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By Michael Marshall
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.
3.8 billion years ago?
This is our current “best guess” for the beginning of life on Earth. It is distinctly possible that this date will change as more evidence comes to light. The first life may have developed in undersea alkaline vents, and was probably based on RNA rather than DNA.
At some point far back in time, a common ancestor gave rise to two main groups of life: bacteria and archaea.
How this happened, when, and in what order the different groups split, is still uncertain.
3.5 billion years ago
The oldest fossils of single-celled organisms date from this time.
3.46 billion years ago
Some single-celled organisms may be feeding on methane by this time.
3.4 billion years ago
Rock formations in Western Australia, that some researchers claim are fossilised microbes, date from this period.
3 billion years ago
Viruses are present by this time, but they may be as old as life itself.
2.4 billion years ago
The “great oxidation event”. Supposedly, the poisonous waste produced by photosynthetic cyanobacteria – oxygen – starts to build up in the atmosphere. Dissolved oxygen makes the iron in the oceans “rust” and sink to the seafloor, forming striking banded iron formations.
Recently, though, some researchers have challenged this idea. They think cyanobacteria only evolved later, and that other bacteria oxidised the iron in the absence of oxygen.
Yet others think that cyanobacteria began pumping out oxygen as early as 2.1 billion years ago, but that oxygen began to accumulate only due to some other factor, possibly a decline in methane-producing bacteria. Methane reacts with oxygen, removing it from the atmosphere, so fewer methane-belching bacteria would allow oxygen to build up.
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.
2.15 billion years ago
First undisputed fossil evidence of cyanobacteria, and of photosynthesis: the ability to take in sunlight and carbon dioxide, and obtain energy, releasing oxygen as a by-product.
There is some evidence for an earlier date for the beginning of photosynthesis, but it has been called into question.
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.
Eukaryotic cells evolved 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.
Later, eukaryotic cells engulfed photosynthetic bacteria and formed a symbiotic relationship with them. The engulfed bacteria evolved into chloroplasts: the organelles that give green plants their colour and allow them to extract energy from sunlight.
Different lineages of eukaryotic cells acquired chloroplasts in this way on at least three separate occasions, and one of the resulting cell lines went on to evolve into all green algae and green plants.
1.5 billion years ago?
The eukaryotes divide into three groups: 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. At this time they were probably all still single-celled organisms.
900 million years ago?
The first multicellular life develops around this time.
It is unclear exactly how or why this happens, but 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 theory.
800 million years ago
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.
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. It has been suggested that they may actually be the last common ancestor of all the animals.
770 million years ago
The planet freezes over again in another “snowball Earth“.
730 million years ago
The comb jellies (ctenophores) split from the other multicellular animals. Like the cnidarians that will soon follow, they rely on water flowing through their body cavities to acquire oxygen and food.
680 million years ago
The ancestor of cnidarians (jellyfish and their relatives) breaks away from the other animals – though there is as yet no fossil evidence of what it looks like.
630 million years ago
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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.
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), 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.
570 million years ago
A small group breaks away from the main group of deuterostomes, known as the Ambulacraria. This 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.
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.
535 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 may simply be an illusion caused by a lack of older fossils.
530 million years ago
The first true vertebrate – an animal with a backbone – appears. It probably evolves from a jawless fish that has a notochord, a stiff rod of cartilage, instead of a true backbone. 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 probably look like eels.
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 the missing link between insects and crustaceans. Nectocaris pteryx, thought to be the oldest known ancestor of the cephalopods – the group that includes squid – lives around this time.
489 million years ago
The Great Ordovician Biodiversification Event begins, leading to a great increase in diversity. Within each of the major groups of animals and plants, many new varieties appear.
465 million years ago
Plants begin colonising the land.
460 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. They eventually include all the sharks, skates and rays.
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.
425 million years ago
The coelacanth, one of the most famous “living fossils” – species that have apparently not changed for millions of years – splits from the rest of the lobe-finned fish.
417 million years ago
Lungfish, another legendary living fossil, follow the coelacanth by splitting 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.
400 million years ago
The oldest known insect lives around this time. Some plants evolve woody stems.
397 million years ago
The first four-legged animals, or tetrapods, evolve from intermediate species such as Tiktaalik, probably in shallow freshwater habitats.
The tetrapods go on to conquer the land, and give rise to all amphibians, reptiles, birds and mammals.
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.
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 evolve into the mammals.
320 to 250 million years ago
The pelycosaurs, the first major group of synapsid animals, 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 100 million years ago
The therapsids, close cousins of the pelycosaurs, evolve alongside them and eventually replace them. The therapsids survive until the early Cretaceous, 100 million years ago. Well before that, a group of them 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 the ecosystem recovers, it undergoes a fundamental shift. Whereas before the synapsids (first the pelycosaurs, then the therapsids) dominated, the sauropsids now take over – most famously, in the form of dinosaurs. The ancestors of mammals 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.
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, paving the way for the dinosaurs to take over from their sauropsid cousins.
Around the same time, proto-mammals evolve warm-bloodedness – the ability to maintain their internal temperature, regardless of the external conditions.
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: they 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.
150 million years ago
Archaeopteryx, the famous “first bird”, lives in Europe.
140 million years ago
Around this time, placental mammals split from their cousins the marsupials. These mammals, like the modern kangaroo, that 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 reach it by an extremely roundabout route. Arising in south-east Asia, they spread into north America (which was attached to Asia at the time), then to south America and Antarctica, before making the final journey to Australia about 50 million years ago.
131 million years ago
Eoconfuciusornis, a bird rather more advanced than Archaeopteryx, lives in China.
130 million years ago
The first flowering plants emerge, following a period of rapid evolution.
105-85 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 how these splits occurred is unclear at present.
100 million years ago
The Cretaceous dinosaurs reach their peak in size. The giant sauropod Argentinosaurus, believed to be the largest land animal in Earth’s history, lives around this time.
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.
65 million years ago
The Cretaceous-Tertiary (K/T) extinction wipes out a swathe of species, including all the giant reptiles: the dinosaurs, pterosaurs, ichthyosaurs and plesiosaurs. The ammonites are also wiped out. 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 tarsier, a primate with enormous eyes to help it see at night, splits from the rest of the haplorrhines: the first to do so.
55 million years ago
The Palaeocene/Eocene extinction. A sudden rise in greenhouse gases sends temperatures soaring and transforms the planet, wiping out many species in the depths of the sea – though sparing species in shallow seas and on land.
50 million years ago
Artiodactyls, which look like a cross between a wolf and a tapir, begin evolving into whales.
48 million years ago
Indohyus, another possible 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
New World monkeys become the first simians (higher primates) to diverge from the rest of the group, colonising South America.
25 million years ago
Apes split from the Old World monkeys.
18 million years ago
Gibbons become the first ape to split from the others.
14 million years ago
Orang-utans branch off from the other great apes, spreading across southern Asia while their cousins remain in Africa.
7 million years ago
Gorillas branch off from the other great apes.
6 million years ago
Humans diverge from their closest relatives; the chimpanzees and bonobos.
Shortly afterwards, hominins begin walking on two legs. See our interactive timeline of human evolution for the full story of how modern humans developed.
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.