  The surface-brightness profiles of galaxies I(R) and the density profiles of
dark-matter halos rho(r) are well represented by the same analytic function,
named after either S\'ersic, I~exp[-(R/R*)^(1/m)], or Einasto,
rho~[exp[-(r/r*)^alpha], where R* and r* are characteristic radii. Systems with
high S\'ersic index m (or low Einasto index alpha) have steep central profiles
and shallow outer profiles, while systems with low m (or high alpha) have
shallow central profiles and steep profiles in the outskirts. We present the
results of idealized numerical experiments which suggest that the origin of
these profiles can be traced back to the initial density fluctuation field:
high-alpha (low-m) systems form in smooth regions via few mergers, while
low-alpha (high-m) systems form in clumpy regions via several mergers.
