Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism.
While Earth was in its earliest stage (Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon.
Species continue to evolve, taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments.
The process of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor.
The history of Earth is divided into four great eons, starting 4,540 mya with the formation of the planet.
Each eon saw the most significant changes in Earth's composition, climate and life.
Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution.
Bacteria begin producing oxygen, shaping the third and current of Earth's atmospheres.
This sudden diversification of life forms produced most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era.
It is estimated that 99 percent of all species that ever lived on Earth, over five billion, The Earth's crust has constantly changed since its formation, as has life has since its first appearance.
Human activity is now a dominant force affecting global change, harming the biosphere, the Earth's surface, hydrosphere, and atmosphere with the loss of wild lands, over-exploitation of the oceans, production of greenhouse gases, degradation of the ozone layer, and general degradation of soil, air, and water quality.
In geochronology, time is generally measured in mya (megayears or million years ago), each unit representing the period of approximately 1,000,000 years in the past.
(In the graphic: Ga means "billion years ago"; Ma, "million years ago".) Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula.