AM fungi are ancestral obligate symbionts, able to germinate, but unable to grow, in the absence of the host. Despite this, they have co-evolved with their host plants for about 400 million year. Many interesting hypotheses have been made on the evolutionary mechanisms which allowed these “living fossils” to survive, and the most important have been here discussed. The wide host range — 80% of land plants — ensures that germinating sporeshave a high probability to contact the roots of a host species, which are unambiguously recognised by AM fungi, which activate an “energy-saving” mechanism regulating the differentiation of infection structures. When no host-derived signals from the surrounding environment are perceived by germinating spores, fungal hyphae undergo a programmed growth arrest and resource reallocation, allowing long-term maintenance of viability and host-infection ability. The ability of hyphae originating from germinated spores to form anastomoses with self-compatible mycelia may provide a further survival strategy for obligately biotrophic AM fungi: the existence of large mycelial networks in soil means that young hyphae produced by spores germinating in the absence of the host may plug into the appropriate web as soon as the hyphal tip contacts a compatible mycelium. In my view, future research should focus on the role played by self-recognition between compatible mycelia in natural situations, where the wide host range of AM fungi and hyphal ability to form anastomosis may lead to the formation and establishment of indeterminate webs, from which young germlings growing far from their hosts could drain resources at an early stage, thus enhancing their chances of survival.