exceeded absorption — a surplus — and free O₂ began to build up in the air. This is the Great Oxygenation Event, about 2.4 billion years ago.
Everything that breathes exists because of that surplus.
There is a real process behind it, though, and it is worth knowing. Once water vapour was in the upper atmosphere, ultraviolet light split some of it apart — a process called photodissociation. That genuinely does release H₂ and O₂. The hydrogen is so light it escaped into space; the oxygen stayed. It was a trickle, and the sinks absorbed it immediately, but it is the honest version of what CAPS is describing.
In an exam: if a question follows CAPS's wording, answer it — heating and cooling of the early Earth released gases including H₂ and O₂. Just know that photosynthesis, not vulcanism, is what actually oxygenated the planet, and that the free oxygen from splitting water vapour was a minor contributor.
The first cells — and CAPS's forgotten word
CAPS concept 12 says organic molecules developed from a liquid "soup", "resulting in the origin of simple chemosynthetic and photosynthetic cells." Photosynthesis usually gets all the attention here — but chemosynthetic cells are named first, and for good reason.
Chemosynthetic cells
Build food using the chemical energy in inorganic compounds such as hydrogen sulphide — no light needed. On an early Earth with a hazy sky, constant volcanism and abundant sulphide, this was arguably the easier way to make a living.
~ their descendants still run the hydrothermal vent ecosystems you met in Topic 11 — the only major ecosystem on Earth not powered by the sun.Photosynthetic cells
Build food using light energy, releasing O₂ as waste. Slower to arrive, but it had access to an effectively unlimited energy supply — and that waste product rebuilt the entire atmosphere.
~ later photosynthetic colonies — the stromatolites — converted still more CO₂ into O₂ (CAPS 13).The two questions CAPS asks you to discuss
"Is Pluto a planet, and why is it not classed as a planet?" — Pluto was reclassified as a dwarf planet in 2006. It meets two of the three tests for planethood: it orbits the Sun, and it is round under its own gravity. It fails the third — it has not cleared its orbital neighbourhood. Pluto shares its orbit with thousands of other Kuiper Belt objects, some nearly as large. Our solar system therefore has eight planets.
"Mars — was or is there any life on Mars?" — No life has ever been found, but the case for past conditions is strong: dry river valleys, deltas, lake-bed clays and minerals that only form in water. Mars had liquid water for hundreds of millions of years. Then, being small, it lost its magnetic field and its atmosphere, and the water froze or escaped. Current view: past habitability is likely; past or present life is unproven. The search now targets briny subsurface water and ancient lake sediments.
Both are set up properly, with the evidence and the marking points, in the practical.
Key Vocabulary
Tap each card to flip it. The atmosphere-and-ocean terms added below carry CAPS key concepts 7 to 12 — the half of this block that is most often examined.
The extremely hot, dense point where all the energy and mass of the universe was concentrated — the starting point of space, time, and the Big Bang 13.7 billion years ago.
A giant cloud of gas and dust in space. Some nebulae are star nurseries — they collapse under gravity and form stars and planets, like the one our Solar System came from.
A luminous sphere of gas held together by gravity, hot enough at its core to run nuclear fusion. The Sun is our nearest star. Every element heavier than helium was made inside one.
A huge group of stars, gas, dust and dark matter held together by gravity. Our galaxy is the Milky Way — spiral-shaped, with about 200 billion stars.
A celestial body in orbit around a star, with enough mass that its own gravity makes it round. Not big enough to start nuclear fusion (otherwise it'd be a star). Eight in our system.
The process where small particles clump together due to gravity, gradually building larger and larger bodies. How dust grains became pebbles, became rocks, became proto-planets, became Earth.
The release of gases trapped in rocks when they melt. Volcanic outgassing on early Earth released water vapour, CO₂ and ammonia — the second atmosphere, and the source of the early ocean.
Able to convert CO₂ + water + sunlight into food + oxygen. Cyanobacteria were the first photosynthetic organisms. Their slow oxygen output over billions of years made the modern atmosphere.
Able to build food using the chemical energy in inorganic compounds such as hydrogen sulphide — no light required. CAPS names chemosynthetic cells before photosynthetic ones as among the first life. Their descendants still power the hydrothermal vent ecosystems (Topic 11).
Anything that absorbs free oxygen as fast as it is produced — on the early Earth, mainly dissolved iron in the oceans, plus volcanic gases and crustal rock. CAPS concept 7: O₂ could not accumulate "until a surplus O₂ was produced than was absorbed into rocks and other compounds."
Layered iron-rich rock laid down when dissolved iron in the early ocean reacted with the first free oxygen and rusted out of the water. It is the physical receipt for all the oxygen the sinks consumed — and the ore most of the world's iron is mined from today.
About 2.4 billion years ago, when photosynthetic production of O₂ finally exceeded what the oxygen sinks could absorb, and free oxygen began accumulating in the atmosphere for the first time. Everything that breathes exists because of that surplus.
Ultraviolet light splitting water vapour apart in the upper atmosphere, releasing H₂ and O₂. The light hydrogen escapes to space; the oxygen stays. A genuine but minor abiotic oxygen source — and the real process behind CAPS's loosely-worded concept 8.
A body that orbits the Sun and is round under its own gravity, but has not cleared its orbital neighbourhood of other objects. Pluto was reclassified as one in 2006 — it shares its orbit with thousands of Kuiper Belt objects. So the solar system has eight planets.
Quick Check
Question 1
Well done!
Move on to Topic 10 when you're ready.