Earth without oxygen?? Rise of oxygen
Oxygen makes up about one-fifth the volume of Earth's atmosphere today, and is a central element of life as we know it.
But that wasn't always the case. Oxygen, although always present in compounds in Earth's interior, atmosphere, and oceans, did not begin to accumulate in the atmosphere as oxygen gas (O2) until well into the planet's history. What the atmosphere was like prior to oxygen's rise is a puzzle that Earth scientists have only begun to piece together.
Earth coalesced a little more than 4.5 billion years ago from bits of cosmic debris. Liquid oceans existed on the planet almost from the beginning, although in all likelihood they were repeatedly vaporized by the massive meteorites that regularly clobbered the planet during its first 700 million years of existence. Things had settled down by 3.8 billion years ago, when the first rocks that formed under water appear in the geologic record. (They exist in what is now southwest Greenland.)
If Earth had water, it must have had an atmosphere, and if it had an atmosphere, it must have had a climate. What was Earth's early atmosphere made of? Nitrogen (N2), certainly. Nitrogen makes up the bulk of today's atmosphere and likely has been around since the beginning. Water vapor (H2O), probably from volcanic emissions. Carbon dioxide (CO2), also emitted by volcanic eruptions, which were plentiful at that time. And methane (CH4), generated inside the Earth and possibly also by methane-producing microbes that thrived on and in the seafloor, as they do today.
Carbon dioxide, water vapor, and methane played an important role in Earth's subsequent development. Four billion years ago, the Sun was 30 percent dimmer, and therefore colder, than it is today. Under such conditions, Earth's water should have been frozen, yet clearly it wasn't. The water vapor, carbon dioxide, and methane acted as greenhouse gases, trapping heat and insulating the early Earth during a critical period in its development.
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