Mars' ancient atmosphere may not have had much oxygen after all

"The scientists, however, stress that although oxygen may not have been present in Mars' ancient atmosphere after all, the planet still could have teemed with microscopic life forms in the past. "There are several life forms even on Earth that do not require oxygen to survive," Mitra said. "I don't think of it as a 'setback' to habitability — only that there was probably no oxygen-based lifeforms."

Formation of manganese oxides on early Mars due to active halogen cycling,

Do not worry, hope abounds in astrobiology to prove Mars had life. Once this is done, Mars will confirm abiogenesis belief 😊 At least that seems the goal on Mars since 1970s Viking landers 😊 The astrobiology department must be held to the same standard as the heliocentric solar system astronomy was. Provide necessary demonstration from nature that the paradigm is true. So far, nothing from the abiogenesis labs or astrobiology. Waiting patiently to see that the paradigm is true in nature, not just another probability creation story :)
Looking at Earth’s life cycle, we see that all life was anaerobic for quite a long time before something learned how to do photosynthesis. Then there was little free diatomic oxygen because the Earth’s minerals reacted with it and became oxides as fast as it was produced for another very long time. It was not until the minerals became more or less ‘saturated’ that O2 became a thing in the atmosphere. This is only conjecture, but, Perhaps I can see the same process happening on Mars, anaerobic life, photosynthesis, oxidation of the mineralogy, then Mars got too cold and not enough atmosphere to continue the process, and the last of the O2 was used up in the rocks.