3 billion years ago the Earth's atmosphere was warm, wet and primarily consisted of carbon dioxide and water vapor. This was the environment that the first primitive life forms on Earth arose.
It is thought that Mars had a similar environment to Earth, but due to the constant bombardment of meteorites, commets and asteroids, the gasses that formed the ancient Martian atmosphere escaped into space.
Now Mars has an atmosphere which consists mainly of carbon dioxide gas and has polar ice caps of carbon dioxide. The general appearance is that of an arid moon, the surface is very cold and is inhospitable to life as we know it.