  We compare the properties of dwarf galaxies in the Local Group with the
simulated galaxies formed before reionization in a cosmological simulation of
unprecedented spatial and mass resolution,including radiative feedback effects.
We find that a subset of the Local Group dwarfs are remarkably similar to the
simulated dwarf galaxies in all their properties already before reionization.
Based on this similarity, we propose the hypothesis that Local Group dwarfs
form in a variety of ways: some of them are ``true fossils'' of the
pre-reionization era, some of them form most of their stars later, after
reionization (we call them ``survivors'' of the reionization era), and the rest
of them form an intermediate group of ``polluted fossils''. We also identify a
simple observational test that is able to falsify our hypothesis.
