For about seven-eights of the Earth’s history, its oceans were extremely rich in sulfides. This would have prevented animals and plants from surviving in 70% of the planet. But it was a great habitat for photosynthetic bacteria that require sulfides and sunlight to live. Known as purple and green sulfur bacteria (because those are the two colors it comes in) these single-celled microbes can only live in environments where they simultaneously have access to sulfides and sunlight.
That they thrived in the sulfide-rich ocean has been confirmed with the finding of fossilized pigments of purple sulfur bacteria in 1.6 billion-year-old rocks from the McArthur Basin in Northern Australia.
