Science is awesome.

Oxygen levels on early Earth rose, fell several times before great oxidation event

Short form: around 2.4 billion years ago, the rise of early plant life on Earth (largely cyanobcteria in the oceans) led to the first appearance of free oxygen in the atmosphere. Prior to this the atmosphere was pretty much oxygen-free, all oxygen being bound up in carbon dioxide. As the cyanobacteria emitted oxygen as a waste gas, it started to build up, but stayed at relatively low levels in the atmosphere for around a *billion* years because the oceans and land surfaces were busily drinking up the free oxygen. It wasn’t until about 850 million years ago that the oxygen “sinks” were filled up and oxygen could really start to accumulate in the atmosphere, finally allowing the development of modern animal life that depends on that oxygen.

New evidence suggests that 150 million years prior to the Great Oxidation Event of 2.4 billion years ago there was another “pulse” in the oxygen levels that fell back down to near-zero levels.The early history of life on Earth was clearly complex.

For billions of years Earth was a living place, but it would not have *looked* like a living place to modern astronomers. Right now people looking for signs of life in the universe are looking for places with free oxygen, but for Earth that’s only been the most recent quarter of the history of life on Earth. For a vast stretch of time the oceans were filled with dissolved iron; it took *forever* for available biologically emitted oxygen to finally bind with all that iron and sink to the ocean floor (forming the iron oxide deposits such as the one shown below). Only after all the iron was consumed could free oxygen really begin to accumulate. So there could well be other planets out there with oceans and life and atmospheres filled with carbon dioxide… and your efforts to terraform them will meet with the serious issue of the whole *planet* greedily gobbling up all the oxygen you try to fill the atmosphere with.