*Scientists do not know how life began on Earth, but they do know that the early Earth’ atmosphere was very different from the atmosphere now.**
Geologists estimate that the Earth formed around 4.54.5 billion years ago. This estimate comes from measuring the ages of the oldest rocks on Earth, as well the ages of moon rocks and meteorites, by radiometric dating (in which decay of radioactive isotopes is used to calculate the time since a rock’ formation). For many millions of years, early Earth was pummeled by asteroids and other celestial objects. Temperatures also would have been very high (with water taking the form of a gas, not a liquid). The first life might have emerged during a break in the asteroid bombardment, between 4.44.4 and 4.04.0 billion years ago, when it was cool enough for water to condense into oceans. However, a second bombardment happened about 3.93.9 billion years ago. It’ likely after this final go-round that Earth became capable of supporting sustained life.
