Which of the following correctly describes the state of Earth's oceans and atmosphere when our planet was young, 4 billion years ago? Four billion years ago, Earth's atmosphere was already very similar to now, made up of mostly nitrogen and with a substantial fraction of oxygen. However, Earth had no oceans at that time: liquid water oceans did not form on Earth until fairly recently, within the last billion years. Liquid water oceans began to form on Earth about 4 billion years ago. At that time, Earth's atmosphere was very different from now, containing a lot of nitrogen and carbon dioxide, but little to no oxygen. Liquid water oceans began to form on Earth about 4 billion years ago. At that time, Earth's atmosphere was already very similar to now, made up of mostly nitrogen and with a substantial fraction of oxygen. Four billion years ago, Earth's atmosphere was very different from now, containing a lot of nitrogen and carbon dioxide, but little to no oxygen. Earth also had no oceans at that time: liquid water oceans did not form on Earth until fairly recently, within the last billion years.