  Questions regarding how primordial or pristine the comets of the solar system
are have been an ongoing controversy. In this review, we describe comets'
physical evolution from dust and ice grains in the solar nebula to the
contemporary small bodies in the outer solar system. This includes the phases
of dust agglomeration, the formation of planetesimals, their thermal evolution
and the outcomes of collisional processes. We use empirical evidence about
comets, in particular from the Rosetta Mission to comet
67P/Churyumov--Gerasimenko, to draw conclusions about the possible thermal and
collisional evolution of comets.
