Billions of years ago, life on Earth was mostly just large slimy mats of microbes living in shallow water. Sometimes, these microbial communities made carbonate minerals that over many years cemented together to become layered limestone rocks called stromatolites. They are the oldest evidence of life on Earth. But the fossils don’t tell researchers the details of how they formed.
Today, most life is supported by oxygen. But these microbial mats existed for a billion years before oxygen was present in the atmosphere. So what did life use instead?
Our team of geologists, physicists and biologists had found hints in fossilized stromatolites that arsenic was the chemical of choice for ancient photosynthesis and respiration. But modern-day versions of these microbial communities still live on Earth today. Perhaps one of these used arsenic and could offer proof for our theory?
So we joined a surveying expedition of Chilean and Argentinian scientists to look for living stromatolites in the extreme conditions of the High Andes. In a small stream deep in the Atacama Desert, we found a big surprise. The bottom of the channel was bright purple and made of stromatolite-building microbial mats that thrive in the complete absence of oxygen. Just as the clues we’d found in ancient fossils suggested, these mats use two different forms of arsenic to perform photosynthesis and respiration. Our discovery offers the strongest evidence yet for how the oldest life on Earth survived in a pre-oxygen world.
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Image via Pieter Visscher.