  Acceleration of hadrons in relativistic shocks has been long expected and
invoked to model GRB high-energy photon and neutrino emissions. However, so far
there has been no direct observational evidence of hadronic emission from GRBs.
The B.O.A.T. ("brightest of all time") gamma-ray burst (GRB) 221009A had
extreme energies (with an isotropic energy exceeding $10^{55}$ erg) and was
detected in broad-band including the very-high-energy (VHE, $>100\,\rm GeV$)
band up to $>10$ TeV. Here we perform a comprehensive spectral analysis of the
GRB from keV to TeV energy range and perform detailed spectral and light curve
modelings considering both the traditional synchrotron self-Compton process and
the electromagnetic (EM) cascade process initiated by hadronic interactions by
accelerated cosmic rays in the external shock. We find that the leptonic
scenario alone is not adequate to account for the observations, whereas the
proposed scenario with the combination of hadronic and leptonic components can
well reproduce the multi-wavelength spectra and the light curve. This result
reveals the existence of the accelerated hadronic component in the early
afterglow of this extreme burst. According to this scenario, the observed TeV
light curve should contain imprints of the prompt MeV emission.
