This timeline of the evolution of life outlines the major events in the development of life on the planet Earth. For a thorough explanatory context, see the history of Earth, and geologic time scale. The dates given in this article are estimates based on scientific evidence.
In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation. Its occurrence over large stretches of time explains the origin of new species and ultimately the vast diversity of the biological world. Contemporary species are related to each other through common descent, products of evolution and speciation over billions of years.
65 million years since the non-avian dinosaurs died out and
2 million years since humans started looking like they do today.
Note that Ma means "million years ago".
3800 Ma and earlier.
The planet Earth forms from the accretion disc revolving around the young Sun.
The planet Earth and the planet Theia collide, sending countless moonlets into orbit around the young Earth. These moonlets eventually coalesce to form the Moon. The gravitational pull of the new Moon stabilises the Earth's fluctuating axis of rotation and sets up the conditions for the formation of life.
The surface of the Earth cools enough for the crust to solidify. The atmosphere and the oceans form.
Between 4500 and 2500 Ma
The earliest life appears, possibly derived from self-reproducingRNA molecules. The replication of these organisms requires resources like energy, space, and smaller building blocks, which soon become limited, resulting in competition. Natural selection favours those molecules which are more efficient at replication. DNA molecules then take over as the main replicators. They soon develop inside enclosing membranes which provide a stable physical and chemical environment conducive to their replication: proto-cells.
Late Heavy Bombardment: peak rate of impact events upon the inner planets by meteors. This constant disturbance probably obliterated any life that had already evolved, as the oceans boiled away completely; conversely, life may have been transported to Earth by a meteor. 
Somewhere between 3900 - 2500 Ma
Cells resembling prokaryotes appear. These first organisms are chemoautotrophs: they use carbon dioxide as a carbon source and oxidize inorganic materials to extract energy. Later, prokaryotes evolve glycolysis, a set of chemical reactions that free the energy of organic molecules such as glucose. Glycolysis generates ATP molecules as short-term energy currency, and ATP continue to be used in almost all organisms, unchanged, to this day.
Bacteria develop primitive forms of photosynthesis which at first do not produce oxygen. These organisms generate ATP by exploiting a proton gradient, a mechanism still used in virtually all organisms.
Photosynthesizing cyanobacteria evolve; they use water as a reducing agent, thereby producing oxygen as waste product. The oxygen initially oxidizes dissolved iron in the oceans, creating iron ore. The oxygen concentration in the atmosphere subsequently rises, acting as a poison for many bacteria. The moon is still very close to the earth and causes tides 1000 feet high. The earth is continually wracked by hurricane force winds. These extreme mixing influences are thought to stimulate evolutionary processes. (See Oxygen Catastrophe)
A global glaciation may have reduced the diversity of life. Opinion is divided on whether it increased or decreased the rate of evolution.
The Ediacaran biota represent the first large, complex multicellular organisms - although their affinities remain a subject of debate.
Most modern groups begin to appear in the fossil record during the Cambrian explosion.
Around 540 Ma
The accumulation of atmospheric oxygen allows the formation of an ozone layer. This blocks ultraviolet radiation, permitting the colonisation of the land.
542 Ma - present
The Phanerozoic eon, literally the "period of well-displayed life", marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, which are divided by major mass extinctions.
542 Ma - 251.0 Ma
The first known footprints on land date to 530 Ma, indicating that early animal explorations may have predated the development of terrestrial plants.
The first primitive plants move onto land, having evolved from green algae living along the edges of lakes. They are accompanied by fungi, which may have aided the colonisation of land through symbiosis.
By the start of the Carboniferous period, the Earth begins to be recognisable. Insects roamed the land and would soon take to the skies; sharks predated the oceans, and vegetation covered the land, with seed-bearing plants and forests soon to flourish.
Four-limbed tetrapods gradually gain adaptions which will help them occupy a terrestrial life-habit.
The Mesozoic Marine Revolution begins: increasingly well-adapted and diverse predators pressurise sessile marine groups; the "balance of power" in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
Gymnosperm forests dominate the land; herbivores grow to huge sizes in order to accommodate the large guts necessary to digest the nutrient-poor plants.
The first accepted evidence for viruses - the group Geminiviridae at least exists. Viruses are still poorly understood and may have arisen before "life" itself, or may be a more recent phenomenon.
The rise of the Angiosperms: These flowering plants boast structures that attract insects and other animals to spread pollen. This innovation causes a major burst of animal evolution through co-evolution.
65.5 Ma - present
The Cretaceous–Tertiary extinction event eradicates about half of all animal species, including all non-avian dinosaurs.
Grasses evolve from among the angiosperms; grassland dominates many terrestrial ecosystems.
14,000 years ago
The term Anthropocene has been used to describe the period of time during which Man has had a major impact on the planet and its inhabitants. Its beginning is marked by the megafaunal extinction in the Americas which signify the onset of the Holocene extinction event. Fierce debate rages about the influence of man in the initiation of this extinction, but no one can deny that humanity is contributing to its propagation.
With a human population approaching 6.67 billion, the impact of humanity is felt in all corners of the globe. Overfishing, anthropogenic climate change, industrialisation, intensive agriculture, clearance of rain forests and other activities contribute to a dramatically rising extinction rate. At current rates, humanity will have eradicated one-half of Earth's biodiveristy over the next hundred years.
^ Planetary Science Institute page on the Giant Impact Hypothesis. Hartmann and Davis belonged to the PSI. This page also contains several paintings of the impact by Hartmann himself.
^ "However, once the Earth cooled sufficiently, sometime in the first 700 million years of its existence, clouds began to form in the atmosphere, and the Earth entered a new phase of development." How the Oceans Formed (URL accessed on January 9, 2005)
^ " Between about 3.8 billion and 4.5 billion years ago, no place in the solar system was safe from the huge arsenal of asteroids and comets left over from the formation of the planets. Sleep and Zahnle calculate that Earth was probably hit repeatedly by objects up to 500 kilometers across" Geophysicist Sleep: Martian underground may have harbored early life (URL accessed on January 9, 2005)
^ "'Experiments with sex have been very hard to conduct,' Goddard said. 'In an experiment, one needs to hold all else constant, apart from the aspect of interest. This means that no higher organisms can be used, since they have to have sex to reproduce and therefore provide no asexual control.' Goddard and colleagues instead turned to a single-celled organism, yeast, to test the idea that sex allows populations to adapt to new conditions more rapidly than asexual populations." Sex Speeds Up Evolution, Study Finds (URL accessed on January 9, 2005)
^ "The oldest fossils of footprints ever found on land hint that animals may have beaten plants out of the primordial seas. Lobster-sized, centipede-like or slug like animals such as Protichnites and Climactichnites made the prints wading out of the ocean and scuttling over sand dunes about 530 million years ago. Previous fossils indicated that animals didn't take this step until 40 million years later." Oldest fossil footprints on land
^ "The oldest fossils reveal evolution of non-vascular plants by the middle to late Ordovician Period (~450-440 m.y.a.) on the basis of fossil spores" Transition of plants to land
^ "The land plants evolved from the algae, more specifically green algae, as suggested by certain common
biochemical traits" The first land plants
^ "The ancestry of sharks dates back more than 200 million years before the earliest known dinosaur. Introduction to shark evolution, geologic time and age determination
^ "Viruses of nearly all the major classes of organisms—animals, plants, fungi and bacteria/archaea—probably evolved with their hosts in the seas, given that most of the evolution of life on this planet has occurred there. This means that viruses also probably emerged from the waters with their different hosts, during the successive waves of colonisation of the terrestrial environment." Origins of Viruses (URL accessed on January 9, 2005)
^ An United States Census Bureau estimate of the number of people alive on Earth at any given moment. United States census bureau
^ The American Museum of Natural History National Survey Reveals Biodiversity Crisis (URL accessed on February 23, 2006)