Timeline

The Formation of Earth
The way Earth formed is still a matter of debate. The timeline below shows one of the more widely accepted theories.

5 billion years ago (BYA)

 * The solar system is formed by clumps of gas and dust. The largest of these clumps ignites and becomes the sun. Its gravity makes it absorb most of the gas in the young solar system.
 * Larger dust/gas clouds clump together to form the first planets.

4.5 BYA

 * The dust and gas in some of the young planets hardens, forming Mercury, Mars, Venus, and Earth. The young Earth is bombarded by comets and meteorites.
 * Volcanoes are formed by the Earth's plate tectonics and erupt, forming a primitive atmosphere of methane and carbon dioxide. The atmosphere is too hostile for life to evolve.

Ca. 4 BYA
Another mysterious planet crashes into Earth. The smaller planet (around the size of Mars) shatters and chunks are moulded onto the Earth; these form the continents. A smaller piece of the planet cools to form Earth's Moon. The Moon's pull on the Earth pulls the scattered water on Earth's surface into the oceans. The crater left by the impact formed into the Pacific ocean.

Precambrian
The Precambrian saw the Earth as a hostile place. However, in the waters, the first life formed.

2.4 BYA

 * Acids within the waters of Earth form the basis of life. The first life, in the form of microbes like bacteria, form and populate the oceans. The first predators - viruses - evolve to feed on the bacteria.
 * Later, algae evolve and begin to fill the oceans with oxygen. The oxygen in the ocean makes the waters more hospitible to complex organisms.

600 million years ago (MYA)

 * Algae grows into giant kelp and other seaweeds. Oxygen spills into the air.
 * The first animals, including jellyfish, starfish, and snails, evolve and begin feeding on tiny organisms in the water. Arthropods, such as trilobites, evolve too.
 * Primitive worms, like Spriggina populate the ocean floor. Some of these worms may have given rise to the first vertebrates.

Cambrian Period
During the Precambrian, the first life evolved and simple plants and animals populated the oceans. In the Cambrian, life would diversify still further. This sudden bloom in species is known as the Cambrian Explosion.

545 MYA

 * Arthropods diversify into many shapes and forms. Powerful predators, like Anomalocaris, evolve to feed on the abundant food on the ocean floor.


 * Corals form some of the earliest large reefs, which provide a new habitat for all kinds of wildlife.


 * One special soft-bodied animal, known as Pikaia, develops a primitive notochord (a flexible, rod-shaped structure inside the body) that may have become the first backbones.

Ca. 530 MYA

 * Coral reefs grow and develop, soon surrounding most continental shelves.


 * The aquatic predatory arthropods diversify to take on different specialized roles. Possibly because of a loss of food, some species become extinct and leave their fossils remarkably well-preserved.


 * Other groups of arthropods, like the ancestors of horshoe crabs and sea scorpions, begin to evolve.


 * The first fish also evolve. They stay small for the most part, and would have likely fed in a similar way to today's jawless fish.

485 MYA

 * Sea urchins and starfish evolve. They are presumably preyed on heavily.
 * Molluscs diversify into different forms.
 * Orthocones evolve, notably Orthoceras.

445 MYA

 * Fish numbers slowly increase. Jawed fish are nonexistant; most fish are algae-eating bottom feeders.
 * Eurypterids evolve and diversify.