What did creatures breathe before there is the oxygen on earth?

Billions of years before oxygen, highly toxic arsenic may have been the compound that brought new life to Earth.

In Laguna La Brava, an area of Chile's Atacama Desert, scientists have been studying a purple photosynthetic microorganism that lives in permanently oxygen-free, high-Salt Lake water.

Geologist Pieter Visscher from University of Connecticut says, “I’ve been studying this underwater microbial mat for about 35 years.”

“This is the only system on earth where I can find a microbial mat that can survive in complete anaerobic conditions.”

Microbes have been living on earth for at least 3.5 billion years, fossilized to form stromatolites. But for the first billion years that microbial mat existed, there was no oxygen on Earth for them to photosynthesize.

How to survive for these life forms under such extreme conditions is unknown, but by studying stromatolites and living extremophiles, researchers have managed to come up with some possible answers.