  One of the main aims of the ESA Rosetta mission is to study the origin of the
solar system by exploring comet 67P/Churyumov-Gerasimenko at close range. In
this paper we discuss the origin and evolution of comet
67P/Churyumov-Gerasimenko in relation to that of comets in general and in the
framework of current solar system formation models. We use data from the OSIRIS
scientific cameras as basic constraints. In particular, we discuss the overall
bi-lobate shape and the presence of key geological features, such as layers and
fractures. We also treat the problem of collisional evolution of comet nuclei
by a particle-in-a-box calculation for an estimate of the probability of
survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar
system. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus
are derived from two distinct objects that have formed a contact binary via a
gentle merger. The lobes are separate bodies, though sufficiently similar to
have formed in the same environment. An estimate of the collisional rate in the
primordial, trans-planetary disk shows that most comets of similar size to
67P/Churyumov-Gerasimenko are likely collisional fragments, although survival
of primordial planetesimals cannot be excluded. A collisional origin of the
contact binary is suggested, and the low bulk density of the aggregate and
abundance of volatile species show that a very gentle merger must have
occurred. We thus consider two main scenarios: the primordial accretion of
planetesimals, and the re-accretion of fragments after an energetic impact onto
a larger parent body. We point to the primordial signatures exhibited by
67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the
collisional evolution.
