The ubiquity of arsenic resistant genes across all of life’s variety suggests a close intimacy between arsenic biogeochemistry and evolution, over geological time scales. However, the behaviour of arsenic in past environments where life originated and its impact on our evolution is essentially unknown. Arsenic is of particular importance because of its toxic properties, prevalence in tight association with ubiquitous iron and sulfide minerals and as a major component of sulfide-rich waters, all common features of Precambrian oceans. Arsenic obstructs the synthesis of the building blocks of life, exhibiting both chronic and acute toxicity at very low concentrations. These properties make arsenic an agent capable of exerting strong selective pressure on the distribution, success and diversity of life. This is exemplified by when the release of arsenic into groundwater following rock-weathering processes results in widespread poisoning. Using the state of the art stable isotopes tools, coupled to biomass production, bacterial iron, arsenic and sulfur cycling under ancient oceanic conditions, this project will open a new discussion on the much debated relationship between ocean chemistry and evolution, by introducing a new arsenic framework. This will be achieved under three majors themes: 1) Does there exist a biogeochemical connection between arsenic and the timing and transition from the iron-rich to the hypothesized sulfide-rich oceans that are linked to the rise of atmospheric oxygen? 2) Does arsenic and sulfide show concomitant cyclicity during the Precambrian? 3) Could arsenic thus serve as a proxy for the calibration of key transitional steps in the timing of biological innovation?