A little less than four billion years ago, humans would have been able to live and breathe on Mars.

Oxford University's Bernard Wood authored a study published in the journal Nature based on surface rocks collected by the Spirit rover and compared them to meteorites.

Both the meteorites and the surface rocks showed signs of volcanic beginnings, but the surface rocks were richer with oxygen than the meteorites were. In 2004, NASA launched the Opportunity and Spirit rovers and the latter operated until 2010.

"What we have shown is that both meteorites and surface volcanic rocks are consistent with similar origins in the deep interior of Mars, but that the surface rocks come from a more oxygen-rich environment, probably caused by recycling of oxygen-rich materials into the interior," he told BBC News.

The rovers landed and collected sample in the Gusev Crater, a Martian region that is around 3.7 billion years old.

"This result is surprising because while the meteorites are geologically 'young', around 180 million to 1.4 billion years old," Wood said. "The Spirit rover was analyzing a very old part of Mars, more than 3.7 billion years old."

The Oxford professor theorized that Mars had oxygen before Earth, but not all of his contemporaries agree. Francis McCubbin of the University of New Mexico told BBC News that the oxidation of Mars' surface could have occurred without oxygen present.

"As oxidation is what gives Mars its distinctive color, it is likely that the 'red planet' was wet, warm and rusty billions of years before Earth's atmosphere became oxygen-rich," Wood said.

Despite his counter to Wood's theory, McCubbin agreed with the conclusion of a "substantial redox gradient with depth." Redox is a shortened term for the process reduction-oxidation, which could support certain types of life.

"The principal way we would expect to get oxygen is through photolysis of water - water vapour in Mars' atmosphere interacting with radiation from the Sun breaks down to form hydrogen and oxygen," Wood said. "But the gravity on Mars is one third of that on Earth, so hydrogen would be lost more easily. So the oxygen build-up could be enhanced on Mars relative to Earth."