Researchers think they have some evidence:
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.
Pieter Visscher, Brendan Paul Burns, Kimberley L. Gallagher, “Ancient microbial life used arsenic to thrive in a world without oxygen” at The Conversation
Subsequent research showed a “vigorous arsenic cycle” in some modern microbes living in high-stress conditions in the Atacama Desert.
Well, there has to be some use for the chemical element arsenic. This one makes sense.