Our planet has existed for 4.5 billion years, and it has been a busy few eons. Here are the 25 biggest milestones in Earth's history. From leaps forward in evolution to devastating asteroid impacts, these were the turning points that shaped our world.
Earth grew from a cloud of dust and rocks surrounding the young Sun. Earth formed when some of these rocks collided. Eventually they were massive enough to attract other rocks with the force of gravity, and vacuumed up all the nearby junk, becoming the Earth. The Moon probably formed soon after, when a planet-sized chunk of rock smashed into the Earth and threw up a huge cloud of debris. This condensed into the Moon.
Nobody knows exactly when life began. The oldest confirmed fossils, of single-celled microorganisms, are 3.5 billion years old. Life may have begun a bit earlier than that, but probably not while huge rocks were still raining down on Earth. Life may have begun in warm alkaline vents on the seabed, or in open water, or on land. We don't know, and we don't know what the first organisms were like.
All life needs energy to survive, and the biggest source of energy for life on Earth is the Sun. Some of the early microorganisms evolved a way to use the energy from sunlight to make sugars out of simpler molecules. This process is called photosynthesis. But unlike green plants today, the first photosynthesising organisms did not release oxygen as a waste product, so there was no oxygen in the air.
Today, Earth's surface is divided into a few dozen plates of rock, one of which sometimes ploughs under another to be destroyed in the planet's molten heart. This process, called plate tectonics, is thought to have begun around 3 billion years ago. Only when plate tectonics had come into operation could the first continent, nicknamed 'Ur', come into being.
For the first half of Earth's history, there was hardly any oxygen in the air. But then some bacteria began harnessing sunlight to make sugar from carbon dioxide and water, just like green plants today. These microbes pumped out oxygen as a waste product, creating the oxygen-rich atmosphere we have today. But the first oxygen may have caused the entire planet to freeze over into a 'Snowball Earth', by stripping the greenhouse gas methane from the air.
The first organisms were simple cells like modern bacteria, but some of them became much more internally complex. These 'eukaryotes' developed lots of specialised equipment within their cells. They also had a new source of energy: sausage-shaped objects called mitochondria that were once free-living bacteria, but which were absorbed in a process called endosymbiosis. Every animal and plant you've ever seen is a eukaryote.
Between 1.8 billion and 800 million years ago, the fossil record looks fairly dull – so much so that the period is called the 'Boring Billion'. But behind the scenes plenty was happening. For one thing sex may have evolved for the first time. It's not clear why, or when, some organisms stopped simply dividing in two and started the messy business of sex. But it was definitely going on 1.2 billion years ago: there are fossils of red algae from that time that were clearly forming specialised sex cells such as spores.
For the first time, life was not just made up of single cells. Now cells were teaming up to form larger organisms with things like mouths, limbs and sense organs. It's hard to say when this happened: there are fossils of large organisms dating back 2.1 billion years, but these may simply have been colonies of bacteria. Different groups of organisms probably evolved multicellularity independently, with plants managing it before animals.
Earth froze over again, twice, in the space of 200 million years. The ice may well have stretched all the way from the poles to the equator. This second Snowball period may have triggered the evolution of the first complex animals. The first complex organisms, weird tube- and frond-shaped things called the Ediacarans, appeared soon after.
Soon after animals evolved, evolution went through two major growth spurts. In the Cambrian Explosion, it seems almost every group of modern animals appeared within tens of millions of years. This apparent 'explosion' may be partly down to better fossilisation, as many animals now had hard shells. Then 489 million years ago, each animal group expanded in the Great Ordovician Biodiversification Event.
Some animals ventured onto land as far back as 500 million years ago, but they only visited briefly – perhaps to lay eggs in a place without predators. Plants were the first to take up permanent residence on land. The first land plants were relatives of green algae, but they rapidly diversified.
The Ordovician period was a time when life flourished. But towards its end, the world cooled dramatically and ice sheets spread from the poles. The ensuing ice age is called the Andean-Saharan, because the evidence of it comes from the Andes mountains and the Sahara desert. The deep freeze led to the second-worst mass extinction on record, the Ordovician-Silurian. Most life was still confined to the sea, and 85% of marine species were wiped out. In the aftermath, fish became much more common.
With plants well-established on land, the next step was for animals to move out of the water. Insects were among the first, around 400 million years ago. But they were followed soon after by big, backboned animals such as Tiktaalik, a fish that looked a bit like a salamander. Fish like Tiktaalik would eventually evolve four limbs, and give rise to amphibians, reptiles and mammals. It may be a good thing it left the water when it did, as soon afterwards the Late Devonian Extinction wiped out many marine animals, including some terrifying-looking armoured fish.
When the first reptiles appeared, Earth was in the middle of a long cold snap called the Late Paleozoic Ice Age. Reptiles evolved from newt-like amphibians. Unlike their ancestors they had tough, scaly skin and laid eggs with hard shells that did not have to be left in water. Thanks to these advantages, they quickly became the dominant land animals. The reptile-like Dimetrodon reached 4.5m long – but despite what you may have heard, it was not a dinosaur.
For the last time, all Earth's continents came together to form one giant supercontinent. Known as Pangaea, it was surrounded by a world-spanning ocean called Panthalassa. It lasted until 175 million years ago, when it began to tear itself apart over tens of millions of years. Its shattered remnants became the familiar modern continents.
Just as the reptiles were flourishing, life on Earth faced perhaps its greatest challenge. The Permian extinction was the worst mass extinction in the planet's history, obliterating up to 96% of marine species and similar numbers of land animals. We don't know for sure what caused it, but massive volcanic eruptions – creating what is now the Siberian Traps – may have been to blame. In the aftermath, the first dinosaurs evolved.
At the same time that the dinosaurs were spreading and diversifying, the first mammals evolved. Their ancestors were reptiles called cynodonts, whose faces looked a little like those of dogs and may have had fur or whiskers. Early mammals such as Morganucodon were small and shrew-like, and probably only active at night. This may have spurred them to evolve warm-bloodedness: the ability to keep their body temperature constant.
The dinosaurs were flourishing on land, and in the sea giant reptiles called ichthyosaurs had become the top predators. Then another disaster struck. We’re not sure what caused the Triassic extinction, but it killed off around 80% of species. In the aftermath, the dinosaurs became the dominant land animals and eventually reached titanic sizes. The biggest species whose mass is accurately known, Dreadnoughtus schrani, weighed about 59 tonnes.
Birds evolved from feathered dinosaurs – modern birds are essentially Velociraptors with beaks instead of snouts and wings instead of arms. The most famous early bird, Archaeopteryx, lived 150 million years ago. But in recent years slightly older fossils, such as Xiaotingia and Aurornis, have been found in China.
This may sound strange, but flowers are a recent invention. There have been land plants for 465 million years, yet there were no flowers for over two-thirds of that time. Flowering plants only appeared in the middle of the dinosaur era. The equally-familiar grasses appeared even more recently. The oldest fossil grasses are just 70 million years old, although grass may have evolved a bit earlier than that.
Boom, you're extinct. 65 million years ago, a huge chunk of rock from outer space smashed into what is now Mexico. The explosion was devastating, but the longer-term effects were worse. Dust was thrown into the upper atmosphere and blocked out sunlight, and in the ensuing cold and darkness Earth suffered its fifth and last mass extinction. The dinosaurs were the most famous casualties, but pterosaurs and giant marine reptiles were also wiped out.
Almost immediately after the dinosaurs were wiped out, mammals evolved the ability to nourish their young inside their wombs using a placenta, just like modern humans. Soon, some of these early placental mammals evolved into the first primates. They would ultimately give rise to monkeys, apes and humans. But the first ones were small creatures. The oldest known primate skeleton is of a species called Archicebus achilles, which weighed no more than 30 grams. They lived in the hot and humid rainforests of Asia.
Plants have been busily harnessing sunlight to make sugar for hundreds of millions of years – a process called photosynthesis. But fairly recently, some plants have found a better way to do it. C4 photosynthesis is far more efficient than normal photosynthesis, allowing C4 plants to cope with harsh conditions. Today scientists are trying to engineer rice to use C4 photosynthesis, to help feed the growing population.
The first apes appeared in Africa around 25 million years ago. Then at some point, the group split into the ancestors of modern humans and the ancestors of modern apes. It's hard to say exactly when, but thanks to modern genetics and a host of fossil discoveries, we have a rough idea. The oldest known hominid was Sahelanthropus tchadensis, which lived about 7 million years ago.
Our species, Homo sapiens, is ridiculously young. We have only existed for a fifth of a million years. In that time we have expanded from our African birthplace to reach every continent, and even outer space. Our activities have precipitated the sixth mass extinction and unleashed the fastest episode of climate change in Earth's history. Yet we are also the only species that has ever managed to piece together the history of Earth.