  We combine old and new quantum field theoretic arguments to show that any
theory of stable or metastable higher spin particles can be coupled to gravity
only when the gravity sector has a stringy structure. Metastable higher spin
particles, free or interacting, cannot couple to gravity while preserving
causality unless there exist higher spin states in the gravitational sector
much below the Planck scale $M_{\rm pl}$. We obtain an upper bound on the mass
$\Lambda_{\rm gr}$ of the lightest higher spin particle in the gravity sector
in terms of quantities in the non-gravitational sector. We invoke the CKSZ
uniqueness theorem to argue that any weakly coupled UV completion of such a
theory must have a gravity sector containing infinite towers of asymptotically
parallel, equispaced, and linear Regge trajectories. Consequently,
gravitational four-point scattering amplitudes must coincide with the closed
string four-point amplitude for $s,t\gg1$, identifying $\Lambda_{\rm gr}$ as
the string scale. Our bound also implies that all metastable higher spin
particles in 4d with masses $m\ll \Lambda_{\rm gr}$ must satisfy a weak gravity
condition.
