1. The first life ↑
We still don't know how life came into existence on Earth, although all life forms are built from amino acids, which can arise naturally. The first life consisted of prokaryotes, simple cells that may have evolved about 3.8 billion years ago, within a billion years of the formation of the Earth.
Around this time, a common ancestor gave rise to two groups: bacteria and archaea. There's evidence that viruses have existed for at least 3 billion years, although viruses are not generally considered to be alive.
During this time, the atmosphere of the Earth was mostly composed of carbon dioxide, water, and molecular nitrogen. This is thought to have rapidly changed about 2.5 billion years ago, when photosynthesising bacteria began to excrete oxygen for the first time. This is thought to have poisoned almost all life that had evolved before, and is known as 'the great oxidation event'.
2. Eukaryotic cells ↑
Over 1.5 billion years ago, one bacterial cell engulfed another, and eukaryotic cells were formed. Eukaryotic cells contain nuclei and store genes in the form of DNA. The engulfed bacteria eventually became mitochondria, which provide the cell with energy.
Eukaryotic cells later engulfed photosynthetic bacteria, and evolved into chloroplasts, which are found in green algae and green plants.
Eukaryotes divided into three groups - the ancestors of plants, fungi, and animals - within a few hundred million years.
Image credit: Three domains by AJ Cann/CC-NC-SA, animal cell by OpenStax College/CC-A, plant cell by LadyofHats/Public domain, fungi cell by Frankie Robertson/CC-SA, bacteria by LadyofHats/Public domain, & archaea by Helen Klus/Public domain.
3. Multicellular life ↑
Multicellular life may have developed up to 2 billion years ago. Sponges evolved about 800 million years ago, and the Cambrian explosion began about 530 million years ago. Many new species evolved during this time, including the first animal with a backbone, and the first trilobites. Trilobites had primitive eyes, gills, limbs, and a simple brain.
Animals moved from the sea to the land about 500 million years ago, and plants followed within about 25 million years. Once on land, animals could no longer float weightlessly, and so had to grow stronger spinal cords, and evolve to take their oxygen straight from the air, giving up their redundant gills.
4. Mass extinction events ↑
The Ordovician event, the first of at least five mass extinction events, occurred about 440 million years ago, and may have been due to an ice age. About 86% of species were wiped out[16a]. Tetrapods, the first four legged animals, evolved about 400 million years ago. These are the ancestors of all amphibians, reptiles, birds, and mammals.
The second mass extinction, the Devonian event, occurred about 360 million years ago, and about 75% of species were wiped out[16b]. The third, and most devastating mass extinction, the Permian mass extinction, occurred about 250 million years ago. About 96% of species were wiped out[16c]. It's not clear what caused these events, but they may have been caused by comet or asteroid impacts.
The first dinosaurs evolved about 225 million years ago, and they became dominant after a fourth mass extinction, the Triassic-Jurassic mass extinction. This occurred about 200 million years ago, and may have been due to volcanic activity. About 80% of species were wiped out[16d].
Archaeopteryx, evolved about 150 million years ago, around the time that mammals and marsupials diverged. Plants began to flower about 130 million years ago, and grass evolved about 70 million years ago.
The fifth mass extinction event, the Cretaceous event, occurred about 65 million years ago, and wiped out 76% of species, including most of the different species of dinosaurs[16e]. This was most probably due to the Chicxulub object, which may have been an asteroid or a comet.
The Chicxulub object was about 10 km wide and struck the Yucatan Peninsula in Mexico about 65 million years ago. The resulting explosion is thought to have released the same amount of energy as 100,000 billion tonnes of TNT, making it 2 million times more powerful than the most powerful hydrogen bomb ever detonated.
The Chicxulub object is thought to have landed on a bed of sulfur, 10 metres below the surface of the ocean. The atmosphere contained more oxygen than it does today, and so the sky was more combustible. It set alight, mixing with the sulfur to create a rain of sulfuric acid.
Light from the Sun was blocked out, and so there was very little photosynthesis on Earth for several months. This meant that most of the creatures that survived were those in food chains dependent on dead plant material - detritus. The remaining dinosaurs evolved into modern birds, and mammals began to prosper.
5. Hominids ↑
Over 99% of our evolutionary history follows the same line as chimpanzees, that is until about 6 million years ago, when our ancestors diverged. About 4 million years ago, a new species of hominid, or great ape, Australopithecus, evolved in the tropical forests of Africa.
Australopithecus were one of the first hominids to walk upright on two legs, and possibly the first to create tools out of stone. They became the dominant species of hominid until Homo habilis evolved about 2.4 million years ago.
The skulls of Homo habilis revel that they had larger brains than the Australopithecus, and it's thought that they were better equipped to learn by imitation. Within a million years, Australopithecus had vanished.
Human family tree. Image credit: The Smithsonian National Museum of Natural History/CC-NC-A.
Almost 1.7 million years ago, another species, Homo erectus, evolved, possibly directly from habilis. Their lives overlapped until just under 1.4 million years ago, when habilis died out. Homo erectus explored the Earth, with specimens found in Africa, Europe, Indonesia, and China. They were also thought to hunt, to use fire, to make complex tools, and to build campsites. They may have looked after each other when they were weak or frail, and may have begun to develop simple language skills. It's possible that language developed about 300,000 years ago. Homo erectus died out about 150,000 years ago.
6. Homo sapiens ↑
The void left by Homo erectus was filled by two new types of hominid, Homo neanderthalensis, Neanderthals, and Homo sapiens, human beings. Neanderthals evolved about 400,000 years ago, and Homo sapiens evolved about 200,000 years ago. Humans and Neanderthals were able to reproduce, and began interbreeding about 100,000 years ago.
Neanderthals proved to be tougher than humans in some respects, living in colder areas, occupied by cave lions, cave bears, woolly rhinos, and woolly mammoths, while most humans were still living in a tropical climate. The brains of Neanderthals were just as large as the brains of humans. They made tools, clothes, and jewellery, and there is evidence that they placed flowers in the graves of their dead, but they couldn't master the same long-range weapons that the humans had. Neanderthals died out about 30,000 years ago.
The first dogs may have been domesticated about 35,000 years ago. By this time, humans had begun creating images, and by about 15,000 BCE, humans had created paintings, drawings, engravings, sculptures, and ceramics.
Humans began farming in about 10,000 BCE, and towns and then cities began to develop as people formed larger and larger groups. The city of Jericho, in the West Bank, formed in about 9,000 BCE, and cows were domesticated in Africa and the Middle East in about 8,000 BCE, around the time most woolly mammoths became extinct. Within 500 years, the city of Çatalhöyük formed in Turkey, and wheat was cultivated in the Middle East[48a].
Sheep were domesticated in the Middle East in about 7,000 BCE, and rice was cultivated in China. Chickens were domesticated in about 6,000 BCE in Southeast Asia[48b]. Finally, people began to develop a written language in about 3000 BCE.
Although humans appear to dominate the Earth, we're still living in an age of bacteria. There are thought to be more bacteria, by weight, than all other life forms combined, and humans contain at least as many bacterial cells as human cells.
UPDATE: This page has been updated because new evidence shows that eukaryotic cells evolved over 1.5 billion years ago, rather than 2.7 billion years ago, and that Eukaryotes divided into three groups, within a few hundred million years of this. It has also been updated to reflect the fact that Neanderthals evolved about 400,000 years ago, and interbreed with humans about 100,000 years ago, the first dogs may have been domesticated about 35,000 years ago, rather than about 12,000 years ago, and humans contain at least as many bacterial cells as human cells.