  Two alternative scenarios concerning the origin and evolution of extremely
metal-poor halo stars are investigated. The first one assumes that the stars
have been completely metal-free initially and produced observed carbon and
nitrogen overabundances during the peculiar core helium flash typical for
low-mass Population-III stars. The second scenario assumes that the initial
composition resulted from a mixture of primordial material with ejecta from a
single primordial supernovae. Both scenarios are shown to have problems in
reproducing C, N, and O abundances simultaneously, and both disagree with
observed 12C/13C-ratios,though in different directions. We concentrate on the
most iron-poor, carbon-rich object of this class, HE0107-5240, and conclude
that the second scenario presently offers the more promising approach to
understand these objects, in particular because evolutionary tracks match
observations very well.
