The acquisition of cellular carbon from inorganic carbon is a prerequisite for life and marked the transition from the inorganic to the organic world. Recent theories of the origins of life assume that chemo-evolution took place in a hot volcanic flow setting through a transition metal-catalysed, autocatalytic carbon fixation cycle. Many archaea live in volcanic habitats under such constraints, in high temperatures with only inorganic substances and often under anoxic conditions. In this Review, we describe the diverse carbon fixation mechanisms that are found in archaea. These reactions differ fundamentally from those of the well-known Calvin cycle, and their distribution mirrors the phylogenetic positions of the archaeal lineages and the needs of the ecological niches that they occupy.