There are all sorts of ways to reconstruct the history of life on Earth. Pinning down when specific events occurred is often tricky, though. For this, biologists depend mainly on dating the rocks in which fossils are found, and by looking at the “molecular clocks” in the DNA of living organisms.

There are problems with each of these methods. The fossil record is like a movie with most of the frames cut out. Because it is so incomplete, it can be difficult to establish exactly when particular evolutionary changes happened.

Modern genetics allows scientists to measure how different species are from each other at a molecular level, and thus to estimate how much time has passed since a single lineage split into different species. Confounding factors rack up for species that are very distantly related, making the earlier dates more uncertain.

These difficulties mean that the dates in the timeline should be taken as approximate. As a general rule, they become more uncertain the further back along the geological timescale we look. Dates that are very uncertain are marked with a question mark.

4.5 billion years ago

Earth forms.

Soon afterwards it is struck by a Mars-sized body dubbed Theia, which vaporises the surface and blasts it into space. This ejected material condenses to form the Moon.

Massive bodies continue to strike the Earth, at a declining rate, for the next 1.5 billion years, ending about 3 billion years ago. The impacts reshape the planet surface and may help drive the onset of plate tectonics.

However, the idea that there is an especially intense “Late Heavy Bombardment” between about 4 and 3.8 billion years ago, which obliterates any life, no longer seems to be true. This means life may have begun in the first few hundred million years of Earth’s existence.

4.4 billion years ago

By this time, Earth has cooled down considerably and has substantial amounts of water. The origin of this water is mysterious. It may have been contained in the rocks that accreted to form the planet, in which case Earth was born wet.

Alternatively, the planet may have been dry at first, and received water from impactors like comets and asteroids. Either way, the early presence of water also points to an early origin of life.

4.29 billion years ago

Chemical traces in rocks from Canada may represent evidence of life from a deep-sea vent, but the results are contested.

When and how life begins remains highly uncertain. There are multiple competing hypotheses for the nature of the first life, for instance that it was based on RNA rather than DNA.

There are also many proposed settings, such as undersea alkaline vents and geothermal ponds on land. The question of life’s origin fuses a range of scientific disciplines from chemistry to ecology.

4.2 billion years ago?

The first large oceans may form at this time. These primordial seas may be much deeper than today, leaving little or no exposed land.

4.1 billion years ago

Flecks of carbon trapped inside grains of zircon suggest that life exists at this time, but the evidence is limited.

3.9 billion years ago?

The Last Universal Common Ancestor (LUCA), the species from which everything alive today is descended, may live at this time according to genetic evidence. Attempts to reconstruct its genome suggest it lives in a volcanic or geothermal setting. LUCA seems to be a relatively complex microorganism, indicating that the origin of life occurred significantly earlier.

LUCA gives rise to two main groups of life, both single-celled: bacteria and archaea. How and when this split happens is still uncertain.

3.7 billion years ago

Rocks in Greenland from this time contain mysterious structures that may be fossilised microorganisms, or may just be distorted rocks.

The first large land masses may emerge from the ocean around this time, following a change in the behaviour of tectonic plates.

3.5 billion years ago

The oldest fossils of single-celled organisms, from Pilbara in Western Australia, date from this time.

They may live in freshwater hot springs in a volcanic region on land. The ecosystem is complex and thriving, suggesting life is already well-established.

3.46 billion years ago

Some single-celled organisms may be feeding on methane by this time.

3.4 billion years ago

Some bacteria are performing photosynthesis: they take in sunlight and carbon dioxide, and obtain energy. However, this is not photosynthesis as we know it today, because the bacteria do not release oxygen as a waste product. This anoxygenic photosynthesis remains common for a billion years.

3.2 billion years ago

Fossil microorganisms preserved in rocks from South Africa offer undisputed evidence of life on land.

3 billion years ago

Viruses are present by this time, but they may be as old as life itself, and have influenced the course of its evolution.

2.5 to 2.2 billion years ago

The Great Oxidation Event. Some cyanobacteria evolve a new form of photosynthesis that releases oxygen. This toxic waste starts to build up in the seas and atmosphere – though concentrations remain below modern levels for over a billion years.

Dissolved oxygen makes the iron in the oceans “rust” and sink to the seafloor, forming striking banded iron formations.

Oxygenic photosynthesis may well have evolved earlier, but the resulting oxygen was initially confined to small “oases”.

Once oxygen becomes widespread, it may have cause a mass extinction among microbes that are unable to cope with it. It also drives evolutionary innovations. Today almost all animals breathe it and it may be behind the origin of circadian clocks.

2.3 billion years ago

Earth freezes over in what may have been the first “snowball Earth”, possibly as a result of a lack of volcanic activity. When the ice eventually melts, it indirectly leads to more oxygen being released into the atmosphere.

2.1 billion years ago

Burrow-like structures in rocks from Gabon suggest simple multicellular organisms have evolved and are moving, but this is disputed.

2 billion years ago?

Eukaryotic cells – cells with internal “organs” (known as organelles) – come into being. One key organelle is the nucleus: the control centre of the cell, in which the genes are stored in the form of DNA.

Fossils resembling eukaryotic cells are known from 2 billion years ago, and there are disputed claims of older fossils.

Eukaryotic cells evolve when one simple cell engulfed another, and the two lived together, more or less amicably – an example of “endosymbiosis”. The engulfed bacteria eventually become mitochondria, which provide eukaryotic cells with energy. The last common ancestor of all eukaryotic cells had mitochondria – and had also developed sexual reproduction.

The origin of eukaryotes is a turning point in evolution. It is not clear why it happened, but microbes called Asgard archaea may offer clues. Of all known archaea, they are the most closely related to eukaryotes, so ancient archaea similar to them may have taken part in the endosymbiosis.

Later, some eukaryotic cells engulf photosynthetic bacteria and form a symbiotic relationship with them. The engulfed bacteria evolve into chloroplasts: the organelles that give green plants their colour and allow them to extract energy from sunlight.

1.6 billion years ago

Fossils resembling primitive seaweed suggest some eukaryotes have evolved into large, multicellular organisms. The timing of the origin of multicellularity is uncertain and it may have occurred multiple times.

It is unclear exactly how or why multicellularity arises. One possibility is that single-celled organisms go through a stage similar to that of modern choanoflagellates: single-celled creatures that sometimes form colonies consisting of many individuals. Of all the single-celled organisms known to exist, choanoflagellates are the most closely related to multicellular animals, lending support to this idea.

1.5 billion years ago?

The eukaryotes undergo major splits: the ancestors of modern plants, fungi and animals split into separate lineages, and evolve separately. We do not know in what order the three groups broke with each other.

1 billion years ago

Some simple multicellular organisms appear to have distinct cell types, the first step towards internal organs.

890 million years ago

The first recognisable animals may have evolved by this time, much earlier than was once believed. In particular, fossils indicate that sponges are present.

800 million years ago?

Around this time, fungi evolve multicellular body plans.

Meanwhile, the early multicellular animals undergo their first splits. First they divide into, essentially, the sponges and everything else – the latter being more formally known as the Eumetazoa.

However, some genetic studies indicate that a group called the comb jellies or ctenophores was the first to break away, not the sponges. The debate is ongoing.

Around 20 million years later, a small group called the placozoa breaks away from the rest of the Eumetazoa. Placozoa are thin plate-like creatures about 1 millimetre across, and consist of only three layers of cells.

717 to 636 million years ago

The planet freezes over again in another “snowball Earth“.

660 million years ago

Chemical traces in rocks suggest sponges are living in the sea.

630 million years ago

Around this time, some animals evolve bilateral symmetry for the first time: that is, they now have a defined top and bottom, as well as a front and back.

Little is known about how this happened. However, small worms called Acoela may be the closest surviving relatives of the first ever bilateral animal.

It seems likely that the first bilateral animal was a kind of worm. Vernanimalcula guizhouena, which dates from around 600 million years ago, may be the earliest bilateral animal found in the fossil record.

600 million years ago

Fossils that may be comb jellies date from this time.

590 million years ago

The Bilateria, those animals with bilateral symmetry, undergo a profound evolutionary split. They divide into the protostomes and deuterostomes.

The deuterostomes eventually include all the vertebrates, plus an outlier group called the Ambulacraria. The protostomes become all the arthropods (insects, spiders, crabs, shrimp and so forth), molluscs, various types of worm, and the microscopic rotifers.

Neither may seem like an obvious “group”, but in fact the two can be distinguished by the way their embryos develop. The first hole that the embryo acquires, the blastopore, forms the anus in deuterostomes, but in protostomes it forms the mouth.

580 million years ago

The earliest known fossils of cnidarians, the group that includes jellyfish, sea anemones and corals, date to around this time – though the fossil evidence has been disputed.

575 million years ago

Strange life forms known as the Ediacarans appear around this time and persist for about 33 million years. Some may be early animals.

570 million years ago

The Ambulacraria breaks away from the main group of deuterostomes. This small group eventually becomes the echinoderms (starfish, brittle stars and their relatives) and two worm-like families called the hemichordates and Xenoturbellida.

Another echinoderm, the sea lily, is thought to be the “missing link” between vertebrates (animals with backbones) and invertebrates (animals without backbones), a split that occurred around this time.

565 million years ago

Fossilised animal trails suggest that some animals are moving under their own power.

558 million years ago

Dickinsonia, an Ediacaran that is thought to be an early animal, lives around this time.

540 million years ago

As the first chordates – animals that have a backbone, or at least a primitive version of it – emerge among the deuterostomes, a surprising cousin branches off.

The sea squirts (tunicates) begin their history as tadpole-like chordates, but metamorphose partway through their lives into bottom-dwelling filter feeders that look rather like a bag of seawater anchored to a rock. Their larvae still look like tadpoles today, revealing their close relationship to backboned animals.

Around the same time, the Ediacarans die out, perhaps due to competition from newly-evolved species.

539 million years ago

The Cambrian explosion begins, with many new body layouts appearing on the scene – though the seeming rapidity of the appearance of new life forms increasingly looks like an illusion as ever more older fossils come to light.

530 million years ago

The first true vertebrates – animals with a backbone – appear. The exact timing is unclear, and it is difficult to find consensus on which animals are true vertebrates and which are only related to the vertebrates. Possible candidates include Myllokunmingia, known from 518-million-year-old rocks in China.

The early vertebrates have a notochord, protected by a stiff rod of cartilage: they do not have a spine made of bone. The first vertebrate is probably quite like a lamprey, hagfish or lancelet.

Around the same time, the first clear fossils of trilobites appear. These invertebrates, which look like oversized woodlice and grow to 70 centimetres in length, proliferate in the oceans for the next 200 million years.

520 million years ago

Conodonts, another contender for the title of “earliest vertebrate”, appear. They look a little like eels – although whether they are truly vertebrates is still debated.

515 million years ago?

Plants begin colonising the land – an event that may have been enabled by genes obtained from microorganisms.

512 million years ago

A fossilised worm indicates that parasites are now present on Earth, but this lifestyle may be much older.

500 million years ago

Fossil evidence shows that animals were exploring the land at this time. The first animals to do so were probably euthycarcinoids – thought to be related to centipedes and millipedes.

Nectocaris pteryx lives around this time. It is thought by some researchers to be the oldest known ancestor of the cephalopods – the group that includes squid – although others are not so sure.

Some comb jellies develop complex nervous systems.

489 million years ago

The Great Ordovician Biodiversification Event begins, leading to a great increase in the diversity of life. Within each of the major groups of animals and plants, many new varieties appear.

450 to 400 million years ago

Fish split into two major groups: the bony fish and cartilaginous fish. The cartilaginous fish, as the name implies, have skeletons made of cartilage rather than the harder bone. Today, they include all the sharks, skates and rays.

Fish with teeth appear in the fossil record at this time, including Qianodus, thought to have been a cartilaginous fish.

Oxygen reaches its modern level in the atmosphere for the first time – although the exact level will continue to fluctuate.

445-443 million years ago

The Late Ordovician mass extinction, the first of the “big five” extinction events and probably the second most severe in terms of the number of genera that went extinct.

440 million years ago

The bony fish split into their two major groups: the lobe-finned fish with bones in their fleshy fins, and the ray-finned fish. The lobe-finned fish eventually give rise to amphibians, reptiles, birds and mammals. The ray-finned fish thrive, and give rise to most fish species living today.

The common ancestor of lobe-finned and ray-finned fish probably has simple sacs that function as primitive lungs, allowing it to gulp air when oxygen levels in the water fall too low. In ray-finned fish, these sacs evolve into the swim bladder, which is used for controlling buoyancy.

432 million years ago

The oldest unambiguous large fossil of a land plant dates from this time.

425 million years ago

The coelacanths, a group of famous “living fossils” – species that have apparently not changed for millions of years – may have split from the rest of the lobe-finned fish at this time. Coelacanths begin appearing in the fossil record by 409 million years ago.

417 million years ago

The lungfish, another group of legendary living fossils, split from the other lobe-finned fish. Although they are unambiguously fish, complete with gills, lungfish have a pair of relatively sophisticated lungs, which are divided into numerous smaller air sacs to increase their surface area. These allow them to breathe out of water and thus to survive when the ponds they live in dry out.

410 million years ago

For the first time, plants with elaborate root systems that stabilise the soil leave their mark in the fossil record.

400 million years ago

Insects may start appearing in the fossil record around this time – although some researchers now suspect the fossils are misidentified millipedes.

Some plants evolve woody stems.

397 million years ago

Fossilised footprints are left by four-legged animals, or tetrapods, for the first time. But the fossils are controversial: they seem to predate, by many millions of years, some of the fish-like animals thought to have evolved into tetrapods, and so some researchers question whether the fossils really are ancient footprints.

385 million years ago

The oldest fossilised tree dates from this period.

375 million years ago

Tiktaalik, an intermediate between fish and four-legged land animals, lives around this time. The fleshy fins of its lungfish ancestors are evolving into limbs, and unambiguous evidence that tetrapods have made it onto land soon follows.

The tetrapods eventually give rise to all amphibians, reptiles, birds and mammals.

But not all Tiktaalik-like animals move on to land. One – Qikiqtania – evolves back into an aquatic swimmer.

372 to 359 million years ago?

The Late Devonian mass extinction occurs. It is the second of the big five. This one may be a long slow decline over tens of millions of years, not an abrupt event.

350 million years ago

The beetles, one of the most successful animal groups, begin to proliferate.

340 million years ago

The first major split occurs in the tetrapods, with the amphibians branching off from the others.

310 million years ago

Within the remaining tetrapods, the sauropsids and synapsids split from one another. The sauropsids include all the modern reptiles, plus the dinosaurs and birds. The first synapsids are also reptiles, but have distinctive jaws. They are sometimes called “mammal-like reptiles”, and eventually some of them evolve into the mammals.

320 to 250 million years ago

Synapsid animals known informally as the “pelycosaurs” dominate the land. The most famous example is Dimetrodon, a large predatory “reptile” with a sail on its back. Despite appearances, Dimetrodon is not a dinosaur.

275 to 201 million years ago

Some pelycosaurs evolve to become the first therapsids. Eventually, the therapsids replace the rest of the pelycosaurs.

About 260 million years ago, a group of therapsids called the cynodonts develops dog-like teeth and eventually evolves into the first mammals.

250 million years ago

The Permian period ends with the greatest mass extinction in Earth’s history, wiping out great swathes of species, including the last of the trilobites.

As ecosystems recover, fundamental shifts occur. Whereas before the synapsids (first the pelycosaurs, then the therapsids) dominated the land, the sauropsids now take over – most famously, in the form of dinosaurs. The therapsids survive as small, nocturnal creatures.

In the oceans, the ammonites, cousins of the modern nautilus and octopus, evolve around this time. Several groups of reptiles colonise the seas, developing into the great marine reptiles of the dinosaur era. The ichthyosaurs adapt rapidly to the marine lifestyle, some becoming as large as modern whales.

233 million years ago?

Proto-mammals evolve warm-bloodedness – the ability to maintain their internal temperature, regardless of the external conditions. The timing of this remains controversial – as is whether some or all dinosaurs were also warm-blooded.

210 million years ago

Bird-like footprints and a badly-preserved fossil called Protoavis suggest that some early dinosaurs are already evolving into birds at this time. This claim remains controversial.

200 million years ago

As the Triassic period comes to an end, another mass extinction strikes.

In the aftermath, the dinosaurs emerge from among the sauropsids and begin to dominate ecosystems. The largest ichthyosaurs are among the marine casualties of the extinction.

180 million years ago

The first split occurs in the early mammal population. The monotremes, a group of mammals that lay eggs rather than giving birth to live young, break apart from the others. Few monotremes survive today: those that do include the duck-billed platypus and the echidnas.

168 million years ago

A half-feathered, flightless dinosaur called Epidexipteryx, which may be an early step on the road to birds, lives in China.

160 million years ago

Possibly around this time, placental mammals split from their cousins the marsupials. The latter, like the modern kangaroo, give birth when their young are still very small, but nourish them in a pouch for the first few weeks or months of their lives.

The majority of modern marsupials live in Australia, but they may have reached it by an extremely roundabout route. Arising in south-east Asia, they spread into North America, which was attached to Asia at the time. From there they travelled to South America and Antarctica, before making the final journey to Australia about 50 million years ago.

Oldest evidence of pollinating insects.

Some dinosaurs take to the sky with membrane wings like those of modern bats, but are soon outcompeted by feathered birds.

150 million years ago

Archaeopteryx, the famous “first bird”, lives in Europe.

131 million years ago

Eoconfuciusornis, a bird rather more advanced than Archaeopteryx, lives in China.

125 million years ago

Flowering plants begin to leave their mark in the fossil record, following a period of rapid evolution – although genetics suggests the group is actually older and evolved slowly. The plants go on to dominate the planet, outcompeting the flowerless gymnosperms.

105 to 85 million years ago

The placental mammals split into their four major groups: the laurasiatheres (a hugely diverse group including all the hoofed mammals, whales, bats, and dogs), euarchontoglires (primates, rodents and others), Xenarthra (including anteaters and armadillos) and afrotheres (elephants, aardvarks and others). Quite when, where and how these splits occurred is unclear at present.

100 million years ago

The Cretaceous dinosaurs reach their peak in size. The giant sauropods Argentinosaurus and Patagotitan, live around this time. They may be the largest land animals in Earth’s history – unless an enigmatic sauropod that lived 50 million years earlier was bigger.

93 million years ago

The oceans become starved of oxygen, possibly due to a huge underwater volcanic eruption. Twenty-seven per cent of marine invertebrates are wiped out.

75 million years ago

The ancestors of modern primates split from the ancestors of modern rodents and lagomorphs (rabbits, hares and pikas). The rodents go on to be astonishingly successful, eventually making up around 40 per cent of modern mammal species.

70 million years ago

Grasses evolve – though it will be several million years before the vast open grasslands appear.

66 million years ago

The Cretaceous-Tertiary (K/T) extinction wipes out a swathe of species. The giant dinosaurs vanish, although some small forms – the birds – survive. Pterosaurs and plesiosaurs are also among the large reptiles that vanish. The ammonites are also killed off. The extinction clears the way for the mammals, which go on to dominate the planet.

63 million years ago

The primates split into two groups, known as the haplorrhines (dry-nosed primates) and the strepsirrhines (wet-nosed primates). The strepsirrhines eventually become the modern lemurs and aye-ayes, while the haplorrhines develop into monkeys and apes – and humans.

58 million years ago

The tarsiers, primates with enormous eyes to help them see at night, split from the rest of the haplorrhines: the first to do so.

55 million years ago

The Palaeocene-Eocene Thermal Maximum. A sudden rise in greenhouse gases sends temperatures soaring, wiping out many species in the depths of the sea – though sparing species in shallow seas and on land. Archicebus, one of the earliest primates to appear in the fossil record, lives at this time.

50 million years ago

Within the artiodactyls – hoofed mammals including deer and bison – one group begins evolving into whales. Early forms include Pakicetus.

48 million years ago

Indohyus, another artiodactyl closely related to the ancestor of whales and dolphins, lives in India.

47 million years ago

The famous fossilised primate known as “Ida” lives in northern Europe.

Early whales called protocetids live in shallow seas, returning to land to give birth.

40 million years ago

The platyrrhines (sometimes known as the “New World monkeys”) become the first simians (higher primates) to diverge from the rest of the group, when they cross the Atlantic and colonise South America.

30 million years ago?

Some plants evolve C4 photosynthesis, which is more efficient than other forms. Today major crops like maize are C4 plants.

25 million years ago

Apes split from the monkeys and diversify.

17 million years ago

Gibbons become the first ape to split from the others.

15 million years ago

The ponginae – great apes including the orang-utans – branch off from the other great apes, spreading across southern Asia while their cousins remain in Africa. The ponginae include the largest great ape ever to have lived: Gigantopithecus.

7 million years ago

Gorillas branch off from the other great apes.

The ancestors of humans diverge from those of their closest relatives: the chimpanzees and bonobos. Later, some hominins begin walking on two legs.

2.8 million years ago

Our human genus, Homo, appears in Africa.

2 million years ago

A 700-kilogram rodent called Josephoartigasia monesi lives in South America. It is the largest rodent known to have lived, displacing the previous record holder: a giant guinea pig.

11,000 years ago to 1950s?

Beginning of the Anthropocene, the geological epoch defined by human domination of the planet. At time of writing the Anthropocene is not an official geological epoch and the start date remains contentious. Earlier dates coincide with the beginning of farming and the rise of city living and empires. Alternatively, the start date could be pinned to the 20^th century – for example, to the onset of nuclear testing.

The Anthropocene has been marked by a high rate of extinctions, especially of large animals or “megafauna”. It may ultimately prove to be the sixth mass extinction. It is also a time of rapid climatic warming, due to greenhouse gas emissions from the burning of fossil fuels and other activities. Like other dramatic shifts before it, the Anthropocene will reshape life on Earth.