  Recently, it was shown that strongly correlated metallic fermionic systems
[Nature Phys. 3, 168 (2007)] generically display kinks in the dispersion of
single fermions without the coupling to collective modes. Here we provide
compelling evidence that the physical origin of these kinks are emerging
internal collective modes of the fermionic systems. In the Hubbard model under
study these modes are identified to be spin fluctuations which are the
precursors of the spin excitations in the insulating phase. In spite of their
damping the emergent modes give rise to signatures very similar to features of
models including coupling to external modes.
