Earth’s geosphere and biosphere have coevolved over time, influencing each other’s stability and keeping our planet habitable over the last ~4 billion years. Biogeochemical cycles play a key role in controlling this interaction, connecting long-term geological cycles and the much faster evolution of the Earth’s outer biologically dominated envelopes. A small set of microbial-encoded proteins containing redox-sensitive transition metals as their core catalytic center carry out the majority of the key biogeochemical reactions. Metals such as Fe, Co, Ni, Zn, Mo, W, V, and Cu are used in these proteins to access diverse redox couples as a function of the changing planetary availability of these elements over time. Despite the importance of this process, the relationship between metal availability and metabolism evolution and diversity has not been investigated in detail. Here, we will present recent data from field and laboratory experiments elucidating the impact of transition metal availability on microbial functional diversity, and its implications for the emergence and evolution of life on Earth, while discussing the implications of these findings for the search for life in the Universe.