  The distribution of heavy elements is anomalously low in the asteroid main
belt region compared with elsewhere in the solar system. Observational surveys
also indicate a deficit in the number of small ($ \le 50$~km size) asteroids
that is two orders of magnitude lower than what is expected from the single
power-law distribution that results from a collisional coagulation and
fragmentation equilibrium. Here, we consider the possibility that a major
fraction of the original asteroid population may have been cleared out by
Jupiter's secular resonance, as it swept through the main asteroid belt during
the depletion of the solar nebula. This effect leads to the excitation of the
asteroids' orbital eccentricities. Concurrently, hydrodynamic drag and
planet-disk tidal interaction effectively damp the eccentricities of sub-100
km-size and of super-lunar-size planetesimals, respectively. These combined
effects lead to the asteroids' orbital decay and clearing from the present-day
main belt region ($\sim 2.1-3.3$~AU). The intermediate-size (50 to several
hundreds of kilometers) planetesimals therefore preferentially remain as main
belt asteroids near their birthplaces, with modest asymptotic eccentricities.
The smaller asteroids are the fragments of subsequent disruptive collisions at
later times as suggested by the present-day asteroid families. This scenario
provides a natural explanation for both the observed low surface density and
the size distribution of asteroids in the main belt. It also offers an
explanation for the confined spatial extent of the terrestrial planet building
blocks without the requirement of extensive migration of Jupiter.
