Timeline

4.6 billion

Geosphere: Formation of Earth
As the Proto-Earth cools, it becomes a planet—mostly a hot, turbulent mass of solid, semi-solid, and liquid minerals known as a mantle, with an iron core at the center. At the surface, a thin crust forms, mainly dark-gray to black rock known as basalt.

Biosphere: Earth’s first atmosphere
Hydrogen and helium surround the planet, but are quickly stripped away by intense heat and the solar wind—streams of high-energy particles ejected from the Sun. Asteroids and comets bombard the surface. All the elements necessary for life are present, but it will have to wait until conditions improve and stabilize

4.5 billion

Geosphere: Formation of Moon
Cataclysmic collision between Earth and another planet the size of Mars sends a huge cloud of debris into orbit around the Earth. The debris coalesces to form the Moon.

Biosphere: Back to Square One
Its new-born crust destroyed, the Earth survives in one piece, sweeps up other debris from the collision, and again begins to cool.

4.5–4.0 billion

Geosphere: Black Earth
The earth again forms a thin crust above layers of magma. Volcanoes eject gasses that create a new atmosphere and streams of molten magma, or lava, which add to the crust.

Biosphere: Earth’s Second Atmosphere
The new atmosphere consists of methane, carbon dioxide, water, and other gases. It’s much denser than ours and contains very little free oxygen (O2). Water in the cooling atmosphere condenses into clouds, rains down, and creates vast oceans.

4.0–3.8 billion

Geosphere: Late Heavy Bombardment
Asteroids and comets bombard the Earth in record numbers and with overwhelming violence. As the bombardment tapers off, the Earth resumes cooling, oceans reappear, and a crust again covers the planet. Convection currents swirling in the mantle begin to break up the crust and force the pieces to collide, split, and merge.

Biosphere: Back to Square One—Again
Much of the crust melts; the oceans boil away. The atmosphere survives—along, perhaps, with some water inside remnants of the crust.

3.8–3.5 billion

Geosphere: Life Emerges
Water from the new oceans seeps through the crust into the circulating magma, to be ejected along with magma at fissures, further breaking up the rocky surface of the crust and initiating chemical reactions leading to new minerals.

Biosphere: Life Emerges: Date Unknown
Single-cell organisms with complex structures already exist at 3.5 billion years, so the first living things must have emerged earlier. No fossil evidence has been found to tell us when; geologic processes seem to have erased all traces.

3.5 billion

Geosphere: First evidence of life
The emergence and proliferation of life begins a continuous, never-ending process of change in the chemistry and composition of the atmosphere, the oceans, the ocean floors, and the land. Interactions between organisms and existing minerals will create thousands of new minerals.

Biosphere: First fossil evidence of living creatures
The earliest organisms likely draw energy from geochemical reactions, only later evolving processes such as photosynthesis—the ability to absorb energy from sunlight and convert it into chemical energy. Only single-cell organisms exist. They will remain the sole form of life for 2 billion years.

3.45 billion

Geosphere: Magnetic field forms
The Earth’s iron core begins to generate a permanent but changing magnetic field.

Biosphere: Magnetic Earth
The magnetic field begins to deflect high-intensity ultraviolet solar radiation that poses a threat to life.

3.2–1.9 billion

Geosphere: First single cell organisms
Tectonic processes continue, forming continents as plates permanently merge and creating faults, rifts, ocean trenches, mountain ranges, volcanoes, and thermal vents.

Biosphere: Energy from light
Photosynthesis appears, allowing cells to absorb energy from sunlight and convert it into chemical energy. The first photosynthesizers don’t generate oxygen as a by-product and are believed not to use water (H20) but hydrogen sulfide (H2S).

2.7 billion

Geosphere: Nitrogen fixation
In the oceans, sulfur and oxygen levels increase, then iron levels decrease. The rise of oxygen makes ammonia unstable, so organisms can no longer rely on geochemistry as a source. They must learn to make their own ammonia—a process known as “nitrogen fixation”—on of evolution’s great innovations. Nitrogen fixation allows organisms to survive the rise of oxygen, to become more independent of geology, and to colonize new environments, eventually including land.

Biosphere: Biochemical processes continue to evolve
Organisms first evolve nitrogen fixation, which many organisms now rely on to produce amino acids.

2.5 billion

Geosphere: Oxygen generation
Cyanobacteria began generating oxygen 200 million years earlier, but most of it was absorbed in the oxidation of the iron-rich rock continuously disgorged by volcanoes. Now, volcanic activity diminishes, large continental land masses develop, ocean shallows expand, and cyanobacteria populations explode. Absorption can’t keep up with generation, and oxygen begins to fill the oceans and the atmosphere

Biosphere: A critical event
Cyanobacteria are the first organisms to produce oxygen as a by-product of photosynthesis. Some cyanobacteria and their ancestors live in shallow water colonies that form structures called stromatolites—large, layered mats of microbes cemented together by mud.

2.0 billion

Geosphere: Red Earth: The great oxidation event
Rising oxygen levels cause iron and iron-containing minerals in the black basalt to oxidize, turning the Earth’s rocky surface shades of red. Rising oxygen levels also create a layer of ozone (O3) in the upper atmosphere, further blocking ultraviolet solar radiation.

Biosphere: Earth’s Third Atmosphere
Rising oxygen levels kill off countless organisms that fail to adapt but open the door to a vast array of new forms of life. Increased protection from the ultraviolet radiation facilitates the evolution of organisms that leave the water and spread across the land.

1.9–1.0 billion

Geosphere: Multicellular organisms
Explosive growth in the kinds and numbers of organisms profoundly changes the chemistry of oceans. The era of banded iron and similar formations comes to an end. At around 1.5 billion years, stromatolites find themselves on the brink of extinction.

Biosphere: Multicellular organisms
Emerging at an unknown date, they lead to oxygen-based eukaryotes such as the plants and animals we know today. But single-cell organisms are still the most abundant form of life on Earth, thriving in environments once thought too extreme—from deep-ocean thermal vents to arctic ice.

750–570 million

Geosphere: Snow ball Earth: Recurring Ice Ages
Rapid fluctuations in the composition of the atmosphere lead to extreme changes in climate, including alternating periods when ice blankets the Earth.

Biosphere: Struggle to survive
Great numbers of species and ecosystems are driven to extinction. Vast coastal algae blooms come and go as the climate warms and cools. The return to stability ignites the Cambrian Explosion—a momentous period when new organisms appear in great numbers and incredible variety.

400 million

Geosphere: Blue Green Earth: Plants cover the surface
The action of plants and fungi breaks down rocks and minerals to create clays and soils.

Biosphere: The Earth we know
Return of a relatively warm and stable climate allows plants and animals to flourish and the Earth to evolve into the planet we know today.