Life on Earth

The timeline of life evolving on Earth
Credit: LadyofHats

Life on Earth has taken billions of years to evolve. Life is characterised as anything which has the following traits:


Earth's atmosphere from the
International Space Station (ISS)
Credit: NASA

An Atmosphere is a thin layer of gas which surrounds a planet or moon. The gravity of a planet or moon holds the Atmosphere in place. Small planets and moons have low mass and a weak gravitational pull.  This means they tend not to hold onto an atmosphere.