  We discuss fermion zero modes within the 3+1 brain -- the domain wall between
the two vacua in 4+1 spacetime. We do not assume relativistic invariance in 4+1
spacetime, or any special form of the 4+1 action. The only input is that the
fermions in bulk are fully gapped and are described by nontrivial
momentum-space topology. Then the 3+1 wall between such vacua contains chiral
3+1 fermions. The bosonic collective modes in the wall form the gauge and
gravitational fields. In principle, this universality class of fermionic vacua
can contain all the ingredients of the Standard Model and gravity.
