  Asymptotically massless towers of species are ubiquitous in the string
landscape when infinite-distance limits are approached. Due to the remarkable
properties of string dualities, they always comprise Kaluza-Klein states or
higher-spin excitations of weakly coupled, asymptotically tensionless critical
strings. The connection between towers of light species and small black holes
warrants seeking a bottom-up rationale for this dichotomoy, dubbed emergent
string conjecture. In this paper we move a first step in this direction,
exploring bottom-up constraints on towers of light species motivated purely
from the consistency of the corresponding thermodynamic picture for small black
holes. These constraints shed light on the allowed towers in quantum gravity,
and, upon combining them with unitarity and causality constraints from
perturbative graviton scattering, they provide evidence for the emergent string
scenario with no reference to a specific ultraviolet completion.
