<p>Magnesium, the third most abundant element in seawater, is thought to have played a fundamental role in the origin and evolution of life on Earth. Experimental and structural evidence supports catalytic and structural functions for magnesium in early molecular assemblies and in the formation of protocell membranes. Here, we review the ancient role of magnesium in biochemistry, which likely predates the emergence of complex enzyme systems. Beyond its involvement in life’s origins, magnesium is central to ATP hydrolysis and thus directly influences cellular energy flux. We propose that over billions of years of Earth history, magnesium’s function expanded from a primordial catalyst in life’s emergence to a persistent regulator of early cellular organization, metabolism, and ultimately the evolution of circadian timekeeping. This progression links planetary geochemistry, bioenergetics, and adaptation to Earth’s diurnal rotation, underscoring magnesium as a key element at the interface of geochemistry and biological evolution.</p>