Our planet experienced a very violent early history. As planetesimals, comets, and asteroids crowded the inner solar system and rocked early Earth with numerous collisions, the infant Earth cooled and began to settle down. Unlike the decades-long perception that early Earth was a hot, volcanic mess, coated with numerous flows of lava, evidence now shows that the early history of the planet was dominated by a cool environment and with ample liquid ocean water.

Planetary scientists believe the scene turned somewhat ugly again during the so-called Late-Heavy Bombardment, a hypothesized period 4.1 to 3.8 billion years ago when scads of asteroids and comets rained in on our planet's surface, and on all the other young bodies of the inner solar system. This was long after Earth and the other planets accreted; but evidence for this period of bombardment exists from the Apollo Moon rock samples, which show a majority of melt-rocks on the Moon forming during this window. During this earliest, violent period of planetary formation, life on Earth was probably impossible.

But following the Late Heavy Bombardment, the story soon changed, and one day life arose on Earth. Our planet's age is thought to be about 4.54 billion years, based on the radiometric dating of meteoritic samples along with the oldest known Earth and Moon rocks. The oldest known Earth rocks, specifically, are those dated to 4.4 billion years from the Jack Hills region of Western Australia, betrayed by radioactive impurities in the zircon crystals they contain. The earliest microfossils known on Earth also come from Western Australia, from the so-called Strelley Pool formation, one of the oldest outcrops of sedimentary rock on the planet, and were discovered in 2011. They are primitive cyanobacteria, some 3.4 billion years old, and are the oldest known life we have. Spherical, oval, and tubular shaped, they span a mere hundredth of a millimeter across. Specimens that are more controversial could push the age back to 3.9 billion years, but they are as yet unconfirmed.

Scientists are just beginning to understand the complexity of how life arose on Earth, and how quickly it might have arisen.