  It has been recently proposed that at each infinite distance limit in the
moduli space of quantum gravity a perturbative description emerges with
fundamental degrees of freedom given by those infinite towers of states whose
typical mass scale is parametrically not larger than the ultraviolet cutoff,
identified with the species scale. This proposal is applied to the familiar
ten-dimensional type IIA and IIB superstring theories, when considering the
limit of infinite string coupling. For type IIB, the light towers of states are
given by excitations of the D1-brane, as expected from self-duality. Instead,
for type IIA at strong coupling, which is dual to M-theory on $S^1$, we make
the observation that the emergent degrees of freedom are bound states of
transversal M2- and M5-branes with Kaluza-Klein momentum along the circle. We
speculate on the interpretation of the necessity of including all these states
for a putative quantum formulation of M-theory.
