  Using cosmological N-body simulations of critical (SCDM) and open
(Omega_0=0.3, OCDM) cold dark matter models we have identified dark matter
halos which are associated with the progenitors of present day bright early
type galaxies. By following their merging history, we show how early type
galaxies that formed within massive halos at redshift around 3 are now
preferentially residing in clusters and groups. On the other hand, those that
formed through major merging events between redshift 1 and the present have not
yet been accreted into larger, virialized structures. This result is in
agreement with analytical predictions in hierarchical clustering models. CDM
models are able to explain both the ancient and uniform population of
ellipticals that dominates in clusters together with the more recent and
heterogeneous population of field ellipticals.
  Predictions for the comoving number density of bright early-type galaxies are
given, and are shown to be consistent with the observed luminosity function. We
predict that the number density of interacting bright binary galaxies, from
which the field population of ellipticals may have originated, is proportional
to (1+z)**4.2+-0.28 and (1+z)**2.5+-0.42 in SCDM and OCDM respectively. This
result is consistent with previous analytical estimates and is discussed
together with recent observational constraints.
