  We point out that the anticommutation properties of the Dirac matrices can be
derived without squaring the Dirac hamiltonian, that is, without explicit
reference to the Klein-Gordon equation. We only require the Dirac equation to
admit two linearly independent plane wave solutions with positive energy for
all momenta. The necessity of negative energies as well as the trace and
determinant properties of the Dirac matrices are also a direct consequence of
this simple and minimal requirement.
